Subject: Principles of Communication Engineering

Difficulty : Medium

Marks : 10

Methods of FM Generation:

1. Direct Method

• Reactance modulator

• varactor diode modulator

1. Indirect Method

• Armstrong method

  a) Heterodyne method

  b) Multiplication method

  Reactance modulator

Explanation-

• A reactance modulator is illustrated in figure. It is basically a standard commonemitter class A amplifier. Resistors R1 and R2 from a voltage divider to bias the transistor into the linear region. R3 is an emitter bias resistor.
• The oscillator signal from the RC phase-shift circuit made up of Cs and Rs.
• The value of Cs is chosen so that its reactance at the oscillator frequency is about 10 or more times of the value of Rs. if the reactance is much greater than the resistance, the circuit will appear predominantly capacitive; therefore the current through the capacitor and Rs will lead the applied voltage by about $90^0$
• Since the collector current is in phase with the base current, which in turn is in phase with the base voltage, the collector current in Q1 leads the oscillator voltage V0 by $90^0$ Of course, any circuit whose current leads its applied voltage by $90^0$ looks capacitive to the source voltage.
• The modulating signal is applied to the modulator circuit through C1 and RFC1. The RFC helps keep the RF signal from the oscillator out of the audio circuit from the modulating signal usually comes. The audio modulating signal will vary the base voltage and current of Q1 according to the intelligence to be transmitted.
• The collector current will also vary in proportion. As the collector current amplitude varies, the phase shift angle changes with respect to the oscillator, voltage, which is interpreted by the oscillator as a change in the capacitance. So as the modulating signal changes, the effective capacitance of the circuit varies and the oscillator frequency is varied accordingly
• An increase in capacitance lowers the frequency, whereas a lower capacitance increases the frequency. The circuit produces direct frequency modulations.