Generation of fm

About the two methods to generate FM. Both direct and indirect methods to generate FM.

1. GENERATION OF FM Submitted by, B.Kaavya

2. Basic requirements to produce FM Variable output frequency with the variation proportional to the instantaneous amplitude of the modulating voltage. The amplitude of the carrier remains constant during modulation.

3. HOW FM IS GENERATED? If either the capacitance or inductance of an LC oscillator tank is varied, FM is generated. Two methods : 1. Direct method 2. Indirect method

4. DIRECT METHOD To provide variation, reactance modulator or varactor diode is placed across the tank , and the tank is tuned so that the oscillating frequency is equal to the desired carrier frequency. The C or L is changed accordance with modulating voltage.

5. FET Reactance Modulator 3 terminal reactance that can be connected across tank circuit. The value of this reactance is proportional to the trasconductance of the device .

6. THEORY To determine Z, a voltage ‘v’ is applied to the terminals A-A between which the impedance is to be measured, and the resulting current ‘I’ is calculated

7. Impedance to be a pure reactance two requirements must be fulfilled. 1. Bias network current ‘ ib ’ must be negligible compared to the drain current. 2. Drain to gate impedance ( Xc ) must be greater than the gate to source impedance (R).

8. Ceq = gmRC Ceq depends on device gm and can be varied with bias voltage. Capacitance can be adjusted by varying the components R and C. This expression has the correct dimensions of capacitance. Xc > R

9. If R is not << than Xc the Vg will no longer be exactly 90 ° out of phase with the ‘v’. Thus the input impedance will no longer be purely reactive.

10. The gate to drain impedance is, in practice made five to ten times the gate to source impedance. Let Xc = nR (at the carrier frequency) in the capacitive RC reactance FET.

11. 4 Different Arrangements of Reactance Modulator

12. In all arrangements drain current must be much greater than bias network current. Commonly used oscillators: 1. Hartley oscillator 2. Colpitts (Clapp Gouriet ) an RC capacitive transistor reactance modulator operates on tank circit .

13. VARACTOR DIODE Junction capacitance varies linearly with the applied voltage when the diode is reverse biased. It is employed together with a reactance modulator to provide automatic frequency correction for an FM transmitter.

14. This diode is back biased to provide the junction capacitance( capacitance associated with charge variation in the depletion layer ) effect. This bias is varied by the modulating voltage which is in series with it. It is often used for automatic frequency control and remote tuning.

15. VARACTOR DIODE MODULATOR

16. INDIRECT METHOD Generating FM through phase modulation , where a crystal oscillator can be used. Operating frequency for crystal oscillator and phase modulator is 1MHz. Armstrong system

17. Crystal oscillator : It is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a precise frequency . Phase modulation : It encodes a message signal as variations in the instantaneous phase of a carrier wave.

18. BLOCK DIAGRAM EXPLANATION The effect of mixing on an FM signal is to change the center frequency only. The effect of frequency multiplication is to multiply center frequency and deviation equally.

19. Frequency mixer : A nonlinear electrical circuit that creates new frequencies from two signals applied to it. Frequency multiplier : An electronic circuit that generates an output signal whose output frequency is a harmonic of its input frequency.

20. PHASE MODULATION VECTOR DIAGRAMS

21. DIAGRAM 1 The resultant of two sideband frequency vectors is always in phase with the unmodulated carrier vector. So there is a phase change of 90 ° of sideband voltages with carrier vector.

22. DIAGRAM 2 If an amplitude modulated voltage is added to an unmodulated voltage of the same frequency and the two are kept 90 ° apart in phase.

23. DIAGRAM 3 The carrier of the amplitude modulated signal has been removed so that only the two sidebands are added to unmodulated voltage. The resultant of the 2 sideband voltages will always be in quadrature with the carrier voltage. Modulation ↑ phase deviation ↑ hence phase modulation is obtained

24. The resultant voltage will be phase modulated but with little amplitude modulation also. If amplitude limiter is used then there will be phase modulation only. The modulating voltage will have to be equalized before it enters the balanced modulator.

25. The modulating voltage will have to be equalized before it enters the balanced modulator. In FM broadcasting, ῳ L =R at 30Hz . As frequency increases above that the output of the equalizer will fall at a rate of 6dB/octave .

26. Effects of frequency changing on an FM signal Armstrong system is often used with reactance modulator. Modulation index is multiplied by the same factor as the center frequency.

27. If a frequency modulated signal fc± is fed to a frequency doubler, the output signal will contain twice each input frequency. (2fc±2) When a frequency modulated wave is mixed with frequency f0 then output will be fc-f0- and fc-f0+. Maximum frequency is a± .

28. Modulating frequency has remained constant in both cases. But the modulating index will be affected. It is possible to raise the modulation index without affecting the center frequency by multiplying both by 9 and mixing the result with a frequency 8 times the original frequency.

29. Characteristics of Phase modulation Angle of phase modulation is proportional to the amplitude of the modulating voltage not the angle. To keep the angle of deviation small both mixing and multiplication must be done.

30. For broadcast purpose, operating frequency is 180MHz with initial fc=1MHz and =60Hz. Hence a deviation 10.8kHz at 180MHz is achieved and that is adequate for FM mobile network.