RECEIVERS
Presented By :Er. Srishtee Chaudhary
Lecturer E.C.E
GPCG,Patiala
REVIEW ( Last Lecture )
 TRF Receiver
 TRF Receiver drawbacks
 Instability
 Variation in BW
 Poor selectivity
 Super-heterodyne Receivers
 Receiver Characteristics
 Selectivity
 Senstivity
 Fidelity
CONTENTS
 FM Receiver
 AGC
 Communication Receiver
FM RECEIVERS
 FM receiver also operates on super-heterodyning
principle.
 Operating frequencies in FM are much higher than
AM.
 FM receivers need circuits like limiter and deemphasis.
 FM demodulators are different from AM detectors.
RF
amplifier
Mixer
IF
amplifier
Limiter
FM
detector
 R F Amplifier : Improves signal to noise ratio
 Matches receiver input impedance to antenna
impedance.
 Mixer : Similar to AM receiver, down convert received
signal to IF
 Tuning range is less than that in AM
broadcasting (88MHz to 108 MHz)
 IF produced at output of mixer is 10.7MHz.
 I F Amplifier : In case of FM receivers IF and bandwidth
required are much higher.
 IF is 10.7MHz
 Bandwidth is 200kHz
 this large BW will provide low gain per stage.
 So more IF amplifiers are required (and are
cascaded).
 Amplitude limiter stage :-
 Transmitted FM wave has constant amplitude,
but unwanted noise and signal added to it change
its amplitude.
 This has to be removed before demodulation
process otherwise distortion appears in
demodulated signal.
 As demodulators react to amplitude changes as
well as frequency changes.
 Limiter removes all unwanted amplitude
variations
 De-emphasis : Artificial boosting given to higher modulating
frequencies in process of pre-emphasis is
nullified
 De-emphasis circuit is used after FM
demodulator
AM RECEIVERS Vs FM RECEIVERS
AM Receiver
FM Receiver
AM: 540 kHz – 1600 kHz
FM: 88 MHz – 108 MHz
AM detector basically envelope
detector
FM demodulator basically a frequency
to amplitude converter
No limiter and de-emphasis stage
limiter and de-emphasis stage needed
IF 455 kHz
IF 10.7MHz
For AM radio, each station occupies a
maximum bandwidth of 10 kHz
For FM radio, each station occupies a
maximum bandwidth of 200 kHz
AM: Bt = 2W
FM: Bt = 2(D + 1)W
(Carson’s Rule)
Input signal is AM wave
Input Signal is FM wave
AUTOMATIC GAIN CONTROL
 Signals receiving at receiver input are not of same
strength
 Signals of strong station are strong and from weak
stations are weak
 If receiver gain constant then receiver o/p will
fluctuate proportional to i/p signal strength which
is not desired
 So AGC adjust receiver gain automatically to have
constant o/p irrespective of i/p strength
Types of AGC
 Simple AGC
 Delayed AGC
 Ideal AGC
 No AGC
SIMPLE AGC
 It
will change overall gain of a receiver
automatically
 Thus keep receiver o/p constant even when i/p
signal strength changes
 Receiver gain is automatically reduced as i/p signal
becomes stronger
 Advantages
 Simplicity
 Low cost
 Disadvantages
 Weak signals are also attenuated
 Applications
 Low cost domestic radio receivers
IDEAL AGC
 For the portion ‘OA’ i.e for weak signal no AGC is
applied
 After point ‘A’ AGC is applied, keeping receiver
o/p constant
 Thus there is no gain reduction for weak signals
DELAYED AGC
 In this case AGC bias is not applied until i/p signal
strength reaches a predetermined level point B
 After this point is reached AGC bias is applied like
simple AGC but more strongly
 Thus reducing receiver gain for weak signals is
avoided
Operation
 Here an adjustable positive bias is applied to
cathode of AGC diode
 Anode of diode is connected to o/p of last IF
amplifier through coupling capacitor Cc
 R3C3 filter, filters out high frequency appearing at
anode of AGC diode
 When i/p signal is weak, then anode of AGC diode
is at less potential than cathode. So it is off
 So AGC o/p is zero
 When i/p signal is strong then AGC diode is




forward biased
AGC o/p will be constant (Vb + Vd)
Thus delayed AGC reduces gain for strong signal
and not for weak signals
We can say characteristics of delayed AGC is
similar to ideal AGC
But here we can adjust point B ,by adjusting delay
adjust potentiometer
 Advantages
 Weak signals are not attenuated
 Disadvantages
 Complex than simple AGC
 Applications
 High quality communication receivers
COMMUNICATION RECEIVER
Reason for using double radio receiver
 When choosing the intermediate frequency for a
superheterodyne radio receiver there is a trade-off
to be made between the advantages of using a low
frequency IF or a high frequency one
 High frequency IF: The use of a high frequency
IF means that the difference between the wanted
frequency and the unwanted image is much greater
and it is easier to achieve high levels of
performance because the front end filtering is able
to provide high levels of rejection.
 Low frequency IF:
The advantage of choosing a
lower frequency IF is that the filters that provide
the adjacent channel rejection are lower in
frequency. The use of a low frequency IF enables
the performance to be high, while keeping the cost
low.
 Accordingly there are two conflicting requirements
which cannot be easily satisfied using a single
intermediate frequency. The solution is to use a
double conversion superheterodyne topology to
provide a means of satisfying both requirements
 For good image rejection, relatively high IF is
desired. However, for a high gain selective
amplifiers that are stable, a low IF is necessary.
 The solution for above constrain is to use 2
intermediate frequencies, i.e. by using double
conversion AM receiver.
 The 1st IF is a relatively high frequency for good
image rejection.
 The 2nd IF is a relatively low frequency for good
selectivity and easy amplification.
WHY NAMED SO?
 RF signal is down-converted into two IF
 Higher IF and Lower IF
 Therefore named Double conversion receiver
INTERMEDIATE FREQUENCIES
 1st IF = f0 – fs
 1st IF = [(3.7-17.7)]MHz - [(2-16)]MHz
 1st IF = 1.7 MHz
 2nd IF = f0 – fs
 2nd IF = (1.7)MHz - (1.5)MHz
 2nd IF = 200 KHz
OTHER BLOCKS
 Delayed AGC
 BFO ( Beat Frequency oscillator )
 Squelch Circuit ( Muting Circuit )
DELAYED AGC
 The
disadvantage of automatic gain control,
attenuating even the weak signal, is overcome by the
use of delayed automatic gain control .
 This type of system develops no AGC feedback until
an established received signal strength is attained.
 For signals weaker than this value, no AGC is
developed. For sufficiently strong signals, the delayed
AGC circuit operates essentially the same as ordinary
AGC.
 The circuit uses two separate diodes; one is the
detector diode and the other the AGC diode.
 The AGC diode is connected to the primary of the
last IF transformer and the detector diode to its
secondary.
 A positive bias is applied to the cathode of the
AGC diode. This keeps it from conducting until a
prearranged signal level has been reached.
 The adjust delay control allows manual control of
the AGC diode bias. Manual control allows you to
select the signal level at which AGC is applied.
BEAT FREQUENCY OSCILLATOR
 The beat-frequency oscillator (BFO) is necessary
when you want to receive CW signals.
 The action of the RF amplifier, mixer, local
oscillator, and IF amplifier is the same for both CW
and AM; but the CW signal reaches the detector as a
single frequency signal with no sideband
components.
 To produce an AF output, you must heterodyne
(beat) any CW signal with an RF signal of the proper
frequency. This separate signal is obtained from an
oscillator known as a beat-frequency oscillator.
 If the intermediate frequency is 455 kilohertz and
the BFO is tuned to 456 kilohertz or 454 kilohertz,
the difference frequency of 1 kilohertz is heard in
the output. Generally, you will tune the BFO from
the front panel of a receiver.
 When you vary the BFO control, you are varying
the output frequency of the BFO and will hear
changes in the tone of the output audio signal.
 The sensitivity of a receiver is maximum when no
signal is being received.
 This condition occurs, for example, when a
receiver is being tuned between stations. At this
time background noise is picked up by the antenna,
and you will hear noise greatly amplified.
 This noise is highly annoying and occurs because
receiver gain is maximum without a signal.
 You can often overcome this problem by using a
circuit called a SQUELCH, NOISE SILENCER,
NOISE SUPPRESSOR, or NOISE LIMITER. All
of these noise type circuits just clip the peaks of
the noise spikes. Squelch will actually eliminate
noise.
SQUELCH CIRCUIT
 A squelch circuit is a circuit used to turn off the
audio of a receiver amplifier when no RF signal is
being received.
 Without a squelch circuit, such absence of
received signal will be heard on the receiver as an
annoying background noise.
 In Figure , the automatic gain control (AGC)
circuit, which is used to adjust the gain of the
receiver based on the strength of the received
signal, outputs a DC voltage that is proportional to
the received signal's amplitude.
 When carrier is absent i.e there is no signal present




at i/p, receiver produces loud noise
This is due to delayed AGC that disappears in
absence of i/p signal, increasing gain of IF and RF
amplifiers
These amplifiers amplifies noise present at their
i/p’s
Muting circuit avoid this type of condition
In absence of i/p signal AGC will be zero and
squelch circuit will cut-off 1st audio amplifier so
tha no noise can pass through loud speaker
 Thus, in the absence of a received signal, the output of
the AGC's DC amplifier is a very low DC voltage
that's fed into the base of Q1. This low base voltage
causes Q1 to turn off, resulting in the base of Q2
being pulled up 'high' through R1. This turns on
Q2.
 When there's a received signal, Q1 gets a high base
voltage from the AGC amplifier, turning it on. Q2's
base is pulled 'low' by the conducting Q1, causing
Q2 to turn off. With Q2 'off', the audio signal from
Q3's collector is readily passed on to the audio power
amplifier.
Features
 Fine tuning
 Variable senstivity
 Variable selectivity
 Noise limiting
 Better image frequency rejection
 Better adjacent channel rejection
TRF RECEIVER
SUPERHETERODYNE
COMMUNICATION
RXR
No frequency conversion
Single Frequency
conversion
Double Frequency
conversion
No IF frequency
Downconvert RF signal to
lower IF frequency
Downconvert RF signal to
two IF frequency(higher
than lower)
Instability , variation in
BW and poor selectivity
due to high frequencies
No instability, variation in
BW and poor selectivity as
IF introduced.
No instability, variation in
BW and poor selectivity as IF
introduced.
Difficult to design tunable
RF stages.
Main amplifixcation takes
place at IF
Main amplifixcation takes
place at IF stages
Rarely used
Mostly used
Mostly used
No AGC
AGC introduced
AGC,BFO an Squelch circuit
THANKS