Question

A 3kHz audio signal with an amplitude of 20V is modulated by a 75 MHz carrier having an amplitude of 50 V. Then which of the following is/are true ?

• A. modulation factor is $0.4$
• B. equation of modulating wave is $V_a=20Sin\left[6000\pi t\right]$
• C. lower and upper side band frequency are $74997$ kHz and $75003$ kHz
• D. none of these

Answer 1

Answer: (A), (B), (C)

The correct options are
A modulation factor is $0.4$
B equation of modulating wave is $V_a=20Sin\left[6000\pi t\right]$
C lower and upper side band frequency are $74997$ kHz and $75003$ kHz

The modulation factor is given as $m=\frac{E_m}{E_c}=\frac{20}{50}=0.4$
Equation of the modulating wave is given as $E_{m}sin{\omega }_{m}t=20sin\left(2\pi 3000t\right)=20sin6000\pi t$
Now we have $f_m=75000kHz$ and $f_c=3kHz$
Thus we get upper band frequency is $f_m+f_c=75003kHz$ and lower band frequency as $f_m-f_c=74997kHz$