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FM DEMODULATORS. Presented By. Kulwinder Singh Lecturer ECE GPCG, Ludhiana. Contact info: email: kulwinderpannu@gmail.com Phone: 97813-00151. What We Shall Cover. Review of Modulation (with animations) What is demodulation Frequency Demodulation Definition Types of FM Demodulators

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  1. FM DEMODULATORS Presented By Kulwinder Singh Lecturer ECE GPCG, Ludhiana Contact info: email: kulwinderpannu@gmail.com Phone: 97813-00151 Punjab EDUSAT Society (PES)

  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)

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

  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)

  5. Amplitude Modulation Example Transmitted Signal Modulating Signal Punjab EDUSAT Society (PES)

  6. Frequency Modulation Example Transmitted Signal Modulating Signal Punjab EDUSAT Society (PES)

  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)

  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)

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

  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 wave. Punjab EDUSAT Society (PES)

  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)

  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)

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

  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)

  15. SLOPE Detector Detector Output FM FM Source Tank Circuit Punjab EDUSAT Society (PES)

  16. Slope Detector Transfer Characteristics Transfer Curve Output Voltage Voltage t f Input t Punjab EDUSAT Society (PES)

  17. Transfer Curve Output Voltage Voltage fc t f f fc+f Slope Detector Transfer Characteristics fc-f Input t Punjab EDUSAT Society (PES) Punjab EDUSAT Society (PES)

  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)

  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)

  20. Punjab EDUSAT Society (PES)

  21. FM Detector O/P FM Wave Punjab EDUSAT Society (PES)

  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 1800 out of phase. Punjab EDUSAT Society (PES)

  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)

  24. Balanced Slope Detector D1 + fc+f T’ - fc - IN Vo T’’ fc-f + D2 Punjab EDUSAT Society (PES)

  25. Balanced Slope Detector T’ 15V Output of T’ at fc+f (+10 to +15V) 10V fc Output of T’ at fc-f (+5 to +10V) +Ve 5V fc+f fc-f -5V Output of T’’ at fc+f (-5 to -10V) -Ve -10V Output of T’ at fc-f (-10 to -15V) T’’ -15V Punjab EDUSAT Society (PES)

  26. Combined Transfer Curve Vo Useful Range fc-f fc fc+f Punjab EDUSAT Society (PES)

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

  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)

  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)

  30. Phase Discriminator C a’ b + D1 R3 C3 L2 C1 L3 - o Va’b’ L1 - VIN R4 C4 + As C & C4 are coupling & RF Bypass capacitors respectively, therefore VL3 VIN So Voltage across diode= VIN + Secondary voltage/2 a b’ D2 Punjab EDUSAT Society (PES)

  31. Phase Discriminator • Now in Transformer voltage is induced in the secondary as a result of current in primary. And • Where X2= XL2-XC2 Punjab EDUSAT Society (PES)

  32. Phase Discriminator • At resonance i.e. when input frequency is fc, X2=0 • i.e. Vab leads VIN by 900. Punjab EDUSAT Society (PES)

  33. Phase Discriminator • And from the phasor diagram given below : • That as Vao=Vbo, hence discriminator output is zero. Vao Vbo Punjab EDUSAT Society (PES)

  34. 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. Punjab EDUSAT Society (PES)

  35. Phase Discriminator • And from the phasor diagram given below : • That as Vao>Vbo, hence discriminator output is positive. Vao Vbo Punjab EDUSAT Society (PES)

  36. 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. Punjab EDUSAT Society (PES)

  37. Phase Discriminator • And from the phasor diagram given below : • That as Vao<Vbo, hence discriminator output is negative. Vao Vbo Punjab EDUSAT Society (PES)

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

  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)

  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 Vao+Vbo Should remain constant, • and their difference should vary due to variation in input frequency. Punjab EDUSAT Society (PES)

  41. 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. Punjab EDUSAT Society (PES)

  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)

  43. Ratio-Detector C a’ b + D1 R3 C3 R5 L2 C1 L3 - o SUM L1 C5 Vo VIN + C4 R4 R6 - a b’ D2 Change 1: Diode D2 is reversed so that now sum of Vao & Vboappears across points a’ and b’ instead of difference. Punjab EDUSAT Society (PES)

  44. Ratio-Detector C a’ b + D1 R3 C3 R5 L2 C1 L3 - o L1 C5 Vo VIN + C4 R4 R6 - a b’ D2 Change 2: A capacitor C5 with large time constant is connected across a’-b’ in order to keep Vao+Vbo constant. Punjab EDUSAT Society (PES)

  45. Ratio-Detector C a’ b + D1 R3 C3 V1 R5 C1 L3 - o L1 C5 Vo VIN + C4 R4 V2 R6 L2 - a b’ D2 Change 3: Output is taken from o-o’ as the difference of Vao + Vboappears there. Ground is shifted to O’. Punjab EDUSAT Society (PES)

  46. Operation at Resonance • No phase shift occurs at resonance and both Vao & Vboare 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. Punjab EDUSAT Society (PES)

  47. 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. Punjab EDUSAT Society (PES)

  48. Operation Above Resonance • So output voltage in this case will be positive as shown in vector diagram: Vao Vbo Output Punjab EDUSAT Society (PES)

  49. 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 Punjab EDUSAT Society (PES)

  50. Operation Below Resonance • So the output in this case will be negative. Vao Vbo Output Punjab EDUSAT Society (PES)

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