A 1 MHz sinusoidal carrier is amplitude modulated by a symmetrical square wave of period 100u sec. Which of the following frequencies will not be present in the modulated signal?

a) 990 KHz

b) 1010 KHz

c) 1020 KHz

d) 1030 KHz

Answer -c,d

Fc= 1 MHz

Fm=1/(100u sec)=10KHz

For amplitude modulation frequencies will be between Fc-Fm and Fc+Fm

that is they will be in between 990KHZ and 1010KHZ 

So the frequencies  1020 KHz and 1030 KHz will not be present.

fm = 10 Khz

fc = 1000 kHz

So the frequencies are = ( 1000 Khz + 10 kHz = 1010 KHz  and 1000 khz-10 khz = 990 KHz)