Question

Draw a block diagram of a detector for AM signal and show, using necessary processes and the waveforms, how the original message signal is detected from the input AB wave.

Answer 1

Amplitude Modulation (AM) is a modulation technique which is commonly used for transmission of messages with a radio carrier wave. The input wave is rectified and then demodulated. When a message is received, it gets attenuated, amplified, detected and then demodulated. At the final stage an envelope detector is used to retrieve the message.

Complete step by step solution:
When a message is received, it gets attenuated (that is reduced in strength) through the channel. The detected signal may be weak and hence cannot be used as per requirement. As a result the receiver, which is an antenna, has to be an amplifier and a detector. The carrier frequency is usually changed to an intermediate frequency (IF).
In order to obtain the signal, certain processes are followed as shown in the diagram below:

In the first stage A, the AM wave is sent to a rectifier which is then passed through a rectifier. Now the signal reaches stage B, here an envelope signal is produced. This envelope signal is the message signal. In order to retrieve this message, the signal is passed through an envelope detector which detects whether the envelope contains the message or not.

At stage A, we have the AM input wave. At stage B after being passed through a rectifier we have a rectified wave. And finally at stage C, we have the Output without the RF component.

Note: The AM input signal is first passed through a rectifier and then through an envelope detector. It must be noted that the input signal at times is weak and the desired output is not obtained hence the input AM signal is modulated and demodulated to obtain the desired output. Modulation is performed before the signal reaches stage A.