FM RECEIVER AND DETECTION
1
BASIC FM DEMODULATORS
 The FM discriminator (detector) extracts the intelligence that has
modulated onto the carrier via frequency variations.
 FM detection is two stage process:
 FM to AM conversion
 AM to AF conversion using diode detector
vFM t 
y(t)
vFM(t)
t
2
t
t
FOSTER-SEELEY DISCRIMINATOR
3
FOSTER-SEELEY DISCRIMINATOR
4
Circuit Operation at Resonance
5
Circuit Operation above Resonance
When a series- tuned circuit operates at a frequency above
resonance, the inductive reactance of the coil increases and the
capacitive reactance of the capacitor decreases
 Above resonance, the tank circuit acts like an inductor Secondary
current lags the primary tank voltage
The voltage developed across R1 is greater than the voltage
developed across R2, the output voltage is positive
6
Circuit Operation below Resonance
 Below resonance the tank acts like a capacitor and the
secondary current leads primary tank voltage
The voltage drop across R4 is larger than that across R3 and
the output across both is negative.
7
FOSTER-SEELEY DISCRIMINATOR
1. Calculation of VL3
2. Calculation of Secondary voltage V2 and Vab
3. Voltage across diodes D1 and D2
8
Case2 : fin>fc
MV 1 Xc2900
Vab 
L1 Z 2 
Vab 
MV 1 Xc 290  
L1 Z 2
Case3 : fin<fc
MV 1 Xc 2900
Vab 
L1 Z 2   
Vab 
MV 1 Xc 290  
L1 Z 2
Ratio Detector
 By making a few changes in the Foster-Seely discriminator, it is
possible to have a demodulator circuit which has built in capability to
handle the amplitude changes of the input FM signal
 This obviates the need for an amplitude limiter
 This resulting circuit is called the ratio detector
 The same vector diagram of Foster Seeley discriminator applies for
ratio detector
10
RATIO DETECTOR
Va' b'  Vo1  Vo2
Va' b'
Vo  Vb' o'Vb' o 
 V 02
2
Vo1  Vo2
Vo 
 V 02
2
1
1
Vo1  Vo2
Vo 
2
Block Diagram of FM Transmitter
with pre-emphasis
HETERODYNE METHOD OF FREQUENCY UP-CONVERSION
13
MULTIPLICATION METHOD OF UPCONVERSION
14
540KHz to 1640KHz
Ganged
tuning
Problems of Tuned Radio Frequency (TRF) Receiver
1.
Instability- Overall gain of RF amplifiers is very very high so a
very small f/b from o/p to i/p with correct phase can initiate
oscillations . Due to stray capacitance at high freq.
Variation in BW- For 535KHz – 1640KHz Range BW=10KHz
for fc=535 Q=fr/BW=535/10=53.5
for fc=1640 Q=164
But max value of Q is 120 so BW=fr/Q=1640/120=13.7K
So receiver picks adjacent channels.
2.
3. Insufficient Selectivity- Due to variable BW selectivity of TRF
receiver is poor.
Superheterodyne Receivers

Superheterodyne receivers convert all incoming
signals to a lower frequency, known as the intermediate
frequency (IF), at which a single set of amplifiers is used
to provide a fixed level of sensitivity and selectivity.

Gain and selectivity are obtained in the IF amplifiers.

The key circuit is the mixer, which acts like a simple
amplitude modulator to produce sum and difference
frequencies.

The incoming signal is mixed with a local oscillator signal.
Super heterodyne principle
f0
F0>fc
Fc
MIXER
f0+fc
f0-fc
If=f0-fc
AM Wave
If is Intermediate Freq
Fo
Oscillator
fIF is a fixed value (typically 455-kHz for AM radio).
fo is tuned
Superheterodyne Receiver
AGC = Automatic Gain Control
AGC samples the signal strength at the detector and then feeds back a
control signal to adjust the gain of the earlier stages
AGC keeps audio volume fairly constant if signals level vary
IF = LO - Input RF
540KHz to 1640KHz
455KHz
Image Frequency= The image frequency fi is a potentially interfering RF
signal that is spaced 2 times the IF above or below the desired frequency fs.
Which image that occurs depends upon whether the local oscillator
frequency fo is above or below the signal frequency. The mixing process
creates sum and difference frequencies for the desired signal (680 kHz). It
also creates sum and difference frequencies
for the undesired signal (1590 kHz).
1135- 680 =455 kHz
1590- 1135= 455 kHz
Fsi=fs+2IF
680
1135
1590
IF Amplifiers
The primary objective in the design of an IF stage is to obtain
good selectivity.
Choice of IF=
1. High IF – Poor selectivity and poor adjacent channel
rejection tracking problem increases.
2. Very Low If –Image frequency rejection is poor and sharp
selectivity cut the side bands.
3. If must not fall in tuning range of receiver.
So IF is selected as 455KHz.
Local Oscillator

What should be the frequency of the local oscillator used for
translation from RF to IF?
fLO = fc + fIF (up-conversion)
or
fLO = fc  fIF (down-conversion)
 Tuning ratio = fLO, max / fLO, min
 Up-Conversion: (1600 + 455) / (530+455) ≈ 2
 Down-Conversion: (1600–455) / (530–455) ≈ 12

Easier to design oscillator with small tuning ratio.
Block Diagram of FM Receiver
FM Receivers
A limiter is a non-linear circuit that compares the input to a certain threshold value.
The output indicates either comparison is true or false (i.e., binary results).
Typically the output is a saturated minimum or maximum value.
2
8
FM Capture effect
Capture effect
In telecommunication, the capture effect, or FM
capture effect, is a phenomenon, associated with FM
reception, in which only the stronger of two signals at
or near the same frequency will be demodulated
The capture effect is defined as the complete
suppression of the weaker signal occurs at the
receiver limiter, if it has one, where it is not amplified,
but attenuated. When both signals are nearly equal in
strength, or are fading independently, the receiver may
switch from one to the other.
30
540KHz to 1640KHz
Ganged
tuning
Problems of Tuned Radio Frequency (TRF) Receiver
1.
Instability- Overall gain of RF amplifiers is very very high so a very small f/b
from o/p to i/p with correct phase can initiate oscillations . Due to stray
capacitance at high freq.
Variation in BW- For 535KHz – 1640KHz Range BW=10KHz
for fc=535 Q=fr/BW=535/10=53.5
for fc=1640 Q=164
But max value of Q is 120 so BW=fr/Q=1640/120=13.7K
So receiver picks adjacent channels.
2.
3. Insufficient Selectivity- Due to variable BW selectivity of TRF receiver is poor.
Superheterodyne Receivers

Superheterodyne receivers convert all incoming signals to a lower
frequency, known as the intermediate frequency (IF), at which a single
set of amplifiers is used to provide a fixed level of sensitivity and selectivity.

Gain and selectivity are obtained in the IF amplifiers.

The key circuit is the mixer, which acts like a simple amplitude modulator to
produce sum and difference frequencies.

The incoming signal is mixed with a local oscillator signal.
Super heterodyne principle
F0>fc
Fc
MIXER
f0
f0+fc
f0-fc
AM Wave
If=f0-fc
Fo
If is Intermediate Freq
Oscillator
fIF is a fixed value (typically 455-kHz for AM radio).
fo is
Superheterodyne Receiver
AGC = Automatic Gain Control
AGC samples the signal strength at the detector and then feeds back a
control signal to adjust the gain of the earlier stages
AGC keeps audio volume fairly constant if signals level vary
IF = LO - Input RF
540KHz to 1640KHz
455KHz
Image Frequency= The image frequency fi is a potentially interfering RF
signal that is spaced 2 times the IF above or below the desired frequency fs.
Which image that occurs depends upon whether the local oscillator
frequency fo is above or below the signal frequency. The mixing process
creates sum and difference frequencies for the desired signal (680 kHz). It
also creates sum and difference frequencies
for the undesired signal (1590 kHz).
1135- 680 =455 kHz
1590- 1135= 455 kHz
Fsi=fs+2IF
680
1135
1590
Image Frequency rejection Ratio
Gainatsignalfreq
IFRR 
 1  Q2  2
Gainatimagefreq
Where
Q= Loaded Q of the tuned ckt.

fsi
fs

fs
fsi
If two tuned ckts are there then
IFRR  1* 2
Double spotting= Same station gets picked up at two diff.
nearby points, on the receiver dial.
This can be reduced by increasing front end selectivity.
Fo1=1955K
Fo2=1045K
Radio Dial
590K
2*IF
1500K
Tracking= In tuning process the Local oscillator freq tracks the station
freq to have a correct diff of IF(455KHz).
If diff freq is not correct the error is called as Tracking error.
3 Point Tracking=
Padder tracking
950
1500
Trimmer
tracking
The Cp in series with Cosc decreases total capacitance and increases
Freq. in +ve direction making tracking error zero.
The Ct in parallel with Cosc increases total capacitance and decreases
Freq. in -ve direction
Diode Detector
Two distortions of envelope Detector
1. When RC time constant of load is too long.
1
1
 RC 
fc
fm
Modulation index at the o/p side of detector is higher than i/p side.
md  m *
Rc
Zm
Zm=Diode load impedance at
audio freq
Rc=Dc diode load resistance
md max  1
Zm
m max 
Rc
Practical Diode Detector
1.
Negative envelope will be demodulated and a –ve AGC will developed.
2.
R1-C1 LPF removes RF ripple.
3.
C2 prevents dc o/p to reach to volume control.
4.
R3-C3 LPF removes AF from demodulated o/p.
Types of AGC used in receivers
1.
Simple AGC- Receiver gain is
automatically adjusted by –ve AGC.
Gain is reduced for weak signals also.
2.
Ideal AGC- Gain is not reduced for
weak signals only for very strong
signals. Then O/p is constant.
3.
Delayed AGC- AGC bias is provided
after a level of signal. Used in High
quality receivers.