a) How is amplitude modulation achieved?
b) The frequencies of two side bands in an AM wave are 640 kHz and 660 kHz respectively. Find the frequencies of carrier and modulating signal. What is the bandwidth required for amplitude modulation?

 

(a) Process to achieve AM:
Amplitude modulation can be achieved by application of the message signal and the carrier wave to a non-linear device, followed by a band pass filter. Amplitude modulation is the process of varying the amplitude of the carrier signal in accordance with the amplitude of the message signal. Amplitude modulated signals can be produced by adding the carrier signal with the message signal and passing the resultant signal through a square law device (eg: FET).

(c) Given: ${\mathrm{f}}_{\mathrm{c}}+{\mathrm{f}}_{\mathrm{m}}=660$
${\mathrm{f}}_{\mathrm{c}}+{\mathrm{f}}_{\mathrm{m}}=640$
Adding equation (i) and (ii), $2{\mathrm{f}}_{\mathrm{c}}=1300{\mathrm{f}}_{\mathrm{c}}=650\mathrm{khz}$
Putting in equation (i),
$\begin{array}{l}{\mathrm{f}}_{\mathrm{m}}=660-650\\ {\mathrm{f}}_{\mathrm{m}}=10\mathrm{khz}\end{array}$
Bandwidth =$=2{\mathrm{f}}_{\mathrm{m}}$
$\begin{array}{l}=2\times 10\\ =20\mathrm{kHz}\end{array}$