Subject: Priniciples of Communication Engineering

Difficulty : Medium

Marks : 10

• Reactance modulators are used for generation of frequency modulation (FM)
• These reactance modulators do not have a very good frequency stability. But if it is to be used for broadcast applications, than it is must have a high frequency stability. It can be achieved through frequency stabilized reactance modulator.

Operation:

• The reactance modulator gets the modulating signal as input. It operates on the tank circuit of an LC master oscillator.
• The output of this oscillator is FM signal. It is passed through a buffer and amplitude limiter before it is applied to the Class C power amplifier.
• A fraction of FM output is taken from the limiter and fed to the mixer. The mixer also receives signal from a crystal oscillator.
• The difference frequency ( $f_s$ - $f_o$ ) at the output of the mixer is selected.
• It is usually one twentieth of the master oscillator frequency. This signal is amplified using the IF amplifier.
• The amplified voltage is applied to phase discriminator. The o/p of phase discriminator is a dc voltage connected to the reactance modulaor for correcting any drift in the average frequency of the LC master oscillator.

Features:

• The stability of the whole circuit depends on the stability of the discriminator.

• As the discriminaor is turned to frequency which is one twentieth of the drift in the master oscillator frequency. The frequency drift is reduced to a great extent.