Introduction to Analog
Communication &
Reception
MS. RENIA DIAS
01-09-2025
ADCOM --PROF. RENIA DIAS
01-09-2025
ADCOM --PROF. RENIA DIAS
Books
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Module 1
Introduction to Communication Systems
◦ Analog
◦ Digital
Modulation Techniques
◦ AM
◦ FM
Reception
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Introduction to Communication
System
The purpose of a communication system is to deliver
a message signal from an information source in the
recognizable form to a user destination, with the
source and the user being physically separated from
each other.
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Overview of Communication
Communication systems are essential for exchanging
information effectively. They enable connectivity and
collaboration across various sectors, which is vital in today’s
interconnected world.
Types of communication
There are various communication systems, including wired,
wireless, and satellite, each serving distinct purposes and
applications in both personal and professional
environments.
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Analog v/s Digital
Analog communication uses
continuous signals to represent
information. It is characterized by its
simplicity and ability to transmit
audio and video signals without
digital conversion.
Digital communication utilizes
discrete signals for data
transmission. It offers enhanced
clarity, reduces noise interference,
and allows for easy encryption,
making it suitable for a wide range
of applications.
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Need for Modulation
Practical Length of Antenna
Narrow Banding of Signal
Frequency Multiplexing
Effective Power Radiated By
Antenna
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Modulation
Modulation is the process for which some characteristic of a
signal called carrier is varied in accordance with the
instantaneous value of another signal called modulating
signal. Signals containing information are modulating
signals.
This information-bearing signal is also called a baseband
signal.
The carrier frequency is greater than the modulating
frequency. The signal resulting from the process of
modulation is called modulated signal.
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Types of Modulation
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Types of Modulation
1. Continuous Wave Modulation
When the carrier wave is continuous in nature, the
modulation process is known as continuous wave
(CW) modulation or analog modulation.
◦ Amplitude Modulation
◦ Angle Modulation.
when the angle of the carrier is varied according to the
instantaneous value of the modulating signal, it is
called angle modulation.
Angle modulation may be further subdivided into
Frequency modulation (FM) and Phase modulation
(PM), in which the instantaneous frequency and phase
of the carrier, respectively, are varied in accordance
with the message signal.
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Types of Modulation
2. Pulse Modulation
When the carrier wave is a pulse-type waveform,
the modulation process is known as pulse
modulation.
In pulse modulation, the carrier consists of a
periodic sequence of rectangular pulses. Pulse
modulation can be of an analog or digital type.
In analog pulse modulation, the amplitude,
duration, or position of a pulse is varied in
accordance with sample values of the message
signal.
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Modulation Techniques
AM is a technique where the amplitude of a carrier wave is
varied in proportion to the information signal. This method
enables transmission of audio signals over long distances.
Uses of AM
1. broadcasting radio transmissions
2. particularly in the medium and shortwave bands
3. providing a means for delivering news, music, and
entertainment to the public.
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Mathematical Expression of AM
Let the modulating signal be,
and the carrier signal be,
Where,
Am and Ac are the amplitude of the modulating signal and the carrier signal respectively.
fm and fc are the frequency of the modulating signal and the carrier signal respectively.
Then, the equation of Amplitude Modulated wave will be
Modulation Index of AM
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Perfect, Under & Over Modulation
For a perfect modulation, the value of modulation index should be 1, which implies the percentage of
modulation should be 100%.
For instance, if this value is less than 1, i.e., the modulation index is 0.5, then the modulated output would
look like the following figure. It is called as Under-modulation. Such a wave is called as an undermodulated wave.
If the value of the modulation index is greater than 1, i.e., 1.5 or so, then the wave will be an overmodulated wave.
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Bandwidth of AM Wave
Bandwidth (BW) is the difference between the highest and lowest frequencies of the signal.
Mathematically, we can write it as
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Measurement of Modulation index
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Power measurement of AM
The AM signal consists of three frequency components—the carrier signal and the two sidebands:
upper sideband and lower sideband. The easiest way to compute the total average power in an AM
signal is to add the individual average power contents in each of three frequency components. That
is, Total power in AM signal = Carrier power + Lower-sideband power + Upper-sideband power
PAM = Pc + Plsb + Pusb
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Power measurement of AM
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Transmission Efficiency
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Frequency Spectrum of AM
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Limitations of AM
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Numericals on Modulation
A modulating signal m(t)=10cos(2π×103t) is amplitude
modulated with a carrier signal c(t)=50cos(2π×105t). Find the
modulation index, the carrier power, and the power required
for transmitting AM wave.
The equation of amplitude wave is given by
s(t)=20[1+0.8cos(2π×103t)]cos(4π×105t). Find the carrier
power, the total sideband power, and the band width of AM
wave.
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DSBSC Modulation Technique
Double-sideband suppressed carrier (DSBSC) modulation technique is a modified form of
amplitude modulation technique in which the carrier signal is completely suppressed from
amplitude modulated signal.
The main disadvantage of DSBSC signal is that the resultant envelope is not a faithful
representation of the modulating signal. It is merely the sum of the lower-sideband and uppersideband signals. The frequency of its envelope is twice the modulating frequency
DSBSC AM is not often used in analog communication system. However, it forms the basis for
generating single-sideband suppressed-carrier (SSBSC), or just single-sideband (SSB) signals.
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DSBSC Mathematical expression
The bandwidth of the DSBSC signal (BDSBSC) is equal to the difference between the maximum
upper-sideband frequency and the minimum lower-sideband frequency
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Power in DSBSC
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Disadvantage of DSBSC
The main disadvantage of DSBSC signal is that the resultant envelope is not a
faithful representation of the modulating signal.
It is merely the sum of the lower-sideband and upper-sideband signals.
The frequency of its envelope is twice the modulating frequency.
DSBSC AM is not often used in analog communication system. However, it forms
the basis for generating single-sideband suppressed-carrier (SSBSC), or just
single-sideband (SSB) signals.
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SSB
Single-Sideband Suppressed Carrier (SSBSC), or simply single-sideband (SSB), is a form of amplitude modulation in
which the carrier is fully suppressed and one of the sidebands (lower or upper) is also suppressed
With double-sideband transmission, the information contained in the lower-sideband is identical to the
information contained in the upper-sideband.
 In fact, the two sidebands of an AM signal are mirror images of each other, since one consists of the difference
of the carrier and modulating frequencies and the other is the sum of the carrier and modulating frequencies.
The two sidebands are uniquely related to each other because they are symmetrical about the carrier frequency.
If amplitude and phase spectra of one sideband is known, then the amplitude and phase spectra of other
sideband can be uniquely determined.
As far as the transmission of information is concerned using amplitude modulation, then the transmission of one
sideband is necessary.
This means that no information will be lost by suppressing the carrier and one of two sidebands of AM signal.
This is essentially SSB transmission.
The upper and lower sidebands contain the same information, and as such there is no preference of one over
the other.
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BW of SSB
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Power in SSB
How much power can be saved with SSB transmissions as compared to that of conventional AM
and DSBSC signal?
Thus the minimum power increase in single-sideband by suppressing the carrier and the other
sideband (corresponding to ma = 1) will be 6 times or 10 log (6) ª 7.78 dB, as compared to
conventional AM; and 2 times or 3 dB as compared to DSBSC
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Advantages of SSB
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VSB
The Vestigial-SideBand (VSB) modulation is another form of an amplitude-modulated signal (that
is, the carrier signal plus double-sideband) in which a part of the unwanted sideband (called as
vestige, and hence the name vestigial sideband) is allowed to appear at the output of VSB
transmission system
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Comparison of AM,DSBSC,SSB
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SUMS
The power in the sidebands can have a maximum of one-third of the total AM signal transmitted
power for 100% modulation. Calculate the percentage of power in the sidebands for 10%
modulation. How does this justify that AM transmitters should be operated with the modulation
as close to 100% as possible?
The power transmitted by a SSB transmitter is 10 kW. It is required to be replaced by standard
AM transmission having modulation index of 0.8 and same power. Determine the power
contents of the carrier and each of the sidebands. Also compute the transmission efficiency.
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Analog Transmission & Reception
Amplitude modulator is a device which is used to generate an amplitude-modulated signal.
Depending on the level at which the amplitude modulation is carried out, there are two
methods of generation of AM signal. Accordingly, AM transmitters can be broadly classified as
• Low-Level AM Transmitter
• High-Level AM Transmitter
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Low level Transmitter
oWhen the process of amplitude modulation is accomplished at any one of the initial stages
(other than final stage) in AM transmitter, it is referred to as low-level AM modulation
oDue to low-power levels of modulating signal and carrier signal, the amplitude-modulated
output of AM modulator is also at low-power level
oLow-level AM modulation is used in not-so-efficient laboratory AM transmitters. Low-level AM
transmitters also finds applications in low-power, low-capacity radio communication systems
such as wireless paging network, wireless intercoms, short-range walkie-talkie, and remote
control units.
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High Level Transmitter
When the process of amplitude modulation is accomplished at final stage in AM transmitter, it is
referred to as high-level AM modulation.
The high-frequency carrier signal from RF carrier oscillator is applied to narrowband high-power
RF amplifier operating at fixed carrier frequency. • High-level AM modulator performs the
process of amplitude modulation on high-level carrier signal by amplified-modulating signal, to
produce 100 % modulation or desired modulation index.
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AM Modulator
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Square Law AM Modulator
The nonlinear device can be a semiconductor diode or a BJT or FET.
• When the input signal level is small, the diode operates in the highly nonlinear region of its
characteristics curve.
• When the input signal level is large, bipolar junction and field-effect transistors can be
operated in their nonlinear operating regions.
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Balanced Modulator
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Ring Modulator for DSBSC
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Frequency Modulation
Frequency Modulation (FM) is that form of angle modulation in which the instantaneous
frequency of the carrier signal is varied in proportion to the instantaneous value of the
modulating signal.
Consider a sinusoidal modulating signal defined as
according to definition, the FM signal is given by
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Frequency Modulation (FM)
Frequency Modulation involves varying the
frequency of a carrier wave according to the
information signal being sent. FM minimizes
the effects of noise and interference during
transmission.
FM has several advantages over AM,
including better sound quality, greater
resistance to noise, and improved fidelity,
making it the preferred choice for highfidelity music broadcasting and
communication applications.
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Concept of Instantaneous Frequency
The instantaneous frequency of FM wave, as a function of time, can be written as
Let the modulating signal is a sine wave of the form,
From this equation, it can be deduced that peak frequency deviation (maximum change in frequency on
either side of the carrier frequency) will be k f Vm (because maximum value of sine function is unity) .
Therefore, the instantaneous frequency of FM wave can also be expressed as
This is the expression for the frequency of an FM signal with sinusoidal wave modulating
signal
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SUM
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Frequency-Modulation Index
For a single-tone sinusoidal modulating signal, frequency-modulation index (mf ) is defined as the ratio
of peak frequency deviation (d ) and the modulating frequency (fm).
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Sums
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SUM
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Instantaneous Frequency Deviation
The process of frequency modulation causes the carrier signal frequency to vary (deviate) from
its original unmodulated frequency.
In general, the frequency deviation is the maximum change in instantaneous frequency from the
unmodulated carrier frequency, and is directly proportional to instantaneous amplitude of the
modulating signal (as per the basic definition of FM).
Thus, the instantaneous frequency deviation, expressed in Hz, is given
where kf is a deviation constant, and is also known as the deviation sensitivity or frequency
sensitivity of the frequency modulators. It is expressed in units of Hz/V. And vm is the
instantaneous amplitude of the modulating signal.
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Applications
FM broadcast stations use large frequency deviation.
FM signal is meaningful only if the frequency deviation is large enough.
The audio quality of a FM signal increases as the frequency deviation increases.
This is the reason why FM broadcast stations use large frequency deviation.
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Spectrum of Constant BW FM
The following observations can be made:
1.
The FM signal produces an infinite number of sidebands, each one is separated in frequency
from the carrier by multiples of modulating frequency.
2.
This means that the bandwidth required to transmit FM signal is theoretically infinite.
3.
As the value of modulation index increases, the number of sidebands tends to increase
significantly and hence the bandwidth of FM signal also increases.
4.
But actually the amplitude levels of higher and higher sidebands tend to decrease as these
are away from the carrier frequency.
5.
In practice, frequency-modulated signal can be considered to be band limited because
sidebands with amplitude levels less than about 1% of the total power can usually be
ignored.
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FM BW
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Pre-emphasis & De-emphasis circuit
They help to improve the quality of any communication especially audio signals on the
transmitter and receiver sides.
When the amplitude of a high-frequency noise is higher than the current component in the
modulation signal, it causes high-frequency interference.
To deal with this issue, most FM circuits use a technique called pre-emphasis during
transmission and de-emphasis during receiving.
Pre-emphasis and de-emphasis circuits are commonly used in FM transmitters and receivers to
improve the output signal-to-noise ratio.
Pre-emphasis and De-emphasis are used to improve the fidelity of FM transmission of the
audio signal.
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Pre-emphasis Circuit
In the pre-emphasis circuit:
R1, R2, and C form a high-pass filter.
The audio signal is applied to the input.
The output is taken across capacitor C.
Below is a High Pass Filter with transfer function
50µsec pre-emphasis corresponds to frequency response curve
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Advantages of Pre-emphasis
It help us to amplify high frequency signal components so that they have a higher magnitude
than the noise components. It improves Signal to Noise ratio or SNR.
It has an upper cutoff point where signal enhancement begins to phase.
This is a simple high pass filter with amplification
The pre-emphasis has time constant of 50µs.
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Disadvantages of Pre-emphasis
If there are too many constraints, over modulation will occur.
Pre-emphasis amplifies high-frequency components, including noise, which can degrade the
signal-to-noise ratio.
Pre-emphasis is most effective in systems with high signal-to-noise ratios and wide bandwidths,
making it less suitable for some applications.
Implementing pre-emphasis requires additional circuitry and introduces complexity, potentially
increasing the cost of the system.
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De emphasis circuit
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Narrow band and wideband FM
The bandwidth of a frequency-modulated signal, BFM is a function of the modulating signal
frequency (fm) and the modulation index (mf ).
• With FM, multiple sets of sidebands are produced, and as a result, the FM-signal bandwidth
can be significantly wider than that of an AM wave with the same modulating signal.
• Moreover, there are no theoretical limits to the frequency deviation or the modulation index
of an FM signal. The limits, of course, are practical.
• As an instance, larger values for the frequency deviation result in an increased bandwidth as
well as the signal-to-noise ratio (SNR).
• The increased SNR is desirable, but increased bandwidth means more spectrum occupancy
which may not be available for allocation.
• Since FM receivers must be designed for a particular signal bandwidth, there has to be defined
standards for maximum frequency deviation.
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Comparison
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AM Demodulator- Square Law
demodulator
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FM Modulators
The reactance modulator uses a transistor and passive components to create a variable
reactance. This variable reactance, which can be capacitive or inductive, is used to change the LC
oscillator's tank circuit
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Reactance Modulator for FM Generation
In this circuit, capacitor C1 and resistor R1 are used to provide feedback from the collector to the
base of the transistor. For C1, we opt for a small value to ensure its reactance at the frequency of
interest is considerably higher than the resistance of R1. As a result, the current flowing through
the R1C1 branch is essentially set by the capacitor.
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FM Demodulator
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Working
This circuit consists of an inductively coupled double tuned circuit in which both primary and
secondary coils are tuned to the same frequency. The center of the secondary coil is connected
to the top of the primary through a capacitor C. this capacitor performs the functions are:
◦ It blocks the D.C. from primary to secondary
◦ It couples the signal frequency from primary to center tapping of the secondary.
◦ The primary voltage 𝑉3 appears across the inductor in primary side. Nearly entire voltage 𝑉3 appears
across inductor L except a small drop across the capacitor C.
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Phase Modulation
In phase modulation, the phase of the carrier signal varies in accordance with the instantaneous
amplitude of the modulating signal.
Here with the change in phase, the frequency of the signal
also shows variation.
Thus it can be said that while phase modulating any signal,
the phase as well as the frequency of the
carrier signal shows variation
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Radio Receiver
A radio receiver Is a device for converting radio or electromagnetic waves into perceptible or
intelligence- signals.
Radio receiver is an electronic equipment which picks up the desired signal, reject the unwanted
signal and demodulate the carrier signal to get back the original modulating signal.
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Characteristics of radio receivers
Selectivity: Radio receiver should have good selectivity and selectivity of radio receiver is its ability to select
desired signal and reject unwanted signals. Selectivity is obtained by using tuned circuits, which are tuned to
desired frequency. The quality factor of these LC circuits determines the selectivity. It is given by, Q=XL/R. For
better selectivity ‘Q’ should be high.
Sensitivity: Ability to detect weak signals. Broadcast receivers/ radio receivers should have reasonably high
sensitivity so that it may have good response to the desired signal but should not have excessively high sensitivity
otherwise it will pick up all undesired noise signals. Sensitivity of a receiver is expressed in microvolts of the
received signal. Typical sensitivity for commercial broadcast-band AM receiver is 50 μV.
Sensitivity of the receiver depends on.
◦
◦
◦
Noise power present at the input to the receiver.
Receiver noise figure
Bandwidth improvement factor of the receiver The best way to improve the sensitivity is to reduce the noise level.
Fidelity is defined as a measure of the ability of a communication system to produce an exact replica of the original
source information at the output of the receiver. A radio receiver should have high fidelity or accuracy. It is
determined by the high frequency response.
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Classification of radio receivers
Depending upon application
◦ AM Receivers
◦ FM Receivers
◦ Communication Receivers
◦ Television Receivers
◦ Radar Receivers
Depending upon fundamental aspects
◦ Tuned Radio Frequency (TRF)Receivers
◦ Super-heterodyne Receivers
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Functions of radio receivers
1. Detection of weak signals: a radio is able to detect or sense weak signals that come with
noise. The minimum level of signal may be as low as 0.25µv.
2. Amplification of received signals and maintenance of the information contained in minimum
signal strength.
3. Separation of wanted from unwanted signals even when they are at close frequencies.
4. Recovery of the intelligence from the carrier i.e. demodulation
5. Amplification of the recovered audio information to drive the loudspeaker 6. Disabling the
audio amplifier in the absence of signal to cut out electrical noise.
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The Tuned radio frequency (TRF) receiver
• The antenna receives the electromagnetic wave and sends it to the ganged-RF amplifier stage where
selection and amplification of the very weak in-coming RF-signal is done and unwanted signals rejected.
• The signal then goes to the detector or demodulator from where the original information signal is
recovered, amplified and fed to the loudspeaker. This is a typical Tuned Radio Frequency (TRF).
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Advantages of TRF
TRF receivers are simple to design and allow the broadcast frequency 535 KHz to 1640 KHz.
High sensitivity.
Disadvantages of TRF:
•TRF receivers are simple to design and allow the broadcast frequency 535 KHz to 1640 KHz.
•The TRF is not stable at high frequency because of positive feedback which occurs as a result of
high gain achieved by the multi-stage amplifiers. This positive feedback is almost unavoidable at
high frequency and is not conducive for good signal reception.
•At higher frequency, it produces difficulty in design.
•It has poor audio quality.
•The TRF cannot achieve sufficient selectivity at high frequency because of the single tuned or
ganged circuit
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Superhytrodyne receiver
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Superhytrodyne receiver
The shortcomings of the TRF receiver are overcome by the invention of the super heterodyne
receiver.
A super heterodyne receiver converts all incoming radio frequency (RF) signals to a lower
frequency known as an intermediate frequency (IF).
The difference between superhet and the TRF is the presence of the Mixer and the Intermediate
frequency (IF) stages. The antenna picks the weak electromagnetic wave.
The signal is selected and amplified in the RF stage.
The Mixer and the local oscillator form a frequency changer circuit called CONVERTER. The signal
is heterodyned or mixed with the high frequency signal from the oscillator.
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Superhytrodyne receiver
The RF mixing unit develops an Intermediate Frequency (IF) to which any received signal is converted,
so as to process the signal effectively.
RF Mixer is an important stage in the receiver.
Two signals of different frequencies are taken where one signal level affects the level of the other
signal, to produce the resultant mixed output.
The outputs of the mixer are two new frequencies in addition to the original electromagnetic wave
and the carrier frequency.
They are: i) The sum of the frequencies ii) The difference of the frequencies iii) The local oscillator
frequency and iv) The RF frequency.
The mixer output is fed to the intermediate frequency (IF), which accepts only the difference signal.
This signal, which is a constant at 455 KHz for AM and 10.7MHz for FM, will be selected and others
rejected. The detector or demodulator removes the carrier wave from the signal and the audio or
intelligence signal is boosted to a level sufficient enough to drive the loudspeaker which finally
changes the electrical signal back to original sound.
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Superhytrodyne receiver
The RF stage: Consists of a pre-selector and an amplifier. Pre-selector is a broad-tuned band-pass filter with an adjustable center
frequency used to reject unwanted radio frequency and to reduce the noise bandwidth. RF amplifier determines the sensitivity of the
receiver and a predominant factor in determining the noise figure for the receiver.
2. The Converter stage: Consists of the local oscillator and a mixer. Choice of oscillator depends on the stability and accuracy desired.
Mixer is a nonlinear device to convert radio frequency to intermediate frequencies (i.e. heterodyning process). In the mixer, RF signals are
combined with the local oscillator frequency. The local oscillator is designed such that its frequency of oscillation is always above or below
the desired RF carrier by an amount equal to the IF center frequency, therefore the difference of RF and oscillator frequency is always
equal to the IF frequency. Frequency conversion in the mixer stage is identical to the frequency conversion in the demodulator except that
in the receiver, the frequencies are down-converted rather that up-converted.
3. IF stage: Consists of a series of IF amplifiers and band-pass filters to achieve most of the receiver gain and selectivity. Intermediate
frequency filter is a band pass filter, which passes the desired frequency. It eliminates all other unwanted frequency components present
in it. This is the advantage of IF filter, which allows only IF frequency. IF amplifiers are also less likely to oscillate than their RF
counterparts.
4. Detector stage: To convert the IF signals back to the original source information (demodulation). The received AM wave is now
demodulated using AM demodulator. This demodulator uses the envelope detection process to receive the modulating signal.
5. Audio amplifier section: Comprises several cascaded audio amplifiers and one or more speakers. This is the power amplifier stage,
which is used to amplify the detected audio signal. The processed signal is strengthened to be effective. This signal is passed on to the
loudspeaker to get the original sound signal.
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Disadvantages of TRF overcome by the
superhet receiver
Stability: As high frequency is down-converted to IF the reactance of stray
capacitances will not decrease as it was at higher frequencies resulting in
increased feedback.
No variation in Bandwidth as IF range is 438 to 465 KHz (in case of AM receive
mostly 455 KHz.
Better selectivity as no adjacent channels are picked due to variation in
Bandwidth.
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