If a message signal of frequency 10 kHz and peak voltage 12 V is used to modulate a carrier wave of frequency 1 MHz, the modulation lindex is 0.6. To make the modulation index 0.75, the carrier peak voltage should be

Options

A. decreased by 25%
B. increased by 25%
C. decreased by 20%
D. increased by 20%

Answer

C

Explanation

Modulation index, `M=(V_("peak, signal"))/(V_("peak, carrier"))`
Let.s write `V_("peak, carrier")=V`
In first case, `0.6 =(12)/(V)`
`rArr" "V=20 v`
In second case, `0.75 =(12)/(V.)`
`rArr" "V.=(12)/(0.75) =16 V`
Change in peak voltage of carrier wave
`Delta V=20-16=4V`
% change `=(4)/(20)xx100% =20%` (decrement)