1. Punjab EDUSAT Society (PES) Contact info: email: kulwinderpannu@gmail.comkulwinderpannu@gmail.com Phone: 97813-00151 03 Oct, 20121

2. What We Shall Cover Review of Modulation (with animations) What is demodulation Frequency Demodulation Definition Types of FM Demodulators Study of Various FM Demodulators (Slope, Balanced, Foster-Seeley, Ratio and Phase Locked Loop) Punjab EDUSAT Society (PES) 03 Oct, 20122

3. What is Modulation Modulation is the addition of information (or the signal) to an electronic or optical signal carrier. Modulator Carrier Wave Information Modulated Signal Punjab EDUSAT Society (PES) 03 Oct, 20123

4. Types of Modulation Common modulation methods include: Amplitude modulation (AM), in which the voltage applied to the carrier is varied over time Frequency modulation (FM), in which the frequency of the carrier waveform is varied in small but meaningful amounts Phase modulation (PM), in which the natural flow of the alternating current waveform is delayed temporarily Punjab EDUSAT Society (PES) 03 Oct, 20124

5. Amplitude Modulation Example Modulating Signal Transmitted Signal Punjab EDUSAT Society (PES) 03 Oct, 20125

6. Frequency Modulation Example Transmitted Signal Modulating Signal Punjab EDUSAT Society (PES) 03 Oct, 20126

7. What is a Demodulator? Demodulation is the act 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. Punjab EDUSAT Society (PES) 03 Oct, 20127

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 And then original information is extracted with the AM demodulation techniques say diode detector. Punjab EDUSAT Society (PES) 03 Oct, 20128

9. Punjab EDUSAT Society (PES) Types of FM Demodulators FM Demodulation IndirectDirect Slope Detector Balanced Slope Detector Foster-Seeley Phase Discriminator Ratio Detector Phase Lock Loop(PLL) 03 Oct, 20129

10. Basic FM Demodulator Punjab EDUSAT Society (PES) TUNED CIRUIT Frequency Variations Amplitude Variations NOTE: Amplitude Variations are added to wave according to frequency variations, and frequency variations remain present in wave. 03 Oct, 201210

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. Punjab EDUSAT Society (PES) 03 Oct, 201211

12. Basic FM Demodulator This type of circuit converts the FM IF voltage of constant amplitude into a voltage that is both FM and AM. The later is applied to a detector which reacts to amplitude changes and ignores frequency changes. Punjab EDUSAT Society (PES) 03 Oct, 201212

13. Punjab EDUSAT Society (PES) FM Wave Output of Tuned Circuit Basic FM Demodulator 03 Oct, 201213

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. Punjab EDUSAT Society (PES) 03 Oct, 201214

15. FM Source Tank Circuit Detector Output FM SLOPE Detector Punjab EDUSAT Society (PES) 03 Oct, 201215

16. Transfer Curve Output Slope Detector Transfer Characteristics Punjab EDUSAT Society (PES) 03 Oct, 201216

17. Transfer Curve Output Slope Detector Transfer Characteristics Punjab EDUSAT Society (PES) 03 Oct, 2012 Punjab EDUSAT Society (PES) 17

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. Punjab EDUSAT Society (PES) 03 Oct, 201218

19. Limitations of Slope Detector It is inefficient, as it is linear in very limited frequency range. It reacts to all amplitude changes. It is relatively difficult to tune, as tuned circuit must be tuned to different frequency than carrier frequency. Punjab EDUSAT Society (PES) 03 Oct, 201219

20. Punjab EDUSAT Society (PES) 03 Oct, 201220

21. Punjab EDUSAT Society (PES) 03 Oct, 201221

22. 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 180 0 out of phase. Punjab EDUSAT Society (PES) 03 Oct, 201222

23. Balanced Slope Detector The top secondary circuit is tuned above the IF 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. Punjab EDUSAT Society (PES) 03 Oct, 201223

24. Balanced Slope Detector Punjab EDUSAT Society (PES) 03 Oct, 201224

25. Balanced Slope Detector Punjab EDUSAT Society (PES) Output of T’ at f c +  f (+10 to +15V) Output of T’ at f c -  f (+5 to +10V) Output of T’ at f c -  f (-10 to -15V) Output of T’’ at f c +  f (-5 to -10V) 5V 10V 15V -5V -10V -15V 03 Oct, 201225

26. Combined Transfer Curve Punjab EDUSAT Society (PES) Useful Range 03 Oct, 201226

27. Balanced Slope Detector When input frequency = f c – Then output of T’(+Ve)= output of T’’ (-Ve) – So sum of outputs of T’ and T’’ = Zero When input frequency = f c +  f – Then output of T’(+Ve) > output of T’’ (-Ve) – So sum of outputs of T’ and T’’ = +Ve When input frequency = f c -  f – Then output of T’(+Ve) < output of T’’ (-Ve) – So sum of outputs of T’ and T’’ = -Ve Punjab EDUSAT Society (PES) 03 Oct, 201227

28. Balance 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. Punjab EDUSAT Society (PES) 03 Oct, 201228

29. Foster-Seeley (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. Punjab EDUSAT Society (PES) 03 Oct, 201229

30. Punjab EDUSAT Society (PES) As C & C4 are coupling & RF Bypass capacitors respectively, therefore V L3  V IN So Voltage across diode= V IN + Secondary voltage/2 Phase Discriminator 03 Oct, 201230

31. Phase Discriminator Now in Transformer voltage is induced in the secondary as a result of current in primary. And Where X 2 = X L2 -X C2 Punjab EDUSAT Society (PES) 03 Oct, 201231

32. Phase Discriminator At resonance i.e. when input frequency is f c, X 2 =0 i.e. V ab leads V IN by 90 0. Punjab EDUSAT Society (PES) 03 Oct, 201232

33. Phase Discriminator And from the phasor diagram given below : That as V ao =V bo, hence discriminator output is zero. Punjab EDUSAT Society (PES) V ao V bo 03 Oct, 201233

34. Phase Discriminator When input frequency is greater than f c, then X L2 >X C2 & hence X 2 is positive. That is V ab leads V IN by less than 90 0. Punjab EDUSAT Society (PES) 03 Oct, 201234

35. Punjab EDUSAT Society (PES) Phase Discriminator And from the phasor diagram given below : That as V ao >V bo, hence discriminator output is positive. V ao V bo 03 Oct, 201235

36. Phase Discriminator When input frequency is less than f c, then X L2 <X C2 & hence X 2 is negative. That is V ab leads V IN by more than 90 0. Punjab EDUSAT Society (PES) 03 Oct, 201236

37. Punjab EDUSAT Society (PES) Phase Discriminator And from the phasor diagram given below : That as V ao <V bo, hence discriminator output is negative. V ao V bo 03 Oct, 201237

38. Phase Discriminator Punjab EDUSAT Society (PES) Useful Range Beyond which o/p falls due to frequency response of transformer. Useful Range extends upto half-power points of tuned transformer. 03 Oct, 201238

39. 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. Punjab EDUSAT Society (PES) 03 Oct, 201239

40. Ratio- Detector Ratio detector demodulator is modified Foster-Seeley circuit in order to incorporate amplitude limiting. In Foster-Seeley discriminator that sum of voltages V ao +V bo Should remain constant, and their difference should vary due to variation in input frequency. Punjab EDUSAT Society (PES) 03 Oct, 201240

41. Ratio-detector But practically speaking any variation in the amplitude of input signal, also has impact on sum of V ao +V bo, leading to distortion. Ratio-detector circuit eliminates this variation of V ao +V bo, and performs the function of amplitude limiter also. Punjab EDUSAT Society (PES) 03 Oct, 201241

42. 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. Punjab EDUSAT Society (PES) 03 Oct, 201242

43. Ratio-Detector Punjab EDUSAT Society (PES) Change 1: Diode D2 is reversed so that now sum of V ao & V bo appears across points a’ and b’ instead of difference. 03 Oct, 201243

44. Ratio-Detector Punjab EDUSAT Society (PES) Change 2: A capacitor C 5 with large time constant is connected across a’-b’ in order to keep V ao +V bo constant. 03 Oct, 201244

45. Ratio-Detector Punjab EDUSAT Society (PES) Change 3: Output is taken from o-o’ as the difference of V ao + V bo appears there. Ground is shifted to O’. 03 Oct, 201245

46. Operation at Resonance No phase shift occurs at resonance and both V ao & V bo 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 C 5 with large time constant maintains voltage at constant level. Punjab EDUSAT Society (PES) 03 Oct, 201246

47. Operation Above Resonance When a tuned circuit operates at a frequency higher than resonance, the tank is inductive. Secondary voltage V 1 is nearer in phase with primary voltage, while V 2 is shifted further out of phase with primary. Punjab EDUSAT Society (PES) 03 Oct, 201247

48. Operation Above Resonance So output voltage in this case will be positive as shown in vector diagram: Punjab EDUSAT Society (PES) V ao V bo Output 03 Oct, 201248

49. Operation Below Resonance When a tuned circuit operates below resonance, it is capacitive. Secondary current leads the primary voltage and secondary voltage V 2 is nearer in phase with primary voltage and voltage V 1 is shifted away in phase from primary voltage Punjab EDUSAT Society (PES) 03 Oct, 201249

50. Operation Below Resonance So the output in this case will be negative. Punjab EDUSAT Society (PES) V ao V bo Output 03 Oct, 201250

51. Ratio-Detector Advantages Amplitude limiting is possible. Linearity is quite good as compared to others. So quite often used in high quality receivers. Punjab EDUSAT Society (PES) 03 Oct, 201251

52. Ratio-Detector Dis-advantages Under critical noise conditions, such as satellite receivers, where demodulator noise performance becomes very significant, even this demodulator is found wanting. Under these conditions more advanced demodulators such as Phase Locked Loop are used. Punjab EDUSAT Society (PES) 03 Oct, 201252

53. 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. Punjab EDUSAT Society (PES) 03 Oct, 201253

54. 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. Punjab EDUSAT Society (PES) 03 Oct, 201254

55. 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. Punjab EDUSAT Society (PES) 03 Oct, 201255

56. Phase Locked Loop Punjab EDUSAT Society (PES) compare the two input signals and generate an output signal that, when filtered, will control the VCO. adjusts the VCO frequency in an attempt to correct for the original frequency or phase difference. 03 Oct, 201256

57. 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. Punjab EDUSAT Society (PES) 03 Oct, 201257

58. 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. Punjab EDUSAT Society (PES) 03 Oct, 201258

59. 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. Punjab EDUSAT Society (PES) 03 Oct, 201259

60. 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. Punjab EDUSAT Society (PES) 03 Oct, 201260

61. Punjab EDUSAT Society (PES) Transfer Curve Output kulwinderpannu@gmail.com 03 Oct, 201261