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Amplitude Modulation Reception
Electrical Engineering (Nueva Vizcaya State University)
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Amplitude Modulation Reception
I.
UNIT TITLE/CHAPTER TITLE
Amplitude Modulation Reception
II.
LESSON TITLE
1. Introduction
2. Receiver Parameters
3. AM Receivers
4. AM Receiver Circuits
5. Double-Conversion AM Receivers
6. Net Receiver Gain
III.
LESSON OVERVIEW
AM demodulation is the reverse of AM modulation. A conventional double-sideband AM receiver simply
converts a received amplitude-modulated wave back to the original source information. A receiver must
be capable of receiving, amplifying, and demodulating an AM wave. It must also be capable of
bandlimiting the total radio frequency spectrum to a specific desired frequencies. The selection process
is called tuning the receiver.
IV.
DESIRED LEARNING OUTCOMES
1. Define AM demodulation
2. Define and describe the following parameters: selectivity, bandwidth improvement, sensitivity,
dynamic range, fidelity, insertion loss, and equivalent noise temperature.
3. Explain the functions of the stages of a superheterodyne receiver
4. Describe net receiver gain
V.
LESSON CONTENT
Receiver Parameters
Selectivity – a receiver parameter that is used to measure the ability of the receiver to accept a
given band of frequencies and reject all others.
Shape Factor – the ratio of the bandwidth 60 dB below maximum signal level and bandwidth 3
dB below maximum signal level.
𝐵(−60 𝑑𝐵)
𝑆𝐹 =
𝐵(−3 𝑑𝐵)
Bandwidth Improvement – noise reduction ratio achieved by reducing the Bandwidth
𝐵𝑅𝐹
𝐵𝐼 = 𝐵
Where
𝐼𝐹
𝐵𝐼 = bandwidth improvement (unitless)
BRF = RF bandwidth (hertz)
BIF = IF bandwidth (hertz)
Noise Figure improvement – the corresponding reduction in the noise figure due to the reduction
in bandwidth 𝑁𝐹𝑖𝑚𝑝𝑟𝑜𝑣𝑒𝑚𝑒𝑛𝑡 = 10 log 𝐵𝐼
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Sensitivity – is the minimum RF signal level that can be detected at the input to the receiver and
still produce a usable demodulated information signal. Also known as receiver threshold.
Dynamic Range – defined as the difference in decibels between the minimum input level
necessary to discern a signal and the input level that will overdrive the receiver and produce distortion. The 1dB compression point is defined as the output power when the RF amplifier response is 1 dB less than the ideal
linear gain response.
Fidelity – A measure of the ability of a communication system to produce, at the output of the
receiver, an exact replica of the original source information.
Absolute Phase Shift – the total phase shift encountered by a signal and can generally be
tolerated as long as all frequencies undergo the same amount of phase delay.
Differential Phase Shift – occurs when a different frequencies undergo different phase shifts and
have a detrimental effect on a complex waveform.
Insertion Loss (IL) – defined as the ratio of the power transferred to a load with a filter in the circuit
to the power transferred to a load without the filter.
𝑃𝑜𝑢𝑡
𝐼𝐿 = 10 log
𝑃𝑖𝑛
Noise Temperature and Equivalent Noise Temperature – a hypothetical value that cannot be
directly measured. A parameter that is used in low-noise sophisticated radio receivers rather than noise figure.
AM RECEIVERS
Tuned Radio-Frequency Receiver – one of the earliest types of AM receivers and are probably
the simplest designed radio receivers available today.
Skin Effect – a phenomenon at radio frequencies where current flow is limited to the
outmost area of a conductor
Stagger Tuning – a technique where TRF receiver’s instability can be reduced somewhat
by tuning each amplifier to a slightly different frequency, slightly above or below the desired center frequency.
Example: For an AM commercial broadcast-band receiver (535 kHz to 1605 kHz) with an
input filter Q-factor of 54, determine the bandwidth at the low and high ends of the RF spectrum.
Solution: The bandwidth at the low-frequency end of the AM spectrum is centered around
a carrier frequency of 540 kHz and is
𝑓 540 𝑘𝐻𝑧
𝐵= =
= 10 𝑘𝐻𝑧
𝑄
54
The bandwidth at the high-frequency end of the AM spectrum is centered around
a carrier frequency of 1600 kHz and is
𝑓 1600 𝑘𝐻𝑧
𝐵= =
= 29,630 𝐻𝑧
𝑄
54
Superheterodyne Receiver
Heterodyne – means to mix two frequencies together in a nonlinear device or to translate
one frequency to another using nonlinear mixing.
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RF Section – generally consist of a preselector and an amplifier stage. The Preselector is
a broad-tuned bandpass filter with an adjustable center frequency that is tuned to desired carrier frequency.
Several advantages of including RF amplifiers in a receiver are as follows:
1. Greater gain, thus better sensitivity
2. Improved image-frequency rejection
3. Better signal-to-noise ratio
4. Better selectivity
Mixer/Converter Section – includes a radio-frequency oscillator stage (commonly called a
local oscillator) and a mixer/converter stage (commonly called the first detector)
The most common intermediate frequency used in AM broadcast-band receivers
is 455 kHz
IF Section – consists of a series of IF amplifiers and bandpass filters and is often called IF
strip
Intermediate Frequency – refer to frequencies that are used within a transmitter or
receiver that fall somewhere between the radio frequencies and the original source
information frequencies.
Detector Section – to convert the IF signals back to the original source information.
Generally called an audio detector or the second detector in a broadcast-band receiver.
Audio Amplifier Section – comprises several cascaded audio amplifiers and one or more
speakers.
Receiver Operation
- During the demodulation process in a superheterodyne receiver, the received signals undergo
two or more frequency translations: First, the RF is converted to IF, then the IF is converted
to the source information.
Frequency Conversion – in the mixer/converter stage is identical to frequency conversion in
the modulator stage of a transmitter except that, in the receiver, the frequencies are downconverted rather than up-converted.
Gang Tuning – means that the two adjustments are mechanically tied together so that a
single adjustment will change the center frequency of the preselector and, at the same time,
change the local oscillator frequency.
High-side injection / high-beat injection – is when the local oscillator is tuned above
the RF 𝑓𝑙𝑜 = 𝑓𝑅𝐹 + 𝑓𝐼𝐹
Low-side injection / low-beat injection – is when the local oscillator is tuned below
the RF 𝑓𝑙𝑜 = 𝑓𝑅𝐹 − 𝑓𝐼𝐹
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Where
𝑓𝑙𝑜 = local oscillator frequency (hertz)
𝑓𝑅𝐹 = radio frequency (hertz)
𝑓𝐼𝐹 = intermediate frequency (hertz)
Example: For an AM superheterodyne receiver that uses high-side injection and has a
local oscillator frequency of 1355 kHz, determine the IF carrier, upper side frequency, and lower side
frequency for an RF wave that is made up of a carrier and upper and lower side frequencies of 900 kHz,
905 kHz, and 895 kHz, respectively.
Solution:
𝑓𝐼𝐹 = 𝑓𝑙𝑜 − 𝑓𝑅𝐹 = 1355 𝑘𝐻𝑧 − 900 𝑘𝐻𝑧 = 455 𝑘𝐻𝑧
The upper and lower intermediate frequencies are
𝑓𝐼𝐹(𝑢𝑠𝑓) = 𝑓𝑙𝑜 − 𝑓𝑅𝐹(𝑙𝑠𝑓) = 1355 𝑘𝐻𝑧 − 895 𝑘𝐻𝑧 = 460 𝑘𝐻𝑧
𝑓𝐼𝐹(𝑙𝑠𝑓) = 𝑓𝑙𝑜 − 𝑓𝑅𝐹(𝑢𝑠𝑓) = 1355 𝑘𝐻𝑧 − 905 𝑘𝐻𝑧 = 450 𝑘𝐻𝑧
Note: Sideband inversion – the side frequencies undergo a sideband reversal during the
heterodyning process.
Local Oscillator Tracking
Tracking – the ability of the local oscillator in a receiver to oscillate above or below the
selected radio frequency carrier by an amount equal to the intermediate frequency throughout the entire
radio frequency band.
Tracking Error – the difference between the actual oscillator frequency and the desired
frequency.
Image Frequency – any frequency other than the selected radio frequency carrier that, if allowed
to enter a receiver and mix with the local oscillator, will produce a cross-product frequency that is equal
to the intermediate frequency.
For high-side injection, the image frequency (fim) is
𝑓𝑖𝑚 = 𝑓𝑙𝑜 + 𝑓𝐼𝐹
And, because the desired RF equals the local oscillator frequency minus the IF,
𝑓𝑖𝑚 = 𝑓𝑅𝐹 + 2𝑓𝐼𝐹
Image-frequency rejection ratio – a numerical measure of the ability of a preselector to reject
the image frequency
𝐼𝐹𝑅𝑅 = √(1 + 𝑄2𝜌2)
𝑓𝑖𝑚
𝑓𝑅𝐹
Where 𝜌 = ( ) − ( )
𝑓𝑅𝐹
𝑓𝑖𝑚
Example: for an AM broadcast-band superheterodyne receiver with IF, RF, and local
oscillator frequencies of 455 kHz, 600 kHz, and 1055 kHz, respectively, refer to figure 5-11 and determine
a. Image frequency
b. IFRR for a preselector Q of 100
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Solution:
a. 𝑓𝑖𝑚 = 1055 𝑘𝐻𝑧 + 455 𝑘𝐻𝑧 = 1510 𝑘𝐻𝑧
600 𝑘𝐻𝑧
1510 𝑘𝐻𝑧
b. 𝜌 = (
)−(
) = 2.51 − 0.397 = 2.113
600 𝑘𝐻𝑧
1510 𝑘𝐻𝑧
𝐼𝐹𝑅𝑅 = √1 + (1002)(2.1132) = 211.3
Double Spotting – occurs when a receiver picks up the same station at two nearby points on the
receiver tuning dial.
AM RECEIVER CIRCUITS
RF Amplifier Circuits – a high-gain, low-noise, tuned amplifier that, when used, is the first active
stage encountered by the received signal.
The following characteristics are desirable for RF amplifiers:
1. Low thermal noise
2. Low noise figure
3. Moderate to high gain
4. Low intermodulation and harmonic distortion
5. Moderate selectivity
6. High image-frequency rejection ratio
Low-Noise Amplifiers (LNA) – high performance microwave receivers require a LNA as the input
stage of the RF section to optimize their noise figure.
Mesa Semiconductor FET / Semiconductor FET (MESFET) – a FET with a metalsemiconductor junction at the gate of the device, called a Schottky barrier.
Integrated-circuit RF amplifiers
NE/SA5200 – a wideband, unconditionally stable, low-power, dual-gain linear
integrated-circuit RF amplifier manufacture by Signetic Corporation.
Mixer/Converter Circuits – this section purpose is to down-convert the incoming radio
frequencies to intermediate frequencies proportional to bandwidth.
The output of a balanced mixer is the product of the RF and local oscillator frequencies
𝑉𝑜𝑢𝑡 = (sin 2𝜋𝑓𝑅𝐹𝑡)(sin 2𝜋𝑓𝑙𝑜𝑡)
Where
𝑓𝑅𝐹 = incoming radio frequency (hertz)
𝑓𝑙𝑜 = local oscillator frequency (hertz)
Conversion Gain – the difference between the level of the IF output with an RF input signal
to the level of the IF output with an IF input signal
Self-excited Mixer – a configuration where the mixer excites itself by feeding energy back
to the local oscillator tank circuit to sustain oscillations noise figure.
Integrated-circuit mixer/oscillator
NE/SA602A – a low-power VHF monolithic double-balanced mixer with input amplifier, onboard oscillator, and voltage regulator
IF Amplifier Circuits
Intermediate Frequency (IF) Amplifier are relatively high-gain amplifiers that are very
similar to RF amplifiers, except that IF amplifiers operate over a relatively narrow, fixed frequency
band.
Inductive Coupling
Inductive or Transformer Coupling is the most common technique used for coupling where
the voltage that is applied to the primary windings of a transformer is transferred to the secondary
windings.
𝐸𝑠 = 𝜔𝑀𝐼𝑝
Where
𝐸𝑠 = voltage magnitude induced in the secondary windings (votls)
𝜔 = angular velocity of the primary voltage wave (radians per second)
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M = mutual inductance (henrys)
𝐼𝑝 = primary current (amperes)
Self Inductance or simply Inductance – ability of a coil to induce a voltage within its
windings
Mutual Inductance – ability of one coil to induce a voltage in another coil
𝑀 = 𝑘√𝐿𝑠𝐿𝑝
Where
M = mutual inductance (henrys)
𝐿𝑠 = inductance of the secondary winding (henrys)
𝐿𝑝 = inductance of the primary winding (henrys)
k = coefficient of coupling (unitless)
Coefficient of Coupling – the ratio of the secondary flux to the primary flux
𝜙𝑠
𝑘=
𝜙𝑝
Where
k = coefficient of coupling (unitless)
𝜙𝑠 = secondary flux (webers)
𝜙𝑝 = primary flux (webers)
Flux Linkage – the transfer of flux from the primary to the secondary windings and is
directly proportional to the coefficient of coupling
Critical Coupling – the point where the reflected resistance is equal to the primary
resistance and at the Q of the primary tank circuit is halved and the bandwidth is doubled
Double Peaking – is caused by the reactive element of the reflected impedance being
significant enough to change the resonant frequency of the primary tuned circuit.
Optimum Coupling – the coefficient of coupling approximately 50% greater than the critical
value yields a good compromise between flat response and steep skirts.
𝑘𝑜𝑝𝑡 = 1.5𝑘𝑐
Where
𝑘𝑜𝑝𝑡 = optimum coupling
𝑘𝑐 = critical coupling = 1
√𝑄𝑝𝑄𝑠
Where Qp and Qs are uncoupled values
The bandwidth of a double-tuned amplifier is
𝐵𝑑𝑡 = 𝑘𝑓𝑜
Bandwidth Reduction
𝐵𝑛 = 𝐵1 (√21⁄𝑛 − 1)
Where
Where
𝐵𝑛 = bandwidth of n single tuned stages (hertz)
𝐵1 = bandwidth of one single tuned stage (hertz)
n = number of stages (any positive integer)
𝐵𝑛𝑑𝑡 =
1⁄
𝐵1𝑑𝑡 [2 𝑛
1⁄
4
− 1]
𝐵𝑛𝑑𝑡 = overall bandwidth of n double tuned amplifiers (hertz)
𝐵1𝑑𝑡 = bandwidth of double tune amplifier (hertz)
n = number of double tuned stages (any positive integer
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IF Cans – IF transformers come as a specially designed tuned circuits in groundable metal
packges.
Integrated-Circuit IF Amplifiers
CA3028A – a differential cascaded amplifier designed for use in communications and
industrial equipment as an IF or RF amplifier at frequencies from dc to 120 MHz.
AM Detector Circuits – the function of this circuit is to demodulate the AM signal and recover or
reproduce the original source information
Peak Detector – a simple noncoherent AM demodulator using a diode. Also called as
diode, shape, or envelope detector.
𝑉𝑜𝑢𝑡 = 𝑖𝑛𝑝𝑢𝑡 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑖𝑒𝑠 + ℎ𝑎𝑟𝑚𝑜𝑛𝑖𝑐𝑠 + 𝑠𝑢𝑚𝑠 𝑎𝑛𝑑 𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒𝑠
Detector Distortion
Rectifier Distortion – a distortion in the detection process where the RC time
constant is too short, the output waveform resembles a half-wave rectified signal.
Diagonal Clipping – a distortion in the detection process where the RC time
constant is too long, the slope of the output waveform cannot follow the trailing slope of the envelope.
√(1⁄𝑚2) − 1
𝑓𝑚(max) =
2𝜋𝑅𝐶
Where
𝑓𝑚(max) = maximum modulating signal frequency (hertz)
m = modulation coefficient (unitless)
RC = time constant (seconds)
Automatic Gain Control Circuits – a circuit that compensates for minor variations in the received RF
signal
Delayed AGC – it prevents the AGC feedback voltage from reaching the RF or IF amplifiers until
the RF level exceeds a predetermined magnitude.
Forward AGC – is similar to conventional AGC except that the receive signal is monitored closer
to the front end of the receiver and the correction voltage is fed forward to the IF amplifiers.
Squelch Circuit – its purpose is to quiet a receiver in the absence of a received signal
Noise Limiters and Blankers
Limiters / Clippers – are used to remove sporadic, high-amplitude noise transients of short
duration, such as impulse noise in the audio section of a receiver.
Blanking Circuit – is another circuit option commonly used for reducing the effect of high-amplitude
noise pulses.
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Alternate Signal-to-Noise Measurements
Signal-to-Notched Noise Ratio – a method of measuring signal strength relative to noise strength
where an RF carrier modulated 30% by a 1-kHz tone is applied to the input of the receiver.
Linear Integrated-Circuit AM receivers
LM1820 – a national semiconductor corporation linear integrated circuit AM radio chip that has
an onboard RF amplifier, mixer, local oscillator, and IF amplifier stages. An LIC audio amplifier such as the
LM386, and a speaker are necessary to complete a functional receiver.
PLL receivers – these receivers would need only two external components: a volume control and
astation tuning control.
Net Receiver Gain – the ratio of the demodulator signal level at the output of the receiver (audio) to the
RF signal level at the input to the receiver, or the difference between the audio signal level in dBm and RF signal
level in dBm.
𝐺𝑑𝐵 = 𝑔𝑎𝑖𝑛𝑠𝑑𝐵 − 𝑙𝑜𝑠𝑠𝑒𝑠𝑑𝐵
Where
gains = RF amplifier gain + IF amplifier gain + audio amplifier gain
Losses = pre-selector losses + mixer losses + detector losses
System Gain – includes all the gains and losses incurred by a signal as it propagates from the transmitter
output stage to the output of the detector in the receiver and includes antenna gain and transmission line and
propagation losses.
VI.
LEARNING ACTIVITIES
Seatwork 4.1
1. Determine the IF bandwidth necessary to achieve a bandwidth improvement of 16 dB for a radio
receiver with an RF bandwidth of 320 kHz.
2. Determine the equivalent noise temperature for an amplifier with a noise figure of 6 dB and an
environmental temperature T = 27oC.
3. For an AM superheterodyne receiver using high-side injection with a local oscillator frequency of 1200
kHz, determine the IF carrier and upper and lower side frequencies for an RF envelope that is made
up of a carrier and upper and lower side frequencies of 600 kHz, 604 kHz and 596 kHz respectively.
4. For a receiver with IF, RF, and local oscillator frequencies of 455 kHz, 900 kHZ, and 1355 kHz
respectively, determine
a. Image frequency
b. IFRR for a preselector Q of 80
5. Determine the bandwidth improvement for a radio receiver with an RF bandwidth of 60 kHz and an
IF bandwidth of 15 kHz.
6. For an AM commercial broadcast-band receiver with an input filter Q-factor of 60, determine the
bandwidth at the low and high ends of the RF spectrum.
7. For a citizens band receiver using high-side injection with an RF carrier of 27.04 MHz and a 10.645
MHz IF, determine
a. Local oscillator frequency
b. Image frequency
VII.
ASSIGNMENT
Assignment 4.1
1. Determine the improvement in the noise figure for a receiver with an RF bandwidth equal to 40 kHz
and IF bandwidth of 16 kHz.
2. For an AM commercial broadcast-band receiver with an input filter Q-factor of 85, determine the
bandwidth at the low and high ends of the RF spectrum.
3. For a receiver with a ±2.5𝑘𝐻𝑧 tracking error, a 455-kHz IF, and a maximum modulating signal
frequency fm = 6 kHz, determine the minimum IF bandwidth.
4. Determine the maximum modulating signal frequency for a peak detector with the following
parameter: C = 1000 pF, R = 10 k ohms, and m = 0.5. Repeat the problem for m = 0.707.
5. Determine the net receiver gain for an AM receiver with an RF input signal power of -87 dBm and an
audio signal power of 10 .
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