1. FM DEMODULATORS

2. Contents Review of Modulation What is demodulation
Frequency Demodulation Definition
Types of FM Demodulators
Study of Various FM Demodulators (Slope, Balanced, Foster-Seeley, Ratio Detectors and Phase Locked Loop)

3. What is Modulation
Modulation is the process of changing characteristics of carrier w.r.t message signal.
Modulator
Modulated Signal
Carrier Wave
Information

4. Types of Modulation Common modulation methods include:
AM :- in which the Amplitude of the carrier is varied w.r.t message signal.
FM;- in which the frequency of the carrier is varied w.r.t message signal.
PM:- in which the phase of the carrier is varied w.r.t message signal.

5. Amplitude Modulation Example
Transmitted Signal
Modulating Signal

6. Frequency Modulation Example
Transmitted Signal
Modulating Signal

7. What is a Demodulator?
Demodulation is the process of extracting the original information-bearing signal (modulating signal) from a modulated carrier wave.
A demodulator is an electronic circuit used to recover the information content from the modulated carrier wave.

8. What is FM Demodulator
An electronic circuit in which frequency variations of modulated signals are converted to amplitude variations first, with the help of tuned circuit( Discriminator).
And then original information is extracted with the AM demodulation techniques say diode detector.

9. Types of FM Demodulators
FM Demodulation
Direct
Indirect
Phase Lock Loop(PLL)
Slope Detector
Balanced Slope Detector
Foster-Seeley Phase Discriminator
Ratio Detector

10. Basic FM Demodulator TUNED CIRUIT
Frequency Variations
Amplitude Variations
NOTE: Amplitude Variations are added to wave according to frequency variations, and frequency variations remain present in incomingwave.

11. Basic FM Demodulator
The function of FM demodulator is to change the frequency deviation of the incoming carrier into an AF amplitude variation.
The detection circuit should be insensitive to amplitude changes.

12. Basic FM Demodulator
This type of circuit converts the FM IF voltage of constant amplitude into a voltage, that is from FM to AM.
The later is applied to a detector which reacts to amplitude changes and ignores frequency changes.

13. Basic FM Demodulator
FM Wave
Output of Tuned Circuit

14. Basic FM Demodulator
The most basic circuit employed as FM demodulator is parallel tuned LC circuit, often known as slope detector.
The carrier frequency should fall on one side of resonant frequency and
The entire frequencies should fall on linear region of transfer curve of tuned circuit.

15. SLOPE Detector
Detector
Output FM
FM Source
Tank Circuit

16. Slope Detector Transfer Characteristics
Transfer Curve
Output
Voltage
Voltage t f
Slope Detector Transfer Characteristics
Input t

17. Slope Detector Transfer Characteristics
Transfer Curve
Output
Voltage
Voltage fc t f f
fc+f
Slope Detector Transfer Characteristics
fc-f
Input t

18. SLOPE Detector
The output is then applied to a diode detector with RC load of suitable time constant.
The circuit is, in fact, identical to that of AM detector.

19. Limitations of Slope Detector
It is inefficient, as it is linear in very limited frequency range.
It reacts to all amplitude changes(input FM signal).
It is relatively difficult to tune, as tuned circuit must be tuned to different frequency than carrier frequency.

20. Balanced Slope Detector
This circuit uses two slope detectors, connected in back to back fashion, to opposite ends of center-tapped transformer.
And hence fed 1800 out of phase.

21. Balanced Slope Detector
The top secondary circuit is tuned above the IF(carrier frequency)by an amount f, and bottom circuit is tuned below IF by f.
Each circuit is connected to diode detectors with suitable RC loads.
The output is taken across series combination of loads, so that it is sum of the individual outputs.

22. Balanced Slope Detector
FMIN fc
fc+f
fc-f T’
T’’ D2 Vo + - R1 C1
Vo1 R2 C2
Vo2

23. Balanced Slope Detector
When input frequency = fc
Then output of T’(+Ve)= output of T’’ (-Ve)
Vo= Zero
When input frequency = fc+f
Then output of T’(+Ve) > output of T’’ (-Ve)
Vo= +Ve
When input frequency = fc-f
Then output of T’(+Ve) < output of T’’ (-Ve)
So sum of outputs of T’ and T’’ (Vo) = -Ve

24. Transfer Curve of Balanced slope DeterVo
Useful Range
fc-f fc
fc+f

25. Balanced Slope Detector-Drawbacks
Even more difficult to tune, as there are three different frequencies to be tuned.
Amplitude limiting still not provided.
Linearity, although better than single slope detector, is still not good enough.

26. Foster-seley Phase Discriminator
In this all the tuned circuits are tuned to the same frequency.
Balanced Slope Detector circuit with some changes is used.
This circuit yields far better linearity than slope detection.

27. Foster-seley Phase Discriminator
VIN D2
Va’b’ + - L3 R3 R4 C3 C4 L2 L1 a b a’ b’ o C1 C
As C & C4 are coupling & RF Bypass capacitors respectively, therefore VL3 VIN So
Voltage across diode= VIN + Secondary voltage/2

28. Foster-seley Phase Discriminator
Now when Transformer voltage is induced in the secondary as a result of current in primary.
And
Where X2= XL2-XC2

29. Foster-seley Phase Discriminator
At resonance i.e. when input frequency is fc, X2=0
i.e. Vab leads VIN by 900.

30. Foster-seley Phase Discriminator
And from the phasor diagram given below :
That as Vao=Vbo, hence discriminator output is zero.
Vao
Vbo

31. Foster-seley Phase Discriminator
When input frequency is greater than fc, then XL2>XC2 & hence X2 is positive.
That is Vab leads VIN by less than 900.

32. Foster-seley Phase Discriminator
And from the phasor diagram given below :
That as V ao> Vbo, hence discriminator output is positive.
Vao
Vbo

33. Foster-seley Phase Discriminator
When input frequency is less than fc, then XL2<XC2 & hence X2 is negative.
That is Vab leads VIN by more than 900.

34. Foster-seley Phase Discriminator
And from the phasor diagram given below :
That as Vao<Vbo, hence discriminator output is negative.
Vao
Vbo

35. Foster-seley Phase DiscriminatorVo
Useful Range extends up to half-power points of tuned transformer.
Useful Range fc
Beyond which o/p falls due to frequency response of transformer.

36. Foster-seley Phase Discriminator
It is much easier to align, as there are now two tuned circuits and both are tuned to the same frequency.
Linearity is quite better, as circuit relies less on frequency & more on primary-secondary phase relation, which is quite linear.
Only drawback is, there is no provision for amplitude limiting.

37. Ratio- Detector
Ratio detector demodulator is modified Foster-Seeley circuit in order to incorporate amplitude limiting.
In Foster-Seeley discriminator that sum of voltages Vao+Vbo Should remain constant,
and their difference should vary due to variation in input frequency.

38. Ratio- Detector
But practically speaking any variation in the amplitude of input signal, also has impact on sum of Vao+Vbo, leading to distortion.
Ratio-detector circuit eliminates this variation of Vao+Vbo, and performs the function of amplitude limiter also.

39. Ratio- Detector Three changes are made in Foster-Seeley discriminator:
One of the diodes has been reversed.
A large capacitor has been placed between points, from where output was taken.
Output now is taken from elsewhere.

40. SUM Ratio- Detector D1 VIN D2 Vo + - L3 R3 R4 C3 C4 L2 L1 a b a’ b’ o
Change 1: Diode D2 is reversed so that now sum of Vao & Vbo appears across points a’ and b’ instead of difference.

41. Ratio- Detector + Vo1 - Vo Vo2
FM input D2 Vo + - L3 R3 R4 C3 C4 Ls LP
Vo1
Vo2 CP C C5 R5 R6 V
Change 2: A capacitor C5 with large time constant is connected across a’-b’ in order to keep Vao+Vbo constant.

42. Ratio- Detector D1 VIN D2 Vo + - L3 R3 R4 C3 C4 L2 L1 a b a’ b’ o C1 C
Change 3: Output is taken from o-o’ as the difference of Vao + Vbo appears there. Ground is shifted to O’.

43. Operation at Resonance
No phase shift occurs at resonance and both Vao & Vbo are equal. Hence their difference (output) is zero.
During negative part of cycle of input signal, polarity across secondary also changes and both diodes get reverse biased.
But C5 with large time constant maintains voltage at constant level.

44. Operation Above Resonance
When a tuned circuit operates at a frequency higher than resonance, the tank is inductive.
Secondary voltage V1 is nearer in phase with primary voltage, while V2 is shifted further out of phase with primary.

45. Operation Above Resonance
So output voltage in this case will be positive as shown in vector diagram:
Vao
Vbo
Output

46. Operation Below Resonance
When a tuned circuit operates below resonance, it is capacitive. Secondary current leads the primary voltage and
secondary voltage V2 is nearer in phase with primary voltage and voltage V1 is shifted away in phase from primary voltage

47. Operation Below Resonance
So the output in this case will be negative.
Vao
Vbo
Output

48. Ratio-Detector Advantages
Amplitude limiting is possible.
Linearity is quite good as compared to others. So quite often used in high quality receivers.

49. Ratio-Detector Disadvantages
Under critical noise conditions, such as satellite receivers, where demodulator noise performance becomes very significant, even this demodulator is found inefficient.
Under these conditions more advanced demodulators such as Phase Locked Loop are used.

50. Phase Locked Loop (PLL)
It is the best frequency demodulator.
A phase-locked loop (PLL) is an electronic circuit with a voltage- or current-driven oscillator that is constantly adjusted to match in phase (and thus lock on) with the frequency of an input signal.

51. Phase Locked Loop
A basic phase Locked Loop consists of Three components:
Phase discriminator: compares phase of two signals and generates a voltages according to phase difference of two signals.

52. Phase Locked Loop
Loop Filter: A low pass filter to filter the output of phase discriminator.
Voltage controlled Oscillator(VCO): generates RF signals whose frequency depends upon voltage generated by phase discriminator.

53. Phase Locked Loop
adjusts the VCO frequency in an attempt to correct for the original frequency or phase difference.
compare the two input signals and generate an output signal that, when filtered, will control the VCO.

54. Phase Locked Loop
As incoming frequency changes, The phase discriminator generates a voltage to control the frequency and phase of VCO.
This control voltage varies at the same rate as the frequency of the incoming signal.

55. Phase Locked Loop Control Voltage  rate of input freq change
Hence this signal can be directly used as output.
PLL must have low time constant so that it can follow modulating signal.

56. Phase Locked Loop
Free running frequency of VCO is set equal to the carrier frequency of the FM wave.
The lock range must be at least twice the maximum deviation of the signal.

57. Phase Locked Loop
Linearity is governed by voltage to frequency characteristics of VCO.
As it swings over small portion of its bandwidth, the characteristic can be made relatively linear.
Hence the distortion levels of PLL demodulators are normally very low.

58. Transfer Curve
Output
Voltage f
THANKS
Input t

59. FM DEMODULATORS By
P.Lakshmi Prasanna