Question

The role of envelope detector in an AM receiver is to:
A. retrieve the message signal
B. rectify the AM signal
C. modify the AM signal
D. modulate the message signal
E. retrieve the AM signal

Answer 1

Hint An envelope detector is an electronic circuit that takes a (relatively) high-frequency amplitude modulated signal as input and provides an output which is the envelope of the original signal.

Complete step-by-step solution :
Let us first discuss about AM receiver
An AM receiver detects amplitude variations in the radio waves at a particular frequency, then amplifies changes in the signal voltage to operate a loudspeaker or earphone.
An envelope detector is an electronic circuit that takes a (relatively) high-frequency amplitude modulated signal as input and provides an output which is the envelope of the original signal.
This can be clarify as below:
Any AM signal $x\left( t \right)$ can be written in the following form
$x\left( t \right) = R\left( t \right)\cos \left( {\omega t + \phi \left( t \right)} \right)$
In the case of AM, $\phi \left( t \right)$ (the phase component of the signal) is constant and can be ignored. In AM, the carrier frequency $\omega$ is also constant. Thus, all the information in the AM signal is in $R\left( t \right)$ . It is called the envelope of the signal. Hence an AM signal is given by the function
$x\left( t \right) = \left( {C + m\left( t \right)} \right)\cos \omega t$
With $m\left( t \right)$ representing the original audio frequency message, $C$ the carrier amplitude and $R\left( t \right)$ equal to $C + m\left( t \right)$ . So, if the envelope of the AM signal can be extracted, the original message can be recovered.

Therefore, the role of envelope detector in an AM receiver is to retrieve the message signal. Hence, option A is correct.

Note: Amplitude modulation is a modulation technique used in electronic communication, most commonly for transmitting messages with a radio carrier wave. In amplitude modulation, the amplitude of the carrier wave is varied in proportion to that of the message signal, such as an audio signal.
An envelope detector is sometimes referred to as an envelope follower in musical environments. It is still used to detect the amplitude variations of an incoming signal to produce a control signal that resembles those variations. However, in this case the input signal is made up of audible frequencies.