This document provides information on FM modulation and FM receivers. It discusses the key components of an FM receiver including the RF amplifier, mixer, local oscillator, IF amplifier, amplitude limiter, FM demodulator, de-emphasis circuit and audio amplifier. It then describes various FM demodulation techniques including the single tuned slope detector, balanced slope detector, Foster-Seeley discriminator, and ratio detector. For each technique, it provides the basic principles, circuit diagrams and operating characteristics. The document is intended to inform readers about the technology behind FM receivers and different methods for demodulating an FM signal.

1. FM Modulation
Group Coordinators :Group Coordinators :
 Rubab Shafique
 Barira Nashal
 Rimsha Arshad
 Warda Shadab
Hoor Al Ayn

2. Topics to be discussed:
 Block Diagram of FM Receiver
 Single Tuned Slope Detector
Balance Slope Detector OR Dual Slope Detector
 FM Detector OR Demodulator
 Discriminator
 Ratio Detector

3. FM Receiver:
The devices that are able to receive FM signals (i.e. of bandwidth
ranging from 88MHz to 108MHz) is called FM receiver.
Application: used in Radio broadcasting
Difference Between a AM Receiver and FM Receiver:
 Frequencies are much higher
 Totally different methods of Demodulation is used
 different methods for obtaining AGC is used.
 Totally different methods of demodulation is used
 Need for limiting and de-emphasis in FM receiver
is used

4. Block Diagram of FM Receiver
Speaker
RF
Amplifier
Mixer
Multistage
IF Amp
Local
Oscillator
Amplitude
Limiter
Audio
Amplifier
Antenna

5. Construction of FM Receiver:
RF Amplifier:
 same function as AM receiver
 reduce noise figure
 match input impedance of receiver with antenna
 raise signal level before it goes to mixer
 discriminate against the image signal
Frequency Mixer:
 mix the signal frequency voltage and local oscillator
voltage to produce difference frequency

6. Local Oscillator:
 takes the usual form of frequencies that resembles VHF
operation
IF Amplifier:
 multistage RF Amplifier
 operate at intermediate frequency
 provides a large gain
 staggered tuned single tuned circuits produce more gain
bandwidth

7. Amplitude Limiter:
 stage not used in AM receiver
 a class A overdriven amplifier
 limits IF voltage to a certain level
 clips off amplitude variations in signal caused by noise

FM Demodulator:
 extracts original frequency from frequency
modulated carrier voltage

8. De-emphasis Circuit:
 Pre-emphasis_ boosting of high frequencies at transmitter
 It compensate high frequency at receiver
Audio Amplifier:
 able to accommodate a large bandwidth
Loudspeaker:
 capable of reproducing all high frequencies tones up to
15KHz

9. FM DETECTORSFM DETECTORS
OROR
DEMODULATORSDEMODULATORS

10. FM Detectors OR Demodulators:
“An FM detector or demodulator is a circuit used to convert
the FM signal after modulation back to its original form.” i.e. in
the form of modulation voltage.
 Various techniques can be used to demodulate the FM signal,
 Single tuned or slope detector
 Dual slope detector or Balanced slope detector
 Discriminator
 Ratio detector

11. SINGLE TUNEDSINGLE TUNED
OROR
SLOPE DETECTORSLOPE DETECTOR

12. Single Tuned OR Slope
Detector:
 It consists of a tuned circuit that tuned to a frequency slightly
offset of the carrier signal.
 It is used to convert frequency variation to amplitude
variation.
 As the frequency varies up and down according to its
modulation , so the signal of the slope moves up and down of the
tuned circuit . This causes the amplitude variation.

13.  If fin > fc then circuit show inductive behaviour.
Voltage leads and current lags due to this phase shift
occur. As a result the frequency deviation is + fc and
the output is maximum.
 If fin < fc then circuit
show capacitive behaviour.
Voltage lags and current
leads due to this phase shift
occur. As a result the
frequency deviation is - fc
and the output is maximum.

14. Slope Detector OR Single
Tuned Circuit:
It has been observed from
circuit that .....
 Diode is used as
detector.
 This amplitude signal is
applied to a simple detector
circuit .
 Diode provides rectification.
 Where capacitor removes any unwanted
high frequency component.

15. Advantages:
 Simple - can be used to provide FM demodulation when only
an AM detector is present.
 Enables FM to be detected without any additional circuitry.
Disadvantages:
 Not linear as the output is dependent upon the curve of a filter.
Not linear as the output is dependent upon the curve of a filter.
Both frequency and amplitude variations are accepted and
therefore much higher levels of noise and interference
are experienced.

16. FM BALANCE SLOPEFM BALANCE SLOPE
DETECTORDETECTOR
OROR
DUAL SLOPEDUAL SLOPE
DETECTORDETECTOR

17. Introduction:
 A balanced slope detector is an improved version of slope
detector.
 It is used to over come the drawbacks of simple slope detector
, which is
a) Insufficient.
b) Improper linearity.
c) Difficult to arrange primary and secondary winding
tuned circuit.
These drawbacks are overcome by using balanced
slope detector.

18. Construction:
Three tuned circuits are used.
Two diodes are used.
Primary and secondary winding are center tapped with
output.
Two filters which are used to filter frequencies.
 R1 & C1
 R2 & C2

19. Operation:
Two outputs :
 Vo1
 Vo2
FM signal provided from input which is frequency modulated
signal.
Two frequencies produced from secondary winding.
 fc + f
 fc - f
Final output : Vo = Vo1 – Vo2
Where fc = center frequencyfc = center frequency
& f =deviation in frequencyf =deviation in frequency
Where fc = center frequencyfc = center frequency
& f =deviation in frequencyf =deviation in frequency
Change in two output voltages is
equal to total output voltage .
Change in two output voltages is
equal to total output voltage .

20. Circuit Diagram:

21. Conditions:
 1st
Condition: When fin=fcfin=fc
 2nd
Condition: When fin < fcfin < fc
 3rd
Condition: When fin > fcfin > fc

22. a)same voltages on T1 & T2.
b)both diode conduct on same
voltages.
c)after filtration output
( Vo1 & Vo2) also same.
Due to opposite polarity ,
they cancel each other.
Therefore , the output will
equal to zero
11stst
Condition: When fin=fc:Condition: When fin=fc:

23. Center frequency is greater than
input frequency.
Voltages drop across T1 is high.
D1 conducts more than D2.
Vo1 high (positive) because D1
produces larger output across
filter .
Vo2 is negative.
Then total output voltage (Vo) is
positive.
22ndnd
Condition: When fin < fcCondition: When fin < fc

24. 33rdrd
Condition: When fin > fcCondition: When fin > fc
 Center frequency is less than
input frequency.
 More voltages across T2.
 D2 conducts more than D1.
 Vo2 high (positive) because
D2 produces larger output
across filter .
 Vo1 will be negative.
 Total output voltage (Vo) is
negative.

25. S- shape Characteristics CurveS- shape Characteristics Curve

26. Advantages:
More efficient than simple slope detector.
Better linearity than simple slope detector.
Drawbacks:
It is difficult to tuned because three tuned circuit used
at different frequency.
Amplitude limiting is not provided

27. FOSTERFOSTER
SEELEYSEELEY
DISCRIMINATORDISCRIMINATOR

28. Foster Seeley Discriminator:
 Also called phase shift discriminator  operation very
similar to the balanced slope detector
 It uses a double-tuned RF transformer to convert frequency
variations in the received fm signal to amplitude variations.
 This type of circuit requires a transformer center tapped
secondary generally called the discriminator transformer, two
diodes and a no. of resistors and capacitors

29.  Because of the center-tap arrangement, the voltages at each
end of the secondary winding of T1 are 180 degrees out of phase
 The diodes conduct on opposite half cycles of the input
waveform and produce a RF ripple . This RF ripple is filtered
out by capacitors C3 and C4.

30. Circuit Diagram

31. Principle Circuit Operation:
1.Have 2 tank circuits :
i) Lp & Cp (primary side of T1)
ii) La, Lb & Co (secondary side of T1)
Conditions Of Circuit Driving:
 Operation at Resonance
 Operation above Resonance
 Operation below Resonance
Both tuned exactly at
resonant frequency

32. Operation at Resonance:
If input frequency (fin) same with resonant freq of the
secondary tank circuit
: fin = fo
:D1 and D2 both conduct
: VD1 & VD2 will have equal
voltages
: Vout = VC1 - VC2 = 0

33. Operation above Resonance:
 A phase shift occurs when an input frequency higher than the
center frequency is applied to the discriminator circuit
 The voltage developed across R3 is greater than the voltage
developed across R4; the output voltage is positive.
If : fin > fo
Therefore, (XL > XC),of tank circuit
:D1 conduct
: Vout = VC1 – VC2 = +ve value

34. Operation Below Resonance:
 When the input frequency is lower than the center
frequency,
 The capacitive reactance increases and the inductive
reactance decreases. 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.
If fin < fo
therefore, (XL < XC)
D2 conduct
VD1 < VD2
Vout = VC1 – VC2 = -ve value

35. Discriminator voltage-versus-frequency Response
Curve:
Figure shows a voltage-versus-frequency response curve for a Foster-Seeley
discriminator, called an S-curve

36. Drawback of Phase Discriminator:
In the presence of noise and other amplitude variation the
demodulator output produce error.
Amplitude limiting is not provide.
Advantages:
It is easy to tuned then the balance slope detector.
It is better linearity.

37. RATIO
DETECTOR

38. Ratio Detector:
 Ratio detector is a modified form of discriminator.
 The difference between ratio detector and discriminator
are :
 Reverse diode polarity
 A large vale of capacitor is used
 Output is taken from different position

39. Construction:
 Cc and C1 are coupling capacitors.
 L1 and L2 act as secondary of center tapped transformer.
 Output (differential) is taken from points A and B.

40. Working:
After few cycles C3 becomes charged to peak value of carrier.
C3 will remain charged and total voltage drop across R1 and R2
will remain constant.
At carrier frequency both diodes will conduct equally and output
will be zero.
If fin < fc output is negative.
If fin > fc output is positive.

41. Basic circuit diagram

42. S-curve

43. Advantages:
 Limiter is not required
Comparatively less components used
Excellent noise free output

44. Phase Locked Loop
 Best frequency demodulator.
 Frequency or phase sensitive feedback control
circuit.
 Basic components :
1.Phase detector
2.Voltage control oscillator
3.Low pass filter

45. Phase
locked loop
(PPL)

46. Phase
comparator
VCO
Low pass
filter
Input
output
Description:
 A PLL frequency demodulator requires no tuned circuits
 It automatically compensates for changes in the carrier frequency due to
instability in the transmit oscillator.

47. Working:
 The VCO natural frequency is equal to the IF
center frequency.
 Therefore, if the PLL input is a deviated FM
signal and the VCO natural frequency is equal to
the IF center frequency,
 The correction voltage produced at the output of
the phase comparator and fed back to the input of
the VCO is proportional to the frequency deviation
and is,
 Thus, the demodulated information signal

48. Thank
You