Question

In an amplitude modulated wave for audio frequency of 500 cycles/second, appropriate frequency will be:
A. 50 cycles/sec
B. 100 cycle/sec
C. 500 cycle/sec
D. 50,000 cycle/sec

Answer 1

In amplitude modulation, only the amplitude of the carrier signal is varied in accordance to the amplitude of modulating signal. After the amplitude modulation, the modulating signal consists of a carrier wave whose frequency is ${\omega _c}$ plus two sinusoidal waves whose frequencies are slightly different from the carrier wave. Recall the condition for frequency of the carrier wave for the amplitude modulation to answer this question.

Complete answer:
We know that the process of modulation is the technique of varying the properties of a modulating wave by superimposing it to another wave known as carrier wave. We generally use three techniques of modulation. These are,
-Amplitude modulation
-Frequency modulation
-Phase modulation
We are going to discuss the amplitude modulation technique. In amplitude modulation, only the amplitude of the carrier signal is varied in accordance to the amplitude of the modulating signal. After the amplitude modulation, the modulating signal consists of a carrier wave whose frequency is ${\omega _c}$ plus two sinusoidal waves whose frequencies are slightly different from the carrier wave.
The difference in the frequencies of carrier waves and sinusoidal waves must be very large so that we can have a broad spectrum and the signals do not overlap. Thus, to get the amplitude modulated wave, the frequency of the carrier wave should be greater than the frequency of the modulating signal.

So, the correct answer is option D.

Note: Though, in amplitude modulation, only the amplitude of the carrier signal is varied that does not mean that the frequency of the modulating signal will not change. In this technique, the frequency of the modulating signal is equal to ${\omega _c} \pm {\omega _m}$, where, ${\omega _c}$ is the frequency of the carrier wave and ${\omega _m}$ is the frequency of sinusoidal wave used to superimpose on the carrier wave.