Unit: 1. Amplitude Modulation and Demodulation
Contents: Introduction to Analog Communication System The Electromagnetic & Optical
Spectrum and its usage; Radio spectrum and frequency allocation. Elements of communication
systems, Need for modulation, Amplitude Modulation principles, AM envelope, frequency
spectrum & BW, phase representation of AM wave, AM modulating circuits: Low level AM
modulation, medium power AM modulation, method Vestigial sideband(VSB). AM transmitters:
Block of low level DSBFC, High level DSBFC, SSB suppression techniques. TRF and Super
heterodyne receiver. Technical specification of AM broadcasting.
Introduction to Communication System
An Analog Communication System is a system where the information or message is
transmitted in the form of continuous signals, typically involving variations in amplitude,
frequency, or phase. These systems are commonly used for transmitting audio, video, or voice
signals. Analog communication systems have been widely used in traditional broadcasting and
telecommunication systems, though digital communication systems are now becoming more
prevalent.
Here’s an overview of key components and characteristics of an analog communication system:
Basic block diagram of communication system
Key Components of an Analog Communication System:
1. Source:
o The source generates the original signal that needs to be transmitted, which can be
an audio or video signal, for example, a human voice, music, or a TV signal.
2. Transmitter:
o The transmitter takes the message signal and encodes it into a form suitable for
transmission. In analog communication, this involves modulation of the signal
onto a carrier wave. Common modulation techniques include:
 Amplitude Modulation (AM): The amplitude of the carrier wave is
varied in accordance with the message signal.
 Frequency Modulation (FM): The frequency of the carrier wave is
varied according to the message signal.

Phase Modulation (PM): The phase of the carrier wave is varied in
accordance with the message signal.
3. Transmission Medium:
o The transmission medium is the physical medium through which the modulated
signal is transmitted from the transmitter to the receiver. This could be:
 Wired media like coaxial cables or optical fibers.
 Wireless media like radio waves, microwaves, or satellite links.
4. Receiver:
o The receiver’s job is to capture the transmitted signal, demodulate it, and extract
the original message signal. The receiver typically performs demodulation,
which is the reverse of modulation, to recover the original information.
5. Destination:
o The destination is where the recovered signal is presented to the user or to a
further system for processing. This could involve playing audio through a speaker
or displaying video on a screen.
Key Characteristics of Analog Communication Systems:
1. Continuous Signals:
o Analog signals are continuous in both time and amplitude. This means they can
take on an infinite range of values, which allows for the transmission of rich,
continuous information like sound or video.
2. Bandwidth Requirements:
o Analog communication typically requires a wider bandwidth than digital systems,
especially for high-quality transmissions like high-fidelity audio or television
signals. This can lead to congestion in frequency spectrum usage.
3. Susceptibility to Noise:
o Analog signals are more susceptible to noise and distortion as they travel through
the transmission medium. Noise can degrade the signal quality and lead to poor
reception or communication errors. Techniques like frequency modulation (FM)
can help mitigate noise to some extent.
4. Quality of Signal:
o Analog systems can transmit high-quality signals with very little delay, making
them ideal for real-time communication, such as in radio or traditional television
broadcasting.
5. Limited Data Transmission Rate:
o While analog systems can carry rich data like voice or video, they are not as
efficient in terms of data transmission rates when compared to modern digital
systems, especially when it comes to error correction and compression.
o
Types of Analog Communication Systems:
1. Amplitude Modulation (AM):
o
In AM, the amplitude of the carrier wave is varied in proportion to the
instantaneous value of the message signal. AM is used in AM radio broadcasting,
where the audio signal (voice or music) modulates a carrier wave.
2. Frequency Modulation (FM):
o In FM, the frequency of the carrier wave is varied according to the message
signal. FM is widely used in FM radio broadcasting and television audio
transmissions due to its greater resistance to noise compared to AM.
3. Phase Modulation (PM):
o In PM, the phase of the carrier wave is changed according to the information
signal. While not as common as AM or FM, it forms the basis of several advanced
communication systems.
Applications of Analog Communication:
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AM/FM Radio Broadcasting: AM and FM radio stations use analog communication to
transmit audio signals to the public.
Television Broadcasting: Analog TV signals use amplitude and frequency modulation
for transmitting video and audio to TV sets.
Public Address Systems: Analog communication is used in PA systems where
continuous audio signals are broadcasted in a local environment.
Telephone Systems: Traditional landline telephones use analog signals to carry voice
communication.
Advantages of Analog Communication:
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Simplicity: Analog systems are generally simpler in design and easier to implement
compared to digital systems.
Continuous Transmission: Analog systems provide continuous transmission, which is
beneficial for transmitting signals that change smoothly, such as voice and music.
Real-Time Communication: Analog systems are often ideal for real-time
communication since they do not require extensive processing or delays for data
encoding/decoding.
Disadvantages of Analog Communication:
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Noise Sensitivity: Analog systems are more susceptible to noise and interference, which
can degrade the quality of the signal.
Limited Data Handling: Analog systems have limited capacity for efficiently handling
large volumes of data.
Difficult to Encrypt: Analog signals are harder to encrypt securely compared to digital
signals, making them more vulnerable to interception.
Summary:
Analog communication systems have been fundamental in the development of mass
communication, including radio, television, and early telephone systems. Despite the rise of
digital communication systems in recent years due to their higher efficiency and noise resilience,
analog systems remain widely used in certain applications. Understanding the principles of
analog communication is essential as they laid the foundation for modern communication
technologies.
The Electromagnetic & Optical Spectrum and its usage
The Electromagnetic Spectrum (EM Spectrum) refers to the entire range of frequencies of
electromagnetic radiation. Electromagnetic waves are a form of energy that travels through space
and can travel through a vacuum. They vary in frequency (how many oscillations per second)
and wavelength (the distance between successive peaks of a wave).
The Optical Spectrum is a subset of the electromagnetic spectrum that is visible to the human
eye and includes light waves that range from ultraviolet to infrared radiation.
Overview of the Electromagnetic Spectrum
The EM spectrum is usually divided into different regions based on wavelength and frequency,
with each region having distinct characteristics and uses. Here's a breakdown of the key regions
in the EM spectrum:
Region
Radio Waves
Microwaves
Wavelength
Range
> 1 meter
Frequency
Range
< 300 GHz
Common Uses
Broadcasting (AM, FM, TV), mobile
communication (cell phones), radar, satellite
communication
1 mm to 1 300 MHz to Microwave
ovens,
radar,
satellite
meter
Infrared (IR) 700 nm to 1
mm
Visible Light 400 nm to 700
nm
10 nm to 400
Ultraviolet
nm
(UV)
0.01 nm to 10
X-rays
nm
Gamma Rays < 0.01 nm
300 GHz
300 GHz to
430 THz
430 THz to
770 THz
770 THz to 30
PHz
30 PHz to 30
EHz
> 30 EHz
communication, Wi-Fi, GPS
Night-vision devices, thermal imaging,
remote controls, heating
Human vision, photography, illumination,
optical communication
Sterilization, black lights, tanning, medical
applications
Medical
imaging, security scanning,
industrial testing
Cancer treatment (radiotherapy), nuclear
reactions, cosmic phenomena
Detailed Overview of Specific Regions and Their Usage
1. Radio Waves (1 meter to 100 km):
o Frequency Range: 3 kHz to 300 GHz
o Applications:
 Broadcasting: Radio and television broadcasts (AM, FM, TV signals).
 Communication: Mobile phone communication, including 4G and 5G
networks, and satellite communication.
 Navigation: Radar systems, GPS, and aviation communication systems.
 Military: Radar systems, secure military communications.
2. Microwaves (1 mm to 1 meter):
o Frequency Range: 300 MHz to 300 GHz
o Applications:
 Communication: Used in satellite communication, mobile networks, WiFi, Bluetooth, and microwave relays.
 Radar: Employed in weather radar, air traffic control, and military radar
systems.
 Ovens: Used in microwave ovens for heating food.
 GPS: Global Positioning System (GPS) uses microwaves for accurate
positioning.
3. Infrared (IR) (700 nm to 1 mm):
o Frequency Range: 300 GHz to 430 THz
o Applications:
 Thermal Imaging: Used in thermal cameras for detecting heat, such as in
night vision devices and industrial inspections.
 Remote Controls: Used in infrared remotes for consumer electronics.
 Medical: Used in therapeutic heating and some diagnostic tools.
 Astronomy: Observing astronomical objects that emit infrared radiation.
4. Visible Light (400 nm to 700 nm):
o Frequency Range: 430 THz to 770 THz
o Applications:
 Human Vision: The only part of the spectrum visible to the human eye,
enabling sight and perception of the world.
 Photography: Used in cameras and imaging systems.

Illumination: Lighting systems for various applications, from homes to
public streets.
 Optical Communication: Fiber-optic communication uses visible light to
transmit data with high speed and reliability.
5. Ultraviolet (UV) (10 nm to 400 nm):
o Frequency Range: 770 THz to 30 PHz
o Applications:
 Sterilization: UV light is used to disinfect air, water, and surfaces in
medical and industrial environments.
 Tanning: Ultraviolet rays are used in tanning beds, although prolonged
exposure can be harmful.
 Medical Applications: Used in phototherapy for conditions such as
jaundice or skin diseases.
 Forensics: UV light is used in forensic analysis to detect substances like
bloodstains.
6. X-rays (0.01 nm to 10 nm):
o Frequency Range: 30 PHz to 30 EHz
o Applications:
 Medical Imaging: X-rays are widely used in medical diagnostics,
including radiography and CT scans, to view the inside of the body.
 Security: X-ray machines are used in airport security to scan luggage.
 Astronomy: X-ray telescopes observe high-energy astronomical objects,
such as black holes and neutron stars.
7. Gamma Rays (< 0.01 nm):
o Frequency Range: > 30 EHz
o Applications:
 Cancer Treatment: Gamma radiation is used in radiotherapy to target and
destroy cancer cells.
 Nuclear Reactions: Gamma rays are emitted during radioactive decay and
nuclear fission.
 Cosmic Observations: Gamma-ray telescopes are used to study highenergy processes in the universe, including supernovae and gamma-ray
bursts.
The Optical Spectrum (Visible + Infrared + Ultraviolet)
The optical spectrum primarily refers to the visible light portion of the EM spectrum but is
often extended to include infrared and ultraviolet radiation. It is crucial in many aspects of daily
life and technology.
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
Visible Light: Enables human vision and forms the foundation of technologies such as
photography, display screens, and optical communications (fiber optics).
Infrared Light: Key to thermal imaging and sensing technologies, it is used in night
vision, remote sensing, and heating systems.

Ultraviolet Light: Used for sterilization, UV lamps, and in scientific fields such as
spectroscopy and chemical analysis.
Summary
The Electromagnetic Spectrum is a critical foundation for all modern communication and
many scientific applications. Its different regions (radio waves, microwaves, infrared, visible
light, ultraviolet, X-rays, and gamma rays) serve diverse purposes, from everyday uses like
mobile communication and medical imaging to specialized functions like cancer treatment and
astronomical observation.
Understanding how each part of the spectrum works, along with its applications, is essential for
harnessing its potential to develop new technologies and improve life across various industries.
A communication system typically consists of several key elements that work together to
transmit information from a source to a receiver. These elements are:
1. Source: The origin of the message or information that needs to be communicated. This
could be a person, device, or system that generates the data.
2. Message: The actual content or information that is being communicated. It could be in
the form of speech, text, audio, video, or any other data.
3. Transmitter: The device or system that encodes and converts the message into a signal
suitable for transmission. The transmitter typically takes the message and converts it into
an electrical signal, which may then be modulated onto a carrier wave for transmission.
4. Transmission Medium: The physical or logical channel through which the signal is
transmitted from the sender to the receiver. This could be air (for wireless
communication), cables (for wired communication), fiber optics, etc.
5. Receiver: The device or system that receives the transmitted signal. The receiver
demodulates and decodes the signal back into its original message for further processing
or presentation.
6. Demodulator: A component within the receiver that converts the transmitted signal back
into a form that the receiver can understand, essentially reversing the encoding or
modulation process.
7. Noise: Any unwanted interference or distortion that can affect the signal during
transmission. Noise can degrade the quality of the transmitted message and may need to
be minimized using various techniques.
8. Destination: The end-point or the recipient of the message, which could be a person,
device, or system that interprets and uses the information.
9. Feedback (optional): In some communication systems, feedback is provided by the
receiver to the sender to indicate receipt, request clarification, or provide additional
information.
These elements work together in various combinations depending on the type of communication
system (e.g., analog, digital, wireless, wired, etc.).
Need (necessity) of Modulation
Modulation is a critical process in communication systems, where the characteristics of a carrier
signal (such as its amplitude, frequency, or phase) are altered to encode information. The need
for modulation arises for several key reasons:
1. Efficient Use of the Frequency Spectrum:
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Different communication signals (such as radio, TV, or mobile signals) need to occupy
specific frequency bands. Modulation allows multiple signals to share the same medium
without interfering with each other by placing them at different frequencies
(multiplexing).
Without modulation, the signal would need to occupy a wide bandwidth, leading to
inefficient use of the available spectrum.
2. Transmission over Long Distances:

Low-frequency signals (such as audio or baseband signals) are not suitable for longdistance transmission due to high attenuation and distortion in the medium. Modulation
allows signals to be shifted to higher frequencies (radio frequencies, for instance) where
propagation characteristics are better suited for long-distance travel.
3. Minimizing Noise and Interference:
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Higher-frequency signals are less susceptible to noise and interference than lowfrequency signals. Modulation helps in reducing the effects of noise during transmission
by shifting the signal to a frequency band that is less prone to degradation.
Additionally, modulation techniques such as frequency modulation (FM) are more robust
against noise compared to other techniques like amplitude modulation (AM).
4. Multiplexing:
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Modulation allows for the transmission of multiple signals simultaneously over the same
channel, a process known as multiplexing. For example, in frequency-division
multiplexing (FDM), different signals are modulated onto different carrier frequencies,
allowing for more efficient use of the communication medium.
Mixing up of signals from different transmitters. Suppose many people are talking at the
same time.
We just cannot make out who is talking what.
Similarly, when many transmitters are transmitting baseband information signals
simultaneously, they get mixed up and there is no way to distinguish between them.
The possible solution is communication at high frequencies and allotting a band of
frequencies to each user.
This is what is being done for different radio and T.V. broadcast stations.
Such a situation can be avoided if every transmitter is assigned different high frequencies
for information transmission.

5. Improved Antenna Size:


Antennas are typically more efficient when their size is proportional to the wavelength of
the signal. Low-frequency signals (e.g., audio) would require very large antennas, which
are impractical for many applications. Modulating low-frequency signals onto higherfrequency carriers allows for practical antenna sizes while still transmitting the original
message.
E.g. Range of frequency of audio signal is from 20 Hz to 20 kHz. What is height of
antenna, if we want to transmit a signal of frequency of 1 kHz.
Minimum
length of
antenna
frequency 10 MHz is 7.5 m
required
to transmit a radio signal of
Here, f = 10 MHz = 107 Hz
• λ= c/f =3×108/107 = 30 m,
• Minimum length of antenna = λ/4=30/4 = 7.5 m
• Such length of antenna in reality is impractical as well as very costly.
• However, the length of the antenna for a signal of frequency 1 MHz turns out to be only
75 m.
• This shows that for effective transmission of high frequency signals, required antenna
length is small and hence such an antenna can be easily constructed.
6. Enabling Digital Communication:

In digital communication systems, modulation techniques such as Quadrature Amplitude
Modulation (QAM) or Phase Shift Keying (PSK) are used to represent binary data over
radio waves or optical fibers. These techniques convert digital signals into analog forms
that can be transmitted effectively over various communication channels.
7. Power Efficiency:
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
Modulating the signal onto a higher-frequency carrier allows for more efficient use of
power. For example, radio transmitters use relatively lower power for signal modulation
compared to direct transmission of baseband signals.
The study of electromagnetic radiation shows that the transmitted power by an antenna
of a given length is inversely proportional to the square of the wavelength λ
• As high powers are needed for good transmission, λ should be small i.e. antenna length
should be small, for which wavelength λ should be small or frequency f should be high.
• Hence, for
this
purpose,
also, the
use
of
high frequency
signal is inevitable.
8. Reduction of Signal Distortion:

Low-frequency signals are susceptible to distortion and attenuation, especially over long
distances or through certain mediums (like the atmosphere). Modulating a signal to
higher frequencies allows the signal to travel better and maintain its integrity during
transmission.
9. Facilitation of Different Types of Modulation:

Modulation provides flexibility in communication systems by allowing different types of
modulation (AM, FM, PM, QAM, etc.) based on the requirements of the system, such as
the desired data rate, signal robustness, and bandwidth efficiency.
In summary, modulation is essential for improving the performance, efficiency, and reliability of
communication systems, enabling long-range transmission, multiplexing, and reducing the
impact of noise and interference.
Amplitude Modulation (AM) Principles
Amplitude Modulation (AM) is a technique used in communication systems to transmit
information by varying the amplitude of a high-frequency carrier wave in proportion to the
instantaneous amplitude of the message signal (the baseband signal). AM is commonly used in
radio broadcasting, aviation communication, and certain two-way communication systems.
Basic Concept of Amplitude Modulation
In AM, the carrier wave (a high-frequency signal) is modulated by the message signal (typically
a low-frequency signal, such as audio or voice). The amplitude of the carrier wave is varied
according to the instantaneous amplitude of the message signal.
The general form of an AM signal can be expressed mathematically as:
s(t)=[Ac+Am⋅m(t)]⋅cos(2πfct)s(t)=[Ac+Am.m(t)] cos(2πfct)
s(t)=[Ac+Am⋅m(t)]⋅cos(2πfct)
Where:
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s(t) = The modulated AM signal.
Ac = The amplitude of the carrier signal.
Am = The amplitude of the modulating signal (message signal).
m(t) = The message signal.
fc= The frequency of the carrier wave.
cos(2πfct) = The carrier wave, a cosine function with frequency fc.
Key Components of Amplitude Modulation
1. Carrier Signal:
o The carrier signal is a high-frequency sinusoidal wave that carries the
information. It is the reference signal that is modulated by the message signal.
o The carrier has a frequency fcand a constant amplitude Ac.
2. Message Signal (Modulating Signal):
o The message signal is the information to be transmitted. It is typically a lowfrequency signal, such as a voice or audio signal.
o The message signal m(t) modulates the amplitude of the carrier.
3. Modulated Signal:
o The resulting AM signal is a combination of the carrier wave and the message
signal. The amplitude of the carrier varies in proportion to the instantaneous
amplitude of the message signal.
Mathematical Representation of an AM Signal
For simplicity, assume that the modulating signal m(t) is a low-frequency sinusoidal wave:
m(t)=Am⋅cos(2πfmt)
Where:


fm is the frequency of the modulating signal.
Am is the amplitude of the modulating signal.
Then, the AM signal becomes:
s(t)=[Ac+Amcos2πfmt)]⋅cos(2πfct)
This can be expanded using trigonometric identities:
s(t)=Ac cos(2πfct)+Am.2[cos(2π(fc+fm)t)+cos(2π(fc−fm)t)]
Thus, the AM signal consists of:
1. The carrier wave at frequency fc
2. Two sidebands:
o Upper sideband at fc+ fm
o Lower sideband at fc−fm
Frequency Components of AM
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Carrier Frequency: The central frequency of the AM signal, fc, remains unaltered.
Upper Sideband (USB): The frequency component at fc+fm.
Lower Sideband (LSB): The frequency component at fc−fm .
The AM signal contains the carrier frequency and two sidebands. The sidebands carry the actual
information, while the carrier is a reference signal that is not typically used for information
transfer but is necessary for demodulation.
Modulation Index (Depth of Modulation)
The modulation index (m), also known as the modulation depth, is a measure of the extent of
modulation and is defined as the ratio of the amplitude of the modulating signal to the amplitude
of the carrier:
m=Am/Acm
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If m=0, there is no modulation, and the carrier signal is just a pure sinusoidal wave.
If m=1, the carrier's amplitude is fully modulated, meaning the modulating signal has the
same amplitude as the carrier.
If m>1, it is called over-modulation, which leads to distortion in the signal and potential
interference, as the carrier may be reduced to zero or even inverted.
Bandwidth of an AM Signal
The bandwidth (BW) of an AM signal is determined by the frequency of the modulating signal
fm . The bandwidth of an AM signal is given by:
Bandwidth (BW)=2⋅fm
This means that the bandwidth of an AM signal is twice the frequency of the modulating signal.
For example, if the modulating signal has a frequency of 1 kHz, the bandwidth of the AM signal
will be 2 kHz.
Power in an AM Signal
The total power in an AM signal is distributed between the carrier and the two sidebands. The
total power Pt in the AM signal can be expressed as:
Pt=Pc(1+m2/2)
Where:
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Pc is the power of the carrier signal.
m is the modulation index.
Thus, the power in the sidebands increases as the modulation index increases, while the total
power remains constant. The carrier itself carries less power when the modulation index is
higher, as more power is distributed to the sidebands.
Advantages and Disadvantages of Amplitude Modulation
Advantages of AM:
1. Simplicity: AM is a relatively simple modulation technique, both in terms of
implementation and demodulation.
2. Compatibility: AM signals can be received by most basic radio receivers, making it easy
to deploy for broadcasting.
3. Long Range: AM signals can travel long distances, especially during nighttime when
lower frequencies tend to travel further.
Disadvantages of AM:
1. Power Inefficiency: A significant portion of the transmitted power is carried by the
carrier, which does not convey information. This makes AM less power-efficient
compared to other modulation techniques like Frequency Modulation (FM).
2. Susceptibility to Noise: AM signals are highly susceptible to noise and interference (e.g.,
from electrical equipment or atmospheric conditions), which can degrade signal quality.
3. Limited Bandwidth: AM signals require a relatively large bandwidth (twice the
frequency of the modulating signal), which can be inefficient in terms of spectrum usage.
Applications of Amplitude Modulation
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AM Radio Broadcasting: AM is widely used for medium-wave (MW) and shortwave
(SW) radio broadcasting.
Two-Way Radios: AM is used in aviation communication and other forms of two-way
communication.
Television Broadcasting: In analog television, AM was used for the transmission of
audio signals (with Frequency Modulation, FM, used for video).
Military and Emergency Services: AM have applications in some older military
communication systems and emergency communications.
Summary
Amplitude Modulation (AM) is a simple but important modulation technique that varies the
amplitude of a carrier wave according to the message signal. It is widely used in radio
broadcasting and other communication systems, but it has limitations such as power inefficiency
and susceptibility to noise. Despite its disadvantages, AM remains important due to its simplicity
and ease of implementation.
Amplitude Modulation (AM) Principles
Amplitude Modulation (AM) is a technique used to encode information onto a carrier wave by
varying the amplitude of the carrier wave in proportion to the instantaneous amplitude of a
baseband signal (also called the message or modulating signal). AM is widely used in analog
audio broadcasting, such as AM radio transmission, as well as in other communication systems.
Basic Concept of Amplitude Modulation
In AM, the high-frequency carrier signal is modified (or modulated) by the low-frequency
message signal (e.g., audio or voice) to carry the information. The frequency of the carrier
remains unchanged, but its amplitude varies according to the variations in the amplitude of the
message signal.
The general equation for an AM signal is:
s(t)=[Ac+Am⋅m(t)]⋅cos(2πfct)
Where:
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s(t) is the modulated signal (AM signal).
Ac is the amplitude of the carrier signal.
Am is the amplitude of the modulating signal (message signal).
m(t) is the message signal (which is a low-frequency signal).
fc is the frequency of the carrier signal.
cos(2πfct) represents the carrier wave, a high-frequency sine wave.
Key Components of Amplitude Modulation
1. Carrier Signal:
o This is a high-frequency signal that carries the information. The carrier signal has
a constant amplitude and frequency.
2. Message (Modulating) Signal:
o This is the signal that contains the information you want to transmit. It typically
represents voice, music, or other audio data, and it operates at a much lower
frequency than the carrier.
3. Modulated Signal:
o The resulting signal is the combination of the carrier signal and the modulating
signal. The amplitude of the carrier varies in response to the message signal's
amplitude.
Mathematical Representation of AM
For simplicity, let's assume the modulating signal m(t)m(t)m(t) is a sinusoidal waveform:
m(t)=Am⋅cos(2πfmt)
Where:


Am is the amplitude of the modulating signal.
fm is the frequency of the modulating signal (the message signal).
Then, the AM signal is:
s(t)=[Ac+Amcos(2πfmt)]⋅cos(2πfct)
This can be expanded using trigonometric identities:
s(t)=Accos(2πfct)+Am/2[cos(2π(fc+fm)t)+cos(2π(fc−fm)t)]

The AM signal consists of three main components:
o Carrier Signal: Accos(2πfct).
o Upper Sideband (USB): Am/2cos(2π(fc+fm)t)
o Lower Sideband (LSB): Am/2cos(2π(fc−fm)t).
o The sidebands carry the actual information (the message signal), while the carrier
wave carries no information but is necessary for transmission and demodulation.
Frequency Spectrum of AM
The frequency spectrum of an AM signal includes:
1. Carrier frequency: fc , a high-frequency sine wave.
2. Upper sideband (USB): A frequency component at fc+fm .
3. Lower sideband (LSB): A frequency component at fc−fm.
The bandwidth of an AM signal is determined by the modulating signal frequency fm and is
given by:
Bandwidth (BW) =2fm
For example, if the modulating signal has a frequency of 1 kHz, the bandwidth of the AM signal
will be 2 kHz (covering both the upper and lower sidebands).
Modulation Index
The modulation index (m) is a measure of the extent of modulation and is defined as the ratio of
the amplitude of the modulating signal to the amplitude of the carrier signal:
m=Am/Ac
When m=0, the carrier is unmodulated, and no information is transmitted.
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When m=1, the carrier is fully modulated, and the modulation is at its maximum without
over-modulation.
When m>1, the modulation is over-modulated, which can result in distortion and
interference.
Power in an AM Signal
The total power in an AM signal is distributed between the carrier and the sidebands. The total
power Pt in an AM signal can be expressed as:
Pt=Pc(1+m2/2)
Where:
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Pc is the power of the carrier signal.
m is the modulation index.
As the modulation index increases, the power in the sidebands increases, while the carrier power
decreases. However, the total power remains constant.
NUMERICALS
Advantages and Disadvantages of AM
Advantages:
1. Simplicity: AM is relatively easy to implement, both in terms of modulating and
demodulating the signal.
2. Long Range: AM signals can travel long distances, especially at lower frequencies, due
to the ionosphere's reflective properties, allowing signals to "bounce" over long distances.
3. Compatibility: AM signals are compatible with a wide range of receivers, making it
suitable for broadcasting to a large audience.
Disadvantages:
1. Power Inefficiency: AM signals are power-inefficient because a large portion of the
transmitted power is carried by the carrier, which does not carry any information.
2. Susceptibility to Noise: AM is highly susceptible to noise and interference, as amplitude
variations caused by noise can distort the signal.
3. Wide Bandwidth: AM requires a relatively wide bandwidth compared to other
modulation schemes (like Frequency Modulation, FM), making it less spectrum-efficient.
Applications of Amplitude Modulation
1. AM Radio Broadcasting:
o AM is primarily used in medium-wave (MW) and shortwave (SW) radio
broadcasting. It allows for the transmission of audio signals, such as news, music,
and talk shows.
2. Aviation Communications:
o AM is used for voice communication in aviation because it is simple and robust
for long-distance communication between aircraft and air traffic control.
3. Two-way Radios:
o Some older two-way communication systems, like walkie-talkies, use AM to
transmit signals.
4. Television Broadcasting (Analog):
o AM was used for audio transmission in analog television broadcasting, although
FM is typically used for video signals.
5. Military Communication:
o AM was widely used in military communication systems, especially in older
radios and systems.
Summary
Amplitude Modulation (AM) is a foundational modulation technique in communications,
especially in radio broadcasting. It involves varying the amplitude of a high-frequency carrier
wave in accordance with a lower-frequency message signal. Despite being power-inefficient and
vulnerable to noise, AM remains important due to its simplicity, historical significance, and wide
usage in specific applications like AM radio and aviation communication.
AM Envelope, Frequency Spectrum, and Bandwidth
Amplitude Modulation (AM) is a widely used method for transmitting information through the
variation of the amplitude of a carrier wave. When a carrier is modulated by a message signal,
the resulting modulated wave has several key characteristics that define its shape, frequency
components, and bandwidth. Let's explore the AM envelope, frequency spectrum, and
bandwidth of an AM signal in detail.
1. AM Envelope
The AM envelope is the smooth curve that traces the peak amplitude of the modulated signal as
it varies over time. It is a graphical representation of the modulation applied to the carrier wave.
Envelope Characteristics:


The envelope follows the variations in the amplitude of the carrier wave, which is
directly determined by the instantaneous amplitude of the modulating signal.
When we perform amplitude modulation, the carrier wave is multiplied by the
modulating signal. The resulting waveform contains a high-frequency carrier oscillating
within a pattern defined by the message signal. The envelope of the AM signal
corresponds to the modulating signal (message signal).
Example:
If the message signal is a sinusoidal signal
m(t)=Am cos(2πfmt)
the AM signal is: s(t)=[Ac+Am cos(2πfmt)]cos(2πfct)
The envelope of the AM signal is Ac+Am cos(2πfmt)Ac.
It is a sinusoidal waveform whose frequency corresponds to the frequency of the message signal
fm, and the amplitude varies according to the modulating signal's instantaneous amplitude.
2. Frequency Spectrum of AM
The frequency spectrum of an AM signal contains components at three distinct frequencies:
1. Carrier Frequency fc: The central frequency of the modulated signal. This is the
frequency of the unmodulated carrier.
2. Upper Sideband (USB): This frequency component is located at fc+fm, where fm is the
frequency of the modulating signal. The upper sideband contains the higher-frequency
components of the modulated signal, which carry the information.
3. Lower Sideband (LSB): This frequency component is located at fc−fm, representing the
lower-frequency components that carry the information from the message signal.
The full frequency spectrum of an AM signal can be represented as follows:



The carrier is located at fc
The upper sideband is at fc+fm
The lower sideband is at fc−fm
Frequency Spectrum Summary:



Carrier Frequency: fc
Upper Sideband: fc+fm
Lower Sideband: fc−fm

This spectrum shows that the AM signal consists of the carrier and two sidebands (upper and
lower) that are symmetrical about the carrier frequency.
3. Bandwidth of AM Signal
The bandwidth of an AM signal is determined by the frequency of the modulating signal fm ,
which is the highest frequency present in the message signal.
For an AM signal with a single-tone message signal (i.e., a sinusoidal modulating signal), the
bandwidth is defined as:
Bandwidth (BW)=2⋅fm
This is because the AM signal contains both the upper sideband and lower sideband, each
contributing a frequency component of fm . Therefore, the total bandwidth required for an AM
signal is twice the frequency of the modulating signal.
Example:

If the modulating signal m(t)m(t)m(t) has a frequency fm=1 kHz, the bandwidth of the
AM signal will be:
Bandwidth=2*1 kHz=2 kHz
This means that the AM signal will have frequencies from fc−fm to fc+fm, which in this case
would be from fc−1 kHz to fc+1 kHz.
Bandwidth in General:

If the message signal has a wider bandwidth (for instance, if it contains a range of
frequencies rather than a single tone), the bandwidth of the AM signal will also increase.
The bandwidth will always be twice the maximum frequency present in the modulating
signal.
Illustration of AM Frequency Spectrum
For a clearer understanding, consider a graphical representation of the AM signal's frequency
spectrum:
1. The carrier is represented as a spike at frequency fc.
2. The upper sideband (USB) is represented by a range of frequencies from fc +fm .
3. The lower sideband (LSB) is represented by a range of frequencies from fc−fm.
The frequency spectrum looks like this:
Summary of AM Envelope, Frequency Spectrum, and Bandwidth
1. AM Envelope: The envelope of an AM signal follows the variations in the amplitude of
the carrier wave and corresponds to the modulating (message) signal.
2. Frequency Spectrum: The AM signal's frequency spectrum contains:
o Carrier Frequency fc
o Upper Sideband at fc+fm ,
o Lower Sideband at fc−fm .
3. Bandwidth: The bandwidth of an AM signal is twice the frequency of the modulating
signal, i.e., BW=2⋅fm .In general, for a modulating signal with a frequency range, the
total bandwidth of the AM signal would be twice the highest frequency present in the
modulating signal.
Phase Representation of an AM Wave
In Amplitude Modulation (AM), the carrier wave's amplitude is modulated according to the
instantaneous amplitude of the modulating signal. To understand the phase representation of an
AM wave, we can visualize the modulated signal in the context of its phase as it varies over time.
Basic Concept of Phase Representation in AM
In AM, the carrier wave’s amplitude varies based on the modulating signal, but its phase
remains constant. However, if we look at the overall complex plane or the phasor
representation, we can visualize how the AM signal behaves in terms of phase and magnitude
as it changes over time.
For a clearer understanding, we need to break down how the carrier and the modulating signal
interact in terms of amplitude and phase.
1. AM Waveform
The general expression for an AM wave is:
s(t)=[Ac+Am⋅m(t)]⋅cos (2πfct)
Where:




Ac is the carrier amplitude.
Am is the modulating signal amplitude.
m(t) is the message signal.
fc is the frequency of the carrier.
The amplitude of the carrier signal cos⁡(2πfct)cos(2 pi fc t)cos(2πfct) changes in accordance
with the message signal m(t)m(t)m(t), but the phase of the carrier signal remains unaffected by
the modulation process in its typical form.
2. Phase Representation of a Pure Carrier
For a pure carrier wave Accos(2πfct)Ac, the signal in phasor form is:
Carrier Phasor: Ac∠0∘.
This means the carrier has a constant amplitude Ac and a phase of 0∘.
3. Phase Representation of Modulated AM Wave
When the carrier is modulated by a message signal, the amplitude of the carrier wave is no
longer constant, but its phase remains unchanged. The AM signal s(t)s(t)s(t) is still expressed as:
s(t)=[Ac+Am⋅m(t)]⋅cos(2πfct)
We can visualize this in a phase diagram as follows:



The amplitude of the carrier varies based on the modulating signal.
The phase of the carrier does not change, as the modulation only affects the amplitude.
The modulation index determines the extent to which the amplitude of the carrier varies.
Phase Representation: Time-domain and Complex-domain
Time-domain:


The instantaneous phase of the AM wave is always the same as that of the carrier wave.
The amplitude of the wave varies according to the modulation signal, but the phase does
not change.
Complex-domain (Phasor Diagram):

The phasor representing the carrier rotates at a constant angular frequency fc, and the
modulation only affects the amplitude of this phasor. The phase of the signal remains
fixed at 0∘0^circ0∘ in the case of AM modulation.
Visualizing Phase Representation of AM Wave
Consider a scenario where we plot the carrier and the modulated signal on the complex plane:
1. Carrier Signal: The carrier is represented as a rotating vector (phasor) at 0∘ with
amplitude Ac.
2. AM Modulated Signal: As the message signal modulates the carrier, the amplitude of
the carrier changes over time, but the phase of the phasor remains constant.
For example, in an AM signal with a modulation index m=0.5m = 0.5m=0.5, the instantaneous
amplitude of the carrier will vary as Ac+Amm(t), but the phase remains the same as the
unmodulated carrier's phase.
Summary: Phase Representation of AM Wave
In the phase representation of an AM wave:



The phase of the AM signal remains constant and identical to the phase of the carrier
signal.
The amplitude varies over time according to the modulating signal, but the phase does
not change.
In phasor terms, the amplitude of the rotating phasor changes in time, but the angle (or
phase) remains constant at 0∘.
This shows that while the AM signal’s amplitude is modulated, its phase remains constant,
unlike in some other modulation schemes (such as Frequency Modulation or Phase Modulation),
where the phase varies directly with the modulating signal.
Low-Level AM Modulation
Low-level AM modulation refers to a method of amplitude modulation (AM) in which the
modulation process takes place before the power amplification stage of the transmission system.
In this approach, the message signal (modulating signal) directly modulates the amplitude of a
low-power carrier signal, and then the modulated signal is passed to a power amplifier to
increase the power for transmission.
This is in contrast to high-level AM modulation, where the carrier signal is first amplified to a
high power level, and then the modulation process occurs.
Key Features of Low-Level AM Modulation
1. Modulation Before Amplification:
o In low-level AM modulation, the modulating signal controls the amplitude of the
carrier at a low power level before it is sent to a power amplifier.
o The modulated signal is then amplified for transmission.
2. Advantages:
o Simplicity: The modulation process is simpler since the carrier signal is
modulated at a low power before amplification.
o Efficient Use of Amplifier Power: Power amplifiers are typically designed to
operate at higher efficiency when they are amplifying a constant-amplitude
signal, such as a carrier, rather than a modulated signal. By modulating the carrier
before amplification, we ensure that the power amplifier operates efficiently.
3. Disadvantages:
o Potential for Non-Linear Distortion: Since the carrier is modulated at a low
power level, the modulation process may result in non-linear distortion when the
signal is later amplified. This can affect the quality of the transmitted signal.
o Low Power Output: The output power of the transmitter is low before
amplification, and additional power amplification is required to transmit the
signal over long distances.
How Low-Level AM Modulation Works
In a low-level AM modulation system, the following steps are typically involved:
1. Carrier Generation: A stable high-frequency carrier signal cos(2πfct) is generated at the
desired carrier frequency fc.
2. Modulating Signal: The baseband signal (which could be audio, voice, or other lowfrequency signals) is applied to control the amplitude of the carrier. The amplitude of the
carrier wave varies in proportion to the amplitude of the modulating signal.
The modulated signal can be represented as:
s(t)=[Ac+Amm(t)]cos(2πfct)
Where:
o
o
o
o
Ac is the carrier amplitude.
Am is the modulation amplitude.
m(t) is the modulating signal.
fc is the carrier frequency.
3. Amplification: After modulation, the low-level modulated signal is fed into a power
amplifier. The amplifier boosts the signal's power to a level suitable for transmission
(e.g., to radiate over long distances via an antenna).
4. Transmission: The amplified signal is then transmitted through an antenna to the
receiver.
Advantages of Low-Level AM Modulation
1. Simpler Modulation Circuits:
o The modulation is performed at low power, which reduces the need for complex
high-power modulation circuitry. This makes it simpler and less expensive
compared to high-level modulation schemes.
2. Efficient Use of Power Amplifier:
o Power amplifiers are most efficient when amplifying a constant signal (like the
carrier), rather than a modulated signal. The low-level modulation approach
allows for efficient use of the amplifier’s power and prevents the amplifier from
being overdriven by the variations in the modulated signal.
3. Less Amplifier Distortion:
o Amplifiers can introduce distortion if they are overloaded, but by modulating at
low power, the power amplifier operates within its linear range, reducing the
potential for distortion.
Disadvantages of Low-Level AM Modulation
1. Lower Output Power:
o The primary limitation of low-level AM modulation is that the output power is
relatively low before amplification. Since the modulation is done at a low power
level, additional amplification is needed to increase the signal's power for longrange transmission.
2. Potential for Non-Linear Distortion During Amplification:
o After modulation, when the signal is amplified, non-linear distortion may occur
if the amplifier is not linear. This could cause issues like harmonics and
intermodulation, leading to degraded signal quality.
3. Complexity of Power Amplification:
o While the modulation process is simple, the need for a separate, high-power
amplification stage can add complexity to the system design.
Low-Level AM vs. High-Level AM Modulation
Feature
Modulation Stage
Power Efficiency
Amplifier
Complexity
Transmission
Power
Distortion
Low-Level AM
Before amplification (at low
power)
More efficient use of power
amplifier
Less complex, amplifies a
constant carrier
Low
power
before
amplification
Less distortion if amplifier is
used linearly
High-Level AM
After amplification (at high power)
Less efficient, as power amplifier
handles modulated signal
More complex, amplifies a modulated
signal
High power after amplification
Potential for more distortion due to
modulated signal
Applications of Low-Level AM Modulation
1. AM Radio Broadcasting:
o In AM radio stations, low-level modulation is often used because it simplifies the
modulation circuitry. The signal is then amplified to the desired power level for
transmission over long distances.
2. Aviation Communication:
o Low-level AM modulation is used in aviation communication systems to transmit
voice or other data between aircraft and ground stations.
3. Communication Systems:
o Low-level AM modulation is also used in certain communication systems where
the design requires efficient modulation and amplification.
Summary
Low-level AM modulation is a technique where the carrier signal is modulated by the message
signal at a low power level, and the modulated signal is then amplified for transmission. This
method has advantages such as simpler modulation circuits and more efficient use of power
amplifiers. However, it also has limitations, such as lower output power before amplification and
potential distortion when the signal is amplified. It is commonly used in applications like AM
radio broadcasting and aviation communication.
Medium Power AM Modulation
Medium power AM modulation refers to a system where the carrier is modulated at a medium
power level, and the modulated signal is directly amplified to a level suitable for transmission
over medium to long distances. This approach lies between low-level and high-level AM
modulation in terms of both the power level at which the modulation occurs and the complexity
of the system.
Key Features of Medium Power AM Modulation
1. Modulation at Medium Power:
o In medium power AM modulation, the carrier is modulated at a power level
higher than low-level modulation but lower than high-level modulation.
o This results in a balance between the simplicity of low-level modulation and the
high output power typically seen in high-level modulation systems.
2. Efficiency of Power Amplification:
o After modulation, the signal is passed to a power amplifier for further
amplification, but this amplification stage is more efficient than in low-level AM
systems because the modulation is already done at a moderate power level.
o The amplifier works at a moderate power level and does not need to handle
extreme power levels, which can reduce distortion and increase efficiency.
3. Use of Intermediate Power Levels:
o The term medium power can vary depending on the specific system or
application, but generally it refers to power levels that are neither too low
(requiring a significant gain from the amplifier) nor too high (leading to high
distortion or inefficiency in amplification).
How Medium Power AM Modulation Works
In a medium power AM modulation system, the process typically works as follows:
1. Carrier Generation:
o A stable high-frequency carrier is generated at the desired frequency, fcf_c.
2. Modulation:
o The modulating signal (message signal) is applied to the carrier to vary its
amplitude. The amplitude of the carrier varies in proportion to the instantaneous
amplitude of the modulating signal.
o This modulation occurs at a medium power level, i.e., the carrier is modulated but
is not amplified to its final transmission level at this stage.
3. Amplification:
o The modulated signal is then passed to a power amplifier, which amplifies the
signal to the appropriate transmission power level.
o The power amplifier used in this stage is chosen to efficiently amplify the signal
without introducing excessive distortion. Since the modulation occurs at a
moderate level, the power amplifier operates in a relatively linear region, ensuring
high fidelity and low distortion.
4. Transmission:
o The amplified signal is transmitted through an antenna to reach the receiver.
Advantages of Medium Power AM Modulation
1. Improved Power Efficiency:
o Medium power modulation strikes a balance between low-level modulation and
high-level modulation. The signal is modulated at a medium power level, which
allows for more efficient power amplification, reducing losses in the amplification
stage compared to low-level modulation systems.
2. Less Distortion in Amplifier:
o Since the modulated signal is already at a moderate power level, the power
amplifier does not need to work as hard as it would in a low-level modulation
system, reducing the chance of distortion due to overdrive.
3. Simplified Design:
o Medium power AM modulation systems are simpler than high-power systems, as
they do not require as complex a power amplification setup to achieve the desired
transmission power.
4. Medium Range Transmission:
o Medium power AM modulation is typically used for transmission over medium
ranges, making it suitable for local to regional broadcasting and communication
systems.
Disadvantages of Medium Power AM Modulation
1. Lower Power Than High-Level AM:
o While more efficient than low-level modulation, medium power AM modulation
still requires additional amplification to reach the high power levels needed for
long-range transmission, making it less suitable for very long-range broadcasting.
2. Moderate Efficiency Trade-offs:
o Although more efficient than low-level AM, medium power AM may still not be
as efficient as high-level AM in terms of power usage.
3. More Complex than Low-Level Modulation:
o It requires a more complex modulation and amplification system than low-level
AM modulation, leading to higher cost and complexity.
Applications of Medium Power AM Modulation
Medium power AM modulation is used in a variety of communication and broadcasting
applications where the transmission power is required to be higher than what low-level AM
systems can provide, but where the very high power levels of high-level AM systems are not
necessary. Some typical applications include:
1. AM Radio Stations:
o Local or Regional AM Broadcasting: Medium power AM systems are
commonly used in local or regional radio stations that need to broadcast over a
medium distance, where high-power transmission is not required but efficiency is
still a concern.
2. Aviation Communication:
o Medium power modulation is sometimes used in aviation communication
systems for voice communication between aircraft and air traffic control, as well
as for other communication needs.
3. Two-Way Radio Communication:
o Systems that require reliable communication over a medium range (for example,
in remote areas or local communication networks) may use medium power AM
systems to ensure clear communication without excessive energy consumption.
4. Military Communication:
o Military communication systems in certain contexts, such as ground-to-air or airto-ground communications, often use medium power AM modulation to provide
reliable communication over moderate distances.
Comparison: Low-Level AM vs. Medium Power AM vs. High-Level AM
Feature
Modulation
Stage
Power Amplifier
Usage
Low-Level AM
Before
amplification
(low power)
Amplifies a low-power
modulated signal
Medium Power AM
Modulation at medium
power level
Amplifies a mediumpower
modulated
signal
Simpler, less complex Moderate complexity
modulation circuit
High-Level AM
After
amplification
(high power)
Amplifies a highpower
modulated
signal
More
complex,
Complexity
requires
robust
circuitry
More efficient than Most efficient for high
Power Efficiency Less efficient
low-level AM
transmission power
Possible distortion due Less distortion due to High distortion if not
Distortion
to low-level modulation moderate power levels well designed
Shorter range due to low Medium range
Long range due to high
Transmission
output power
output power
Range
Summary
Medium power AM modulation offers a balance between the simplicity of low-level AM and
the efficiency of high-level AM. It is used in systems where moderate transmission power is
needed for medium-range communication. This type of modulation is efficient, reduces
distortion compared to low-level modulation, and is simpler than high-level modulation, making
it suitable for applications such as local radio broadcasting, aviation communication, and certain
two-way radio systems. However, the need for further amplification for long-range transmission
and potential trade-offs in efficiency makes it less suited for very high-power applications.
Vestigial Sideband (VSB)
Vestigial Sideband (VSB) modulation is a variation of Amplitude Modulation (AM) that is
often used in systems where bandwidth efficiency is important but the full spectrum of a Double
Sideband AM (DSB-AM) signal is unnecessary. VSB is a compromise between Double
Sideband Suppressed Carrier (DSB-SC) and Single Sideband (SSB) modulation, offering a
more bandwidth-efficient method of transmission compared to traditional AM.
In VSB modulation, one of the sidebands (either the upper or lower) is partially suppressed,
leaving a "vestige" of the sideband. This allows for reduced bandwidth usage while still
retaining most of the original signal information.




Frequencies up to 0.75 MHz of the LSB are fully radiated
Attenuation slope of 0.5 MHz at either end
FM sound signal occupies a frequency spectrum of about ±75 KHz around the sound
carrier
Guard band of 0.25 MHz – allowed on the sound carrier side – for inter channel
separation
Key Features of VSB

Partial Sideband Suppression: In VSB modulation, one sideband (upper or lower) is
suppressed, and the other sideband is partially suppressed or partially retained. This
creates a spectrum with a "vestige" of the sideband.



Bandwidth Efficiency: VSB is more bandwidth-efficient than traditional AM (which
uses both upper and lower sidebands) and is used where bandwidth is limited, such as in
TV broadcasting.
Carrier Presence: VSB modulation typically includes a carrier, unlike Single Sideband
(SSB), which suppresses the carrier completely.
Use in TV Broadcasting: VSB is commonly used in analog television transmission (for
example, in the NTSC, PAL, and SECAM TV systems) for transmitting video signals.
In these systems, the vestigial sideband helps to efficiently use bandwidth while keeping
the required signal quality.
How VSB Works
1. Basic AM Modulation:
o In AM, the signal consists of a carrier and two sidebands (upper and lower),
each containing the same information. The spectrum of an AM signal is
symmetric around the carrier frequency fcf_c.
2. Sideband Suppression:
o In VSB, one of the sidebands is partially suppressed, and the other sideband is left
almost intact. This reduces the overall bandwidth required for transmission.
o Typically, the upper sideband is retained in full, while the lower sideband is
partially suppressed, leaving only a "vestige" of it.
3. VSB Spectrum:
o The VSB signal consists of the full upper sideband (USB) and a reduced version
of the lower sideband (LSB). This creates an asymmetrical frequency spectrum,
with a partial sideband being present at the lower frequencies.
Mathematically, the VSB signal is similar to AM, but with the lower sideband partially
removed.
Mathematical Representation of VSB
A VSB signal can be expressed as:
s(t)=[Ac+m(t)]cos(2πfct)+Am2m(t)sin(2πfct)
Where:




Ac is the amplitude of the carrier.
m(t) is the modulating signal.
Am is the amplitude of the modulating signal.
fc is the carrier frequency.
The difference between VSB and traditional AM lies in how the sidebands are treated. In VSB,
the modulating signal's frequency components in one of the sidebands (typically the lower
sideband) are either suppressed or heavily reduced.
Bandwidth of VSB
The bandwidth of a VSB signal is less than the bandwidth of a traditional AM signal but greater
than that of a Single Sideband (SSB) signal.



AM Signal Bandwidth: For an AM signal, the bandwidth is twice the highest frequency
of the modulating signal (2fm).
VSB Bandwidth: In VSB, the bandwidth is reduced because part of the lower sideband
is suppressed. The bandwidth of a VSB signal is roughly equal to fm+Δf, where Δf Delta
f represents the small portion of the lower sideband that is retained. Typically, this results
in a bandwidth savings of approximately 10-20%.
SSB Bandwidth: For SSB, the bandwidth is equal to the highest frequency of the
modulating signal fm, as it transmits only one sideband (either the upper or the lower
sideband).
Applications of VSB
1. Television Broadcasting:
o VSB is extensively used in analog television transmission, especially for the
video signal in systems such as NTSC, PAL, and SECAM.
o In TV broadcasting, the video signal has a broad frequency spectrum, and using
traditional AM would consume a lot of bandwidth. By using VSB, a portion of the
lower sideband is removed, which results in a more efficient use of available
bandwidth without significantly affecting the quality of the signal.
2. Communication Systems with Limited Bandwidth:
o VSB can be used in communication systems where bandwidth is limited, and
there is a need to transmit signals efficiently while preserving enough information
content.
o For example, video telephony or multimedia transmission systems can benefit
from VSB modulation.
3. Data Transmission:
o In some data transmission schemes, especially when dealing with analog signals
and where bandwidth efficiency is critical, VSB can be employed to improve
spectral efficiency.
Advantages of VSB



Bandwidth Efficiency: VSB uses less bandwidth than traditional AM modulation while
still preserving the necessary information content. This is especially beneficial in
applications like TV broadcasting, where bandwidth is limited.
Better Signal Quality: By suppressing part of the lower sideband, VSB modulation
reduces unnecessary frequency components, which helps in minimizing noise and
distortion without significant loss of information.
Compatibility: VSB can be used in systems that are compatible with traditional AM
demodulation techniques, especially in television broadcasting.
Disadvantages of VSB


More Complex Demodulation: VSB demodulation is more complex than simple AM
demodulation, requiring more sophisticated circuitry to reconstruct the original signal.
Partial Sideband Loss: Since part of the lower sideband is removed, some of the
frequency components of the original signal are lost. While the loss is typically minimal,
it can still affect signal fidelity if the suppression is too aggressive.
VSB v/s SSB
While both VSB and Single Sideband (SSB) aim to reduce bandwidth usage by removing a
sideband, the key differences are:



VSB retains a vestige of the suppressed sideband, whereas SSB completely removes one
sideband.
VSB still retains a carrier, whereas SSB typically suppresses the carrier entirely.
VSB provides a compromise between AM and SSB, offering a more efficient bandwidth
usage than traditional AM, while SSB provides maximum bandwidth efficiency.
Summary
Vestigial Sideband (VSB) modulation is a bandwidth-efficient version of AM modulation that is
used primarily in television broadcasting. It provides a good compromise between traditional
AM and Single Sideband (SSB) by suppressing part of one of the sidebands, which results in
reduced bandwidth consumption while maintaining most of the signal quality. VSB is especially
useful in applications like TV broadcasting where efficient use of bandwidth is crucial.
Block Diagram of Low-Level DSB-FC (Double Sideband with Full Carrier) Modulation
Low-level DSB-FC modulation refers to the Double Sideband Full Carrier modulation
technique where the carrier signal is transmitted along with both upper and lower sidebands of
the modulated signal. In this method, the modulation takes place at a low power level before the
signal is passed to a power amplifier for transmission.
Here's a breakdown of the key components involved in a Low-level DSB-FC (Double Sideband
Full Carrier) modulation system:
Block Diagram
Explanation of Circuit Diagram:
In Figure 3-l5a the carrier is applied to the base and the modulating signal to the emitter.
Therefore, this circuit configuration is called emitter modulation. The modulating signal varies
the gain of the amplifier at a sinusoidal rate equal to the frequency of the modulating signal. The
depth of modulation achieved is proportional to the amplitude of the modulating signal. The
voltage gain for an emitter modulator is expressed mathematically as
Explanation of Each Block
1. Message Signal (m(t)):
o The message signal is the information you want to transmit, such as audio or
voice signals. The message signal m(t) is typically a low-frequency signal that
modulates the carrier wave.
2. Carrier Signal:
o The carrier is a high-frequency signal cos(2πfct) that is generated to serve as the
basis for modulation. This carrier is combined with the message signal to form the
modulated signal. It is a continuous wave of a much higher frequency than the
message signal.
3. Modulator (DSB-FC Modulator):
o
The modulator combines the message signal with the carrier signal using
Amplitude Modulation (AM).
o The modulated signal is given by: s(t)=[Ac+Am m(t)]cos⁡(2πfct)
 Ac is the amplitude of the carrier,
 Am is the amplitude of the modulating signal,
 m(t) is the modulating signal,
 fc is the carrier frequency.
4. Power Amplifier:
o After modulation, the power amplifier boosts the power of the modulated signal
to a level suitable for transmission. This is called low-level modulation because
modulation occurs before the signal is amplified. The power amplifier increases
the signal's power to reach the required transmission levels.
5. Antenna:
o The antenna radiates the amplified signal into space for transmission. The
modulated signal (now at a higher power level) is broadcasted to the receiver via
the antenna.
Steps of Low-Level DSB-FC Modulation
1. Message signal generation: The message signal m(t) is generated, typically from audio
or video sources.
2. Carrier signal generation: The carrier signal cos(2πfct) is generated at a much higher
frequency than the message signal.
3. Modulation: The modulating signal m(t) modulates the carrier using Amplitude
Modulation. This creates a signal with both an upper sideband and lower sideband,
alongside the carrier, resulting in a DSB-FC signal.
4. Amplification: The modulated signal is then passed through a low-level power
amplifier to increase its power, suitable for transmission. The carrier and sidebands are
both amplified.
5. Transmission: The amplified signal is transmitted through the antenna.
Advantages of Low-Level DSB-FC Modulation



Simplicity: The modulation stage is simple because it occurs at a low power level before
amplification.
Efficient use of amplifier: Power amplifiers are typically more efficient when
amplifying a signal with constant amplitude (like the carrier). Thus, low-level modulation
ensures efficient amplifier operation.
Cost-effective: Low-level modulation circuits are usually less expensive and less
complex compared to high-level modulation systems.
Disadvantages of Low-Level DSB-FC Modulation

Lower output power: Since modulation occurs before amplification, the output power
from the modulator is relatively low, and additional amplification is necessary.

Non-linear distortion: The modulated signal may cause distortion if the power amplifier
is not properly designed for the modulation type.
Summary
The Low-Level DSB-FC Modulation system involves modulating the message signal with the
carrier at a low power level before amplification. This allows for simple modulation circuitry
and more efficient use of the power amplifier. After modulation, the signal is amplified and
transmitted via an antenna. This type of system is widely used in communication applications
like AM radio transmission.
High-Level DSB-FC (Double Sideband Full Carrier) Modulation
High-Level DSB-FC (Double Sideband Full Carrier) modulation refers to a system in which
the modulation of the message signal (information) occurs after the carrier has already been
amplified to a high power level. This means the carrier is first amplified and then modulated, in
contrast to low-level modulation, where the carrier is modulated at a low power level before
amplification.
In high-level DSB-FC modulation, both the carrier and the full set of sidebands (upper and
lower) are transmitted. This form of modulation is typical in AM radio broadcasting and other
communication systems where high-power transmission is necessary.
Block Diagram of High-Level DSB-FC Modulation
Figure (a) is drawn for audio transmission. In high-level transmission, the powers of the carrier
and modulating signals are amplified before applying them to the modulator stage, as shown in
figure (a). In low-level modulation, the powers of the two input signals of the modulator stage
are not amplified. The required transmitting power is obtained from the last stage of the
transmitter, the class C power amplifier.
The various sections of the figure (a) are:
·
Carrier oscillator
·
Buffer amplifier
·
Frequency multiplier
·
Power amplifier
·
Audio chain
·
Modulated class C power amplifier
Carrier oscillator
The carrier oscillator generates the carrier signal, which lies in the RF range. The frequency of
the carrier is always very high. Because it is very difficult to generate high frequencies with good
frequency stability, the carrier oscillator generates a sub multiple with the required carrier
frequency. This sub multiple frequency is multiplied by the frequency multiplier stage to get the
required carrier frequency. Further, a crystal oscillator can be used in this stage to generate a low
frequency carrier with the best frequency stability. The frequency multiplier stage then increases
the frequency of the carrier to its requirements.
Buffer Amplifier
The purpose of the buffer amplifier is twofold. It first matches the output impedance of the
carrier oscillator with the input impedance of the frequency multiplier, the next stage of the
carrier oscillator. It then isolates the carrier oscillator and frequency multiplier.
This is required so that the multiplier does not draw a large current from the carrier oscillator. If
this occurs, the frequency of the carrier oscillator will not remain stable.
Frequency Multiplier
The sub-multiple frequency of the carrier signal, generated by the carrier oscillator , is now
applied to the frequency multiplier through the buffer amplifier. This stage is also known as
harmonic generator. The frequency multiplier generates higher harmonics of carrier oscillator
frequency. The frequency multiplier is a tuned circuit that can be tuned to the requisite carrier
frequency that is to be transmitted.
Power Amplifier
The power of the carrier signal is then amplified in the power amplifier stage. This is the basic
requirement of a high-level transmitter. A class C power amplifier gives high power current
pulses of the carrier signal at its output.
Audio Chain
The audio signal to be transmitted is obtained from the microphone, as shown in figure (a). The
audio driver amplifier amplifies the voltage of this signal. This amplification is necessary to
drive the audio power amplifier. Next, a class A or a class B power amplifier amplifies the power
of the audio signal.
Modulated Class C Amplifier
This is the output stage of the transmitter. The modulating audio signal and the carrier signal,
after power amplification, are applied to this modulating stage. The modulation takes place at
this stage. The class C amplifier also amplifies the power of the AM signal to the reacquired
transmitting power. This signal is finally passed to the antenna., which radiates the signal into
space of transmission.
High-Level DSB-FC Modulation Process
1. Carrier Generation: A high-frequency carrier signal cos(2πfct) is generated at the
desired carrier frequency fc.
2. Amplification: The carrier is amplified to a high power level in the power amplifier.
This is crucial for ensuring that the signal can travel over long distances, particularly for
broadcasting applications.
3. Modulation: The message signal m(t) modulates the high-power carrier signal in the
DSB-FC modulator. This produces a Double Sideband Full Carrier (DSB-FC) signal
that consists of the carrier along with both the upper and lower sidebands.
4. Transmission: The modulated signal is then transmitted via an antenna after further
amplification if required. The signal travels through space to be received by a distant
receiver.
Mathematical Expression of High-Level DSB-FC Signal
The high-level DSB-FC modulated signal can be represented as:
s(t)=[Ac+Am.m(t)]cos(2πfct)
Where:




Ac is the amplitude of the high-powered carrier.
Am is the amplitude of the message signal.
m(t) is the modulating signal.
fc is the carrier frequency.
Advantages of High-Level DSB-FC Modulation
1. High Transmission Power:
o Since the carrier is modulated after being amplified to high power, high-level
modulation allows for long-range transmission. This makes it ideal for AM
radio broadcasting, where the signal needs to cover large geographical areas.
2. Simple Modulation:
o The process of modulation in high-level DSB-FC is straightforward and involves
combining the modulating signal with the high-powered carrier. This makes the
system relatively easy to implement and understand.
3. Carrier Preservation:
o The full carrier is transmitted, which makes the signal easier to demodulate at the
receiver. This is beneficial in systems where a stable carrier is needed for proper
signal recovery, such as AM radio.
Disadvantages of High-Level DSB-FC Modulation
1. Inefficient Bandwidth Use:
o Since the carrier and both sidebands (upper and lower) are transmitted, the
signal occupies more bandwidth than necessary. Compared to Single Sideband
(SSB) modulation or Vestigial Sideband (VSB) modulation, DSB-FC is less
bandwidth-efficient.
2. Power Inefficiency:
o A significant amount of power is wasted on the carrier signal. Since the carrier
does not carry any information, transmitting it at high power is inefficient
compared to methods that suppress the carrier (like SSB).
3. Amplifier Requirements:
o High-level modulation requires powerful amplifiers capable of handling high
power levels, making the system more costly and complex.
Applications of High-Level DSB-FC Modulation
1. AM Radio Broadcasting:
o AM radio stations often use high-level DSB-FC modulation to transmit audio
signals over long distances. The high power of the carrier ensures that the signal
can be received clearly at distant locations.
2. Aviation Communication:
o High-level AM modulation is sometimes used in aviation communication systems
to transmit voice communication between aircraft and ground stations over long
distances.
3. Broadcasting Systems:
o Other broadcasting systems where long-range transmission is required may also
use high-level DSB-FC modulation for voice, audio, or video transmission.
Comparison of High-Level DSB-FC and Low-Level DSB-FC
Feature
Modulation
Location
Carrier Power
Power Amplifier
Bandwidth
Efficiency
Application
High-Level DSB-FC
Modulation
after
carrier
amplification
High
power
carrier
before
modulation
High-power amplifier to amplify
modulated signal
Occupies bandwidth of carrier +
two sidebands
Less bandwidth-efficient due to
full carrier
Used for long-range broadcasting
like AM radio
Low-Level DSB-FC
Modulation before amplification
Low power carrier before modulation
Power amplifier amplifies low-power
modulated signal
Same as high-level but requires
additional amplification
More efficient use of amplifier power
Used for low-power transmissions and
simpler systems
Summary
High-Level DSB-FC Modulation is widely used in AM radio broadcasting and other systems
where high-power transmission is necessary for long-range communication. By modulating a
high-powered carrier, this system ensures that the signal can reach distant receivers. However, it
suffers from inefficiencies in terms of bandwidth and power usage compared to methods like
Single Sideband (SSB) modulation. Despite these drawbacks, its simplicity and compatibility
with existing AM radio systems make it a practical choice in many communication applications.
Single Sideband (SSB) Suppression Techniques
Overview of Single Sideband (SSB) Modulation:
In conventional AM (Amplitude Modulation), both the upper and lower sidebands of the
modulated signal carry the same information, leading to inefficient use of bandwidth. In SSB
modulation, one of the sidebands is completely suppressed, reducing the bandwidth by half and
making the system more efficient.


Upper Sideband (USB): The portion of the signal above the carrier frequency.
Lower Sideband (LSB): The portion of the signal below the carrier frequency.
SSB modulation can be used to transmit the message signal more efficiently, requiring less
bandwidth and power.
Single Sideband (SS band) modulation is a form of amplitude modulation (AM) that is
designed to improve bandwidth efficiency by transmitting only one of the sidebands (upper or
lower) and suppressing the carrier entirely. SSB is widely used in communication systems, such
as radio transmissions and telecommunication systems, because it reduces the bandwidth
required for transmission and minimizes power consumption, making it highly efficient
compared to Double Sideband AM (DSB-AM).
SSB suppression techniques focus on effectively removing the unwanted sideband and the
carrier, allowing only one sideband (either upper or lower) to remain. Here’s a breakdown of the
main techniques for achieving SSB suppression:
1. Filter Method (Frequency Selective Filtering)
The filtering method is the most straightforward technique to achieve SSB modulation. This
involves using bandpass filters to isolate and transmit only one sideband.




Process:
o In DSB-AM, the signal contains both the upper and lower sidebands, along with
the carrier. The goal of SSB modulation is to eliminate one sideband and the
carrier.
o A bandpass filter is used to isolate either the upper or lower sideband and pass
it through for transmission, while blocking the carrier and the opposite sideband.
Types of Filters:
o High-pass filters and low-pass filters can be used to pass the desired sideband
while filtering out the other sideband and the carrier.
o A combination of filters (such as a Hilbert transformer) can help achieve a
more precise elimination of the unwanted components.
Advantages:
o The filter method is relatively simple and effective in removing unwanted
sidebands and the carrier.
o Filters can be designed to operate at the baseband frequencies, making it ideal for
systems with analog modulated signals.
Disadvantages:
o Precision issues: Filters may not always perfectly suppress the unwanted
sideband or carrier, leading to residual distortion.
o Fixed bandwidth: The filter method is typically less flexible when dealing with
signals with varying bandwidth.
The Weaver method is one of the most widely used techniques for generating Single Sideband
(SSB) modulated signals in communication systems. It is a relatively simple and efficient
method that uses the concept of modulating two signals with a 90° phase shift to suppress one of
the sidebands (either the upper or lower sideband) while maintaining the integrity of the original
message signal.
2) Weaver Method (Third Method) of SSB Generation:
The Weaver method is a popular technique for generating SSB signals by using two balanced
mixers and phase shifts. The method generates SSB by creating the necessary phase shift and
filtering out the unwanted sideband.
Block Diagram of the Weaver Method for SSB Generation:
The basic steps in the Weaver method are as follows:
Step-by-Step Process for Weaver Method:
1. Message Signal:
o Let the input message signal be denoted as m(t), which is typically a lowfrequency baseband signal (audio, for example).
2. Carrier Signal:
o The carrier signal used for modulation is cos(2πfct) , where fc is the carrier
frequency.
3. Quadrature Phase Shifting:
o Create two signals:


The original message signal m(t)m(t) is multiplied by a carrier
cos(2πfct).
The message signal m(t) is also multiplied by a carrier shifted by 90°
(i.e., a sine wave) sin(2πfct) .
These two signals will serve as the in-phase (I) and quadrature (Q) components
for further processing.
4. Mixing Process:
o Mix the in-phase signal m(t)⋅cos(2πfct) with the 90° phase-shifted carrier
sin⁡(2πfct).
o Mix the quadrature signal m(t)⋅sin(2πfct) with the cosine carrier cos(2πfct).
After this step, you will have:
o
o
m(t)⋅cos(2πfct)m(t) (I channel signal).
m(t)⋅sin(2πfct)m(t) (Q channel signal).
These signals are now in quadrature, meaning they are 90° out of phase with each other.
5. Low-Pass Filtering:
o Apply a low-pass filter to both of the mixer outputs to remove high-frequency
components (the carrier frequency and the unwanted sideband).
o After filtering, you get:
 Filtered I channel m(t)⋅cos(2πfct) (shifted by a cosine phase).
 Filtered Q channel m(t)⋅sin(2πfct) (shifted by a sine phase).
6. Final Combination:
o Combine the filtered signals from the I and Q channels appropriately to produce
the desired SSB signal:
 The final SSB modulated signal is obtained by adding or subtracting
these two signals.
Depending on the sign of the combination, you can either generate the Upper Sideband
(USB) or the Lower Sideband (LSB).
o
o
USB Generation: If you add the I and Q channels.
LSB Generation: If you subtract the I and Q channels.
The resulting signal will have one of the sidebands suppressed (either the USB or the
LSB), depending on the combination.
Advantages of the Weaver Method:
1. Simplicity: The Weaver method is conceptually simple, using basic components such as
mixers and phase shifters to generate an SSB signal.
2. High Efficiency: Since one sideband is suppressed, the system uses less bandwidth,
improving the efficiency of frequency usage.
3. Flexibility: The method allows for easy generation of either the Upper Sideband (USB)
or Lower Sideband (LSB) by simply changing the combination of the signals in the final
step.
Applications of the Weaver Method:



AM Radio Broadcasting: The Weaver method is used in high-efficiency AM radio
systems where bandwidth conservation and power efficiency are critical.
Single-Sideband Telephony: In radio communication systems, especially for longdistance and space communications, where efficient bandwidth usage is important.
Satellite Communication: SSB modulation is frequently employed in satellite
communication due to the need to use limited bandwidth efficiently.
Conclusion:
The Weaver method provides an efficient way of generating SSB signals by using the concept of
quadrature modulation (splitting the message signal into two components that are 90° out of
phase). The method uses two mixers and a phase-shifted carrier to generate the modulated signal,
which is then filtered and combined to produce a signal with only one sideband, making it ideal
for applications that require bandwidth efficiency and power conservation.
3. Phasing (or Hartley) Method
The Phasing method (also known as the Hartley method) is another technique for producing
SSB signals, based on the principle of phase cancellation.
Phase Shift Method or phasing method




Process:
o The message signal is split into two components, and one of the components is
shifted by 90 degrees. Then, the components are combined in such a way that the
carrier is suppressed and only one sideband (upper or lower) remains.
o A phase-shifting circuit or Hilbert transformer is used to create the necessary
phase shift.
Phasing Method Components:
o The process involves two mixers and a 90-degree phase shift to produce two
identical but out-of-phase components. These are then recombined to produce the
desired SSB signal.
Advantages:
o This technique allows for high-quality SSB generation with relatively simple
hardware.
o It is widely used in applications that require high-fidelity, low-distortion SSB
signals.
Disadvantages:
o More complex than filters: Requires careful phase control and mixing, and can
be sensitive to errors in phase adjustment.
o Requires precise synchronization of the components to avoid distortion or
unwanted sideband leakage.
The carrier signal is VcSin2πfct and the modulating signal is VmSin2πfmt. Balanced modulator1
produces the product of these two signals.
(VmSin2πfmt)(VcSin2πfct)
Applying a trigonometric identity. (VmSin2πfmt)(VcSin2πfct)
= 1/2[Cos(2πfc - 2πfm)t - Cos(2πfc + 2πfm)t]...................1
1. It is important to remember that a cosine wave is simply a sine wave shifted by 90o.
2.
A cosine wave has exactly the same shape as a sine wave, but it occurs 90o.
o
3. The 90 phase shifters create cosine waves of the carrier and modulating signal which are
multiplied in balanced modulator2 to produce
(VmCos2πfmt)(VcCos2πfct)
Another common trigonometric identity translates this to
VmCos2πfmt)(VcCos2πfct)
= 1/2[Cos(2πfc - 2πfm)t + Cos(2πfc + 2πfm)t].......2
= 1/2[Cos(2πfc - 2πfm)t - Cos(2πfc + 2πfm)t]...................1
= 1/2[Cos(2πfc - 2πfm)t + Cos(2πfc + 2πfm)t]..................2
Vo(ssb) = Cos(2πfc - 2πfm)t
.................single side band (SSB)
Now if you add these two expressions together the sum frequencies cancel while the
difference frequencies add producing only the lower side band:
Note that these are the sum and difference frequencies or the upper and lower side bands.
Summary of SSB Suppression Techniques
Technique
Filter
Method
Balanced
Modulator
Weaver
Method
Phasing
Process
Advantages
Disadvantages
Use of band pass filters Simple,
low-cost, Can
introduce
some
to isolate one sideband.
effective for analog residual distortion, less
signals.
flexible.
Carrier and modulating Efficient,
precise Requires
specialized
signal are combined to control
over equipment, complex design.
cancel the carrier.
transmitted sideband.
Frequency shift and High-quality,
low- Complex requires dualfiltering to select one distortion
SSB sideband modulation and
sideband.
generation.
filtering.
Phase
shift
and Simple, widely used, Sensitive to phase errors,
Method
DSP-Based
Methods
combining to cancel the
carrier.
Digital filtering and
processing to remove the
unwanted sideband.
produces clean SSB
signals.
Highly
flexible,
precise control over
the signal.
complex
synchronization
needed.
Requires
computational
resources, analog-to-digital
conversion.
Summary
Each SSB suppression technique has its strengths and weaknesses, and the choice of technique
depends on the application, required signal quality, hardware complexity, and available
resources. The filter method is simple and effective for basic applications, while the Weaver
and Phasing methods offer more precision and higher-quality results for sophisticated systems.
Digital Signal Processing (DSP) methods provide flexibility and control, making them suitable
for modern communication systems.
TRF Receiver (Tuned Radio Frequency Receiver)
A Tuned Radio Frequency (TRF) receiver is one of the simplest types of radio receivers that is
used to amplify and demodulate a received signal. In a TRF receiver, the radio signal is first
tuned to the desired frequency using a tuning circuit, then amplified, and finally demodulated
to extract the original message signal (like audio or data).
Working Principle of TRF Receiver
1. Antenna: The radio signal is captured by the antenna, which picks up electromagnetic
waves in the form of radio frequencies.
2. Tuning Stage: The incoming radio signal passes through a tuning circuit (typically a
LC circuit or varactor diode circuit) that selects the desired frequency while rejecting
other unwanted frequencies. The tuning circuit adjusts to resonate with the desired
signal frequency.
3. Amplification: After the desired frequency is selected, the signal is passed through an
amplifier to increase its strength.
4. Demodulation: The amplified signal is then passed to the demodulator, which extracts
the audio (for AM radio) or other types of information from the modulated carrier signal.
5. Output: Finally, the demodulated signal is sent to a speaker or other output device to
reproduce the original sound or message.
Advantages of TRF Receiver


Simple Design: The TRF receiver is easy to design and has fewer components compared
to more advanced receiver types.
Good for Narrowband Signals: It's effective for receiving narrowband signals like AM
radio.
Disadvantages of TRF Receiver


Selectivity: TRF receivers have poor selectivity (ability to tune to a single station while
rejecting others) and sensitivity (ability to detect weak signals).
Amplification Issues: The gain of the amplifier may vary with frequency, leading to
potential distortion and poor performance across a wide frequency range.

Limited Frequency Range: A TRF receiver is generally effective only over a limited
range of frequencies and works best for single-frequency tuning.
Super heterodyne Receiver
The Superheterodyne receiver, often referred to simply as the Superhet receiver, is a more
advanced type of radio receiver that overcomes the limitations of the TRF receiver. It is the most
widely used type of radio receiver, employed in everything from AM radios to television
receivers and wireless communication systems.
Simplified block diagram of typical AM receiver
Working Principle of Super heterodyne Receiver
The key idea of the super heterodyne receiver is to convert the received high-frequency signal
(RF signal) into a lower intermediate frequency (IF) that is easier to process, especially for
amplification and demodulation.
1. Antenna: The incoming radio frequency (RF) signal is captured by the antenna.
2. RF Amplifier: The RF signal is passed through an RF amplifier, which amplifies the
signal to a level suitable for processing.
3. Mixer: The amplified RF signal is then passed to a mixer, which combines the RF signal
with a local oscillator (LO) signal. The LO generates a fixed frequency close to the RF
frequency, and the mixer produces the difference between the RF and LO frequencies.
o If the local oscillator frequency is fLO and the incoming signal frequency is fRF,
the mixer generates an intermediate frequency (IF) fIF that is the difference:
fIF=fRF−fLO
4. Intermediate Frequency (IF) Amplifier: The IF signal is then amplified by an IF
amplifier. The advantage is that the IF is usually a fixed frequency, so the amplification
and filtering stages can be optimized for that specific frequency, resulting in better
performance compared to the TRF receiver.
5. Demodulation: After amplification, the IF signal is passed to the demodulator, which
extracts the information (such as audio or data) from the modulated carrier.
6. Audio/Output: Finally, the demodulated signal is sent to a speaker, headphone, or other
output device, where the original audio or data is reproduced.
Advantages of Super heterodyne Receiver
1. Better Selectivity: The Super heterodyne receiver can use highly selective filters at the
intermediate frequency (IF), which improves its ability to reject unwanted signals
(better selectivity).
2. Higher Sensitivity: Super heterodyne receivers can amplify weak signals effectively
because the IF stages can be optimized for amplification.
3. Stable Performance: The performance of the IF stages is relatively stable because they
operate at a fixed intermediate frequency, regardless of the incoming RF frequency.
4. Wide Frequency Coverage: Superhet receivers can easily be designed to handle a wide
range of frequencies by tuning the local oscillator.
5. Improved Filtering: The ability to filter out unwanted signals at the IF stage improves
the overall quality of the received signal.
Disadvantages of Super heterodyne Receiver
1. Complex Design: The design of a super heterodyne receiver is more complex compared
to a TRF receiver, as it requires additional components such as mixers, local oscillators,
and IF amplifiers.
2. Image Frequency: The mixer can create an image frequency (an unwanted signal that
also produces the same IF), which needs to be filtered out, adding to the complexity of
the design.
3. Local Oscillator Leakage: There can be leakage from the local oscillator, which might
interfere with the signal, requiring careful filtering and design.
AM (Amplitude Modulation) Receiver Parameters
AM (Amplitude Modulation) Receiver Parameters refer to the key characteristics and
performance metrics of an AM radio or communication receiver that influence how effectively it
can demodulate and recover the transmitted signal. These parameters define the receiver's ability
to handle different types of signals and conditions, such as noise, interference, and signal
strength. The main AM receiver parameters are as follows:
1. Sensitivity




Definition: Sensitivity refers to the minimum signal strength that the receiver can detect
and successfully demodulate. In other words, it is the weakest signal level at which the
receiver can still produce a readable output.
Unit: Often expressed in microvolts (µV) or dBm.
Importance: High sensitivity is crucial for detecting weak signals, especially in longrange or low-power AM transmissions.
Typical Range: AM radio receivers typically have a sensitivity of around 50 µV or
lower.
2. Selectivity




Definition: Selectivity is the ability of the receiver to isolate the desired signal from other
signals or interference that may be present on nearby frequencies. A receiver with good
selectivity can differentiate between a strong signal and unwanted adjacent signals
(adjacent channel interference).
Unit: Often measured in dB, representing the ratio between the signal strength of the
desired station and the adjacent station's signal strength.
Importance: High selectivity ensures that the receiver can tune into a particular station
without interference from adjacent channels.
Typical Range: AM radio receivers often have a selectivity of 50 to 60 dB or higher.
3. Image Rejection



Definition: Image rejection is the ability of a superheterodyne receiver to reject unwanted
signals, or "images," that occur due to the mixing process. These unwanted signals can
create spurious outputs, especially when the receiver is tuned to a specific frequency.
Unit: Measured in decibels (dB).
Importance: Effective image rejection ensures that only the intended station's signal is
demodulated, without distortion caused by image frequencies.

Typical Range: AM receivers typically have image rejection ratios of 40-60 dB.
4. Frequency Response



Definition: The frequency response of an AM receiver refers to its ability to handle a
range of audio frequencies (usually 20 Hz to 5 kHz for AM radio). The receiver needs to
accurately reproduce audio signals across this range without distortion.
Importance: A receiver with a good frequency response can accurately reproduce both
low and high-frequency components of the audio signal, ensuring clear sound quality.
Typical Range: 20 Hz to 5 kHz, although some AM receivers may go slightly outside
this range.
5. Selectivity Bandwidth



Definition: Selectivity bandwidth refers to the width of the frequency range around the
carrier frequency that the receiver can tune to and still adequately demodulate the signal.
This bandwidth determines how sharply the receiver can differentiate between adjacent
channels.
Importance: Narrower bandwidth improves selectivity by reducing interference from
adjacent channels, while wider bandwidth might allow more signals to be captured,
potentially causing interference.
Typical Range: For AM radio, selectivity bandwidth is typically 10 kHz to 15 kHz.
6. Signal-to-Noise Ratio (SNR)




Definition: The Signal-to-Noise Ratio (SNR) is the ratio of the strength of the desired
signal to the background noise. A higher SNR means better signal quality and clearer
reception, as the noise has less of an impact on the signal.
Unit: Measured in decibels (dB).
Importance: A high SNR indicates better audio quality and fewer interruptions or
distortions caused by noise.
Typical Range: In AM radio, a typical SNR might range from 30 dB to 50 dB under
ideal conditions.
7. Total Harmonic Distortion (THD)


Definition: Total Harmonic Distortion (THD) measures the distortion that is introduced
into the AM signal by the receiver. THD quantifies the presence of harmonics (multiples
of the carrier frequency) that distort the audio signal.
Unit: Expressed as a percentage.


Importance: Low THD ensures that the audio output is as faithful to the original signal
as possible, without introducing unwanted harmonic noise.
Typical Range: AM receivers typically aim for a THD of less than 1% to ensure clear
and accurate sound.
8. Carrier Detection



Definition: Carrier detection refers to the ability of the AM receiver to correctly identify
the presence of the carrier wave. This is important for demodulating the signal, as the
carrier is the base on which the audio signal is modulated.
Importance: Without accurate carrier detection, the receiver will fail to correctly
demodulate the audio information encoded in the amplitude variations of the carrier
wave.
Typical Performance: Modern AM receivers typically feature automatic carrier
detection and tracking.
9. Audio Output Power




Definition: Audio output power is the amount of power the receiver can deliver to the
speakers or audio output device. It determines how loud the sound will be.
Unit: Measured in watts (W).
Importance: Higher output power can drive larger speakers and produce louder sound,
improving the listening experience.
Typical Range: For consumer AM receivers, output power typically ranges from 0.5 W
to 5 W.
10. Intermodulation Distortion (IMD)



Definition: Intermodulation distortion refers to unwanted signals generated when
multiple frequencies mix in the receiver, leading to the production of spurious
frequencies that can interfere with the desired signal.
Importance: High IMD can cause poor audio quality and interference, making it harder
for the receiver to cleanly demodulate the AM signal.
Typical Range: AM receivers typically aim for IMD levels of less than -40 dB to
minimize distortion.
11. Fidelity



Definition: Fidelity is the quality of the reproduced sound in terms of how accurately the
AM receiver can reproduce the original transmitted audio. It is closely related to the
receiver's frequency response, distortion, and SNR.
Importance: High fidelity ensures clear, distortion-free sound and a pleasant listening
experience.
Typical Performance: AM receivers often provide fidelity sufficient for speech and
music reproduction, but they may be limited by bandwidth and noise.
12. Dynamic Range



Definition: The dynamic range of an AM receiver is the range between the weakest and
strongest signals it can handle while still providing an intelligible output. A wide
dynamic range allows the receiver to capture both weak and strong signals without
significant distortion.
Importance: A higher dynamic range allows the receiver to handle a wider variety of
signal strengths, which is important for receiving signals in areas with varying signal
strength.
Typical Range: AM receivers typically offer a dynamic range of 60 dB to 80 dB.
13. Tuning Accuracy and Stability



Definition: Tuning accuracy refers to how precisely the receiver can tune to a specific
frequency, while tuning stability refers to the ability of the receiver to maintain the
correct frequency over time, despite temperature or other environmental changes.
Importance: Accurate and stable tuning ensures that the receiver stays on the correct
frequency, preventing drift and loss of signal.
Typical Performance: Tuning accuracy is typically within ±1 kHz, and receivers may
have automatic frequency control (AFC) to maintain tuning stability.
Summary:
AM receiver parameters are essential in determining the overall performance and quality of an
AM radio or communication system. Sensitivity, selectivity, and SNR are some of the key
factors that influence the ability of the receiver to detect and process AM signals. Other
parameters like distortion (THD and IMD), carrier detection, and audio fidelity contribute to the
clarity and quality of the received audio. Understanding and optimizing these parameters allows
for the design of high-performance AM receivers suited to various applications, from simple AM
radio receivers to more complex communication systems.
Comparison of TRF and Super heterodyne Receiver
Feature
Principle
Complexity
Selectivity
Sensitivity
Performance
Across
Frequencies
Bandwidth
Applications


TRF Receiver
Directly
amplifies
and
demodulates the received signal at
its carrier frequency.
Simple
design
with
fewer
components.
Poor
selectivity,
can
have
difficulty isolating signals.
Limited sensitivity due to poor
amplification.
Limited performance across a wide
frequency range.
Super heterodyne Receiver
Converts the received signal to a
fixed intermediate frequency (IF) for
processing.
More complex, involving a mixer,
local oscillator, and IF stages.
Excellent selectivity due to fixed IF
stages and filtering.
Better sensitivity, as IF stages can be
optimized for weak signals.
Stable performance due to fixed IF,
with better amplification and
filtering.
Requires careful tuning to avoid Can use better filtering and
distortion.
amplification to handle wide
bandwidths.
Older
AM
radios,
simple Most modern radio and TV
communication receivers.
receivers, communications systems.
TRF Receiver: Simple, but limited in performance. It works well for low-frequency
applications like AM radio, but its lack of selectivity and sensitivity makes it less
effective for more complex systems.
Superheterodyne Receiver: Offers superior performance, especially in terms of
selectivity and sensitivity. It is widely used in modern communication systems, from
radio receivers to television and cell phones. Though more complex, it provides better
reception quality and flexibility in handling a broad range of frequencies.
The superheterodyne receiver is the preferred choice in most modern communication systems
due to its excellent performance, while TRF receivers are typically used in simpler or legacy
systems where complexity and cost are more important than performance.
Technical Specifications of AM Broadcasting
Amplitude Modulation (AM) broadcasting is a well-established method used for transmitting
audio signals, primarily for radio broadcasts. AM broadcasting operates in the medium and
long-wave frequency bands and is characterized by the modulation of the amplitude of a highfrequency carrier signal by the audio or baseband signal. The AM broadcasting system is
regulated by international and national standards to ensure compatibility, interference
management, and efficient transmission.
Key Technical Specifications of AM Broadcasting
1. Carrier Frequency



AM radio transmission typically occurs in the Medium Frequency (MF) band.
Carrier frequency range:
o AM broadcasting is allocated in the frequency range 530 kHz to 1700 kHz in
most countries (such as the United States).
o In some countries, there may be slight variations, but the general allocation
remains within the same range.
Spacing between stations:
o Stations are spaced by a fixed frequency interval (usually 10 kHz in the US) to
avoid interference.
2. Modulation and Bandwidth


AM Modulation:
o The carrier frequency is modulated by the audio signal, and this modulation is
expressed as s(t)=Ac[1+m(t)]cos(2πfct),where:
 Ac is the carrier amplitude,
 m(t) is the modulating signal (audio),
 fc is the carrier frequency.
Bandwidth:
o The bandwidth of an AM signal is approximately 2 times the highest frequency
of the audio signal.
o For example, if the audio signal has a bandwidth of 5 kHz (which is common for
AM radio), the AM signal bandwidth will be around 10 kHz (5 kHz for the
upper sideband and 5 kHz for the lower sideband).
3. Power


Transmission Power:
o The transmitting power of AM radio stations can vary depending on the class of
the station and regulatory requirements.
o Commonly, AM stations can transmit power in the range of 50 W to 50 kW, with
high-power stations reaching up to 500 kW (for long-range transmission).
Effective Radiated Power (ERP):
o The ERP is the total power radiated by the antenna, taking into account antenna
gains and losses.
o In many countries, the maximum allowable ERP for AM broadcast stations is
regulated, and high-power stations may be required to operate with directional
antennas to prevent interference with other stations.
4. Frequency Modulation (Optional for Stereo AM)

AM Stereo:
o
o
o
In some cases, stereo broadcasting is used, where a second audio channel is
transmitted.
C-QUAM (Compatible Quadrature Amplitude Modulation) is the most
common system used for AM stereo.
The stereo AM signal requires additional modulation techniques to combine the
two channels (left and right audio channels) and is transmitted within the same
AM frequency band.
5. Transmission Antenna


Type of Antenna:
o AM broadcasting typically uses vertical monopole antennas or vertical dipole
antennas to radiate the signal. The antenna structure is often large and tall, with
an electrical height of a quarter wavelength of the carrier frequency.
o Directional antennas may be used for high-power stations to control the
radiation pattern and limit interference to other stations.
Antenna Height:
o The antenna height is crucial for efficient transmission, especially for longdistance broadcasting.
o Long-wave AM broadcasting often uses tall towers (e.g., 100 meters or higher),
which can help improve coverage and efficiency.
6. Audio Frequency (Baseband)

Audio Bandwidth:
o The baseband audio signal typically has a bandwidth of up to 5 kHz (although in
some cases it can be up to 10 kHz).
o The audio frequency response for AM transmission is typically between 30 Hz
and 5 kHz for standard AM radio, although high-fidelity AM broadcasts may
extend the response to 10 kHz.
7. Modulation Depth

Modulation Depth:
o The modulation depth refers to the extent to which the carrier is modulated by the
audio signal.
o The ideal modulation depth for AM broadcasting is typically 80% to 90%,
meaning the carrier amplitude varies between 10% to 20% of the maximum
modulation level.
o Modulation depths beyond 100% lead to over modulation, causing distortion and
interference.
8. Carrier Suppression and Sideband Power

Carrier Power:
o

A significant portion of the transmitted power in AM broadcasting is carried by
the carrier signal itself.
o Ideally, only a small portion of the total transmitted power is used for the upper
and lower sidebands, which carry the audio information.
Sideband Power:
o In AM transmission, the total power is divided among the carrier and the
sidebands (upper and lower).
o The sideband power carries the modulated signal and can be used to analyze the
audio quality and transmission efficiency.
9. Signal-to-Noise Ratio (SNR)


Signal Quality:
o The signal-to-noise ratio (SNR) for AM broadcasting determines the clarity and
intelligibility of the received signal.
o Typical SNR values for AM radio vary, with higher power stations providing a
better SNR, which improves reception quality.
Noise:
o AM signals are particularly susceptible to electromagnetic interference (EMI),
atmospheric noise (like static or thunderstorms), and man-made noise (like
engine ignition systems).
o AM radios often employ automatic gain control (AGC) to compensate for signal
fluctuations and improve the listening experience.
10. Regulations and Standards


Frequency Allocation:
o In most countries, AM broadcast frequencies are regulated by national
regulatory bodies such as the Federal Communications Commission (FCC) in
the United States and the International Telecommunication Union (ITU)
globally.
o Stations must adhere to the frequency ranges allocated by these bodies, and any
changes in frequency allocations are subject to international agreements and local
regulations.
International Standards:
o The ITU-R (International Telecommunication Union – Radio communication
Sector) sets the standards for AM radio broadcasting. In particular, ITU-R
Recommendation BS.1114 provides technical characteristics for AM
broadcasting systems.
Summary Table of AM Broadcasting Specifications
Parameter
Frequency Range
Modulation Type
Audio Bandwidth
Specification
530 kHz to 1700 kHz (AM broadcast band)
Amplitude Modulation (AM)
5 kHz (standard), 10 kHz (high-fidelity)
Carrier Power
Sideband Power
Modulation Depth
Channel Spacing
Antenna Type
Antenna Height
Signal-to-Noise Ratio (SNR)
Frequency Stability
Typically ranges from 50 W to 50 kW, up to 500 kW
Upper and lower sidebands carry modulated audio signals
80% to 90%
10 kHz (typical for commercial stations)
Vertical monopole or dipole antennas
Typically hundreds of meters tall for long-distance transmission
Dependent on power and interference conditions
Carrier frequency stability typically ±1 Hz
Summary
AM broadcasting remains a widely used method for transmitting radio signals, especially for
news, music, and talk shows. The technical specifications and regulations for AM broadcasting
ensure that broadcasts are effective, interference is minimized, and listeners experience highquality audio transmissions. Despite being susceptible to noise and interference, AM
broadcasting continues to be a reliable and widely accessible medium for mass communication.