Question

Which statement is true about direct FM modulator?

(a) it integrates the modulating signal

(b) it differentiates the modulating signal

(c) varies the frequency of carrier oscillator

(d) varies the phase of carrier oscillator

The question was asked during an internship interview.

This intriguing question originated from SSB with Large Carrier in section Side Band Techniques of Analog Communications

Answer 1

The correct choice is (c) varies the frequency of carrier oscillator

For explanation I would say: Frequency modulated system can be produced either by direct ways or indirect ways. Armstrong method comes under indirect method. Direct FM modulator is generally used to vary the frequency of carrier oscillator, thus being known as frequency discrimination method, while indirect FM modulator varies the phase of carrier oscillator, thus being called phase discrimination method.