Question

A speech signal of 3 kHz is used to modulate a carrier signal of frequency 1 MHz, using amplitude modulation. The frequencies of the sidebands will be ______.

Options

• 1.003 MHz and 0.997 MHz

• 3001 kHz and 2997 kHz

• 1003 kHz and 1000 kHz

• 1 MHz and 0.997 MHz

Answer 1

A speech signal of 3 kHz is used to modulate a carrier signal of frequency 1 MHz, using amplitude modulation. The frequencies of the sidebands will be 1.003 MHz and 0.997 MHz.

Explanation:

The process of changing the amplitude of a carrier wave in accordance with the amplitude of the audio frequency (AF) signal is known as amplitude modulation (AM).

In AM, the frequency of the carrier wave remains unchanged.

Side band frequencies: The AM wave contains three frequencies f_c, (f_c + f_m) and (f_c - f_m), f_c is called carrier frequency, (f_c + f_m) and (f_c - f_m) are called sideband frequencies.

(f_c + f_m): Upper sideband (USB) frequency
(f_c - f_m): Lower sideband (LSB) frequency

Sideband frequencies are generally close to the carrier frequency.

According to the problem, the frequency of carrier signal is f_c = 1 MHz and the frequency of speech signal = 3 kHz

= 3 × 10^-3 MHz

= 0.003 MHz

We know that, Frequencies of side bands = (f_c ± f_m) = (1 + 0.003) and (1 - 0.003)

So, sideband frequencies are 1.003 MHz and 0.997 MHz.