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Prepared by Prof.V.K.Jain 1 Chapter 5 AM Receivers
Prepared by Prof.V.K.Jain 1
Chapter 5
AM Receivers
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Lecture outcome
After studying this lecture, you should be able to:
Describe the basic superheterodyne systemChoose suitable intermediate frequencies and calculate image rejection for receiverExplain the requirements for each stage in a receiver .
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AM Demodulation
AM demodulation is the reverse process of AM modulationAM demodulator converts the AM signal back to the original information signalTo achieve this, the receiver must be capable of receiving, amplifying and demodulating an AM signalLets look at a typical AM receiver block diagram
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AM Receiver
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AM ReceiverRF section: Often called receiver front end, includes the following operations:
detecting RF Signals, band limiting amplifying
Mixer: Down converts the RF frequencies to intermediate frequencies (IF). IF section: Includes amplification and selectivity functions.AM detector: Demodulates the AM wave and converts it to original information signal.
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Superheterodyne AM Receiver
Lo RF IFf f f= +
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AM Receiver Operation
Lo RF IFf f f= +
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AM Receiver Operation
455 Lo RF IF
Rf
f f Ff kHz
= +
= +
535 1605 kHz to kHz
995 2055 kHz to kHz
450 455 460
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AM Receiver OperationImage frequency
Image Frequency: is any frequency other than the selected carrier that, if allowed to enter receiver and mix with local carrier, will produce a cross product frequency equal to IF.
2image RF IFf f f= +
ImageLORFIFFrequency
fIFfIF
2fIF
ImageLORFIFFrequency
fIFfIF
2fIF
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AM Receiver OperationImage-frequency rejection ratio
Image rejection ratio is a measure of ability of preselector to reject the image frequency.Image rejection ratio is the ratio of voltage gain at input frequency at which receiver is tuned to the gain at image frequency.
2 21sig
image
image sig
sig image
AIR Q x
A
f fx
f f
= = +
= −
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AM Receiver OperationImage-frequency rejection ratio
For an AM broadcast-band superheterodyne receiver with IF, RF, and local oscillator frequencies of 455 kHz, 600 kHz, and 1055 kHz, refer to Figure and determine: Image frequency & IR (consider Q = 100)
2image RF IFf f f= +
2 21 211.320log(211.3)
46.5
IR Q x
dB
= + =
==
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AM Receiver OperationAutomatic Gain Control (AGC)
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SSB Receivers
With SSBSC or DSBSC a product detector can be used.Since DSBSC is seldom used by itself, it would be treated as SSB with one sideband removed in IF before detection.
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SSB Receivers
( ) ( )
( ) ( ){ }
( ) ( ){ }
( )
'
'
'
'
( ) ( ) sin 2 sin 2
( ) cos 2 cos 22
( ) cos cos 2 22
( ) cos ( )2
( )
m c c c c
c cm c c c c
c cm c
c co m
m
v t e t E f t E f t
E Ee t f f t f f t
E Ee t f t
E Ev t e t
e t
π π ϕ
π ϕ π ϕ
ϕ π ϕ
ϕ
α
= +
⎡ ⎤ ⎡ ⎤= − − − + +⎣ ⎦ ⎣ ⎦
⎡ ⎤= − + +⎣ ⎦
= −
=
( ) sin(2 )m c ce t E f tπ×
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Noncoherent SSB-BFO Receivers
Ganged tuning
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Noncoherent SSB-BFO Receivers
An SSBSC signal has a suppressed carrier frequency in the IF of a receiver, of 2 MHz. It is modulated with two audio tones having frequencies of 1500 Hz and 2200 Hz. The receiver is mistuned so that the BFO is at 1.9995 MHz. What will be the output frequencies of thedemodulator if the signal is LSB?
Ganged tuning
1.9978MHz 1.9985 MHz 2MHz
1.9995 MHz
1000 Hz 1700 Hzf
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Noncoherent SSB-BFO Receivers
For the BFO receiver shown in figure below, a received RF frequency band of 30 MHz to 30.005 MHz, an RF local oscillator frequency of 20 MHz, an IF frequency band of 10 MHz to 10.005 MHz, and a BFO frequency of 10 MHz, determine
Demodulated information frequency band.Demodulated information frequency band if the RF local oscillator frequency drifts down 0.001%
Ganged tuning
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Noncoherent SSB-BFO Receivers
Ganged tuning
30 MHz 30.005 MHz
20 MHz 10 MHz
0 5000 Hz
10 MHz 10.005 MHz
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Noncoherent SSB-BFO Receivers
Ganged tuning
30 MHz 30.005 MHz
fLO= 20 MHz – 200 Hz
= 19.9998 MHz
10 MHz
200 Hz 5200 Hz
10.0002 MHz 10.0052 MHz
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Coherent SSB-BFO Receiver
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Coherent SSB-BFO Receiver
For the coherent SSB BFO receiver, an RF reduced carrier frequency of 30 MHz with USB extends from 30 MHz to 30.005 MHz, an RF local oscillator of 20 MHz, an IF center frequency of 10 MHz, and BFO frequency of 10 MHz, determine:
Demodulated information signal,Demodulated information band if the reduced RF carrier drifted upwards 60 Hz producing RF carrier of 30.0006 MHz and USB extends to 30.00056 MHz.
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Coherent SSB-BFO Receiver
30 MHz 30.005MHz
20MHz
10 MHz 10.005MHz
10MHz
0 5 KHz
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Coherent SSB-BFO Receiver
30.0006 MHz 30.00056MHz
20.0004MHz
10.0002 MHz 10.0052MHz
10.0002 MHz
0 5 KHz
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Points to rememberReceiver must separate the desired signal from other signals and noise and then demodulate the signal.The most common receiver type is superheterodyne, which uses a mixer to transfer all incoming signals to a common IF.The RF amplifier is principal responsible for setting noise figure for receiver.Single-sideband receivers require a beat-frequency oscillator to reinsert the carrier and generally use product detector.
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