CBSE Class 12 Physics 2019 Delhi Set 1 Paper

Question : 24

Total: 27

(a) If A and B represent the maximum and minimum amplitudes of an amplitude modulated wave, write the expression for the modulation index in terms of A and B.
(b) A message signal of frequency 20‌kHz and peak voltage 10V is used to modulate a carrier of frequency 2‌MHz and peak voltage of 15V. Calculate the modulation index. Why the modulation index is generally kept less than one?

Solution:

(a) Obtaining the expression for modulation index in terms of A and B
(b) Calculation of µ
Reason
We are given that
A‌=Ac+Am
‌ and ‌B‌=Ac−Am
Ac‌=(A+B)∕2
Am‌=(A−B)∕2
∴µ‌=‌

‌=‌

A−B

A+B

(b) We have
µ‌=‌

‌=‌

=‌

µ is kept less than one to avoid distortion.

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