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 AM wave.

Answer 1

Hint: Try and recall what amplitude modulation is. That is, since there is a superposition of the carrier wave and the modulating wave, the detector should have components to extract out the carrier wave information from the modulated AM signal. Think of which components you would require to resolve the components, since you only need to finally end up with the envelope of the modulated wave which carries all information we need, minus the modulating wave.

Complete step by step answer:
Let us begin by first understanding what an AM wave is.
We know that the amplitude of a wave is the maximum displacement that a particle would get displaced from its mean position.
Now, we have two waves (signals). One is called the carrier wave and it carries the information or message that needs to be transmitted, and the other is the modulating wave.
Modulation is basically the process by which one of the parameters (amplitude, frequency or phase) of the carrier wave is changed by the modulating signal. Therefore, the parameters of the modulating signal determine the output of the carrier wave which would then be the modulated wave.
Amplitude modulation is the process by which the carrier wave and the modulating signal are superposed in such a way that the amplitude of the carrier is varied in proportion to the instantaneous value of the modulating signal, while ensuring there is no change in frequency.
The main purpose of amplitude modulation is to generate a signal suited to the characteristics of the transmission channel and to ensure that if the signal is transmitted or broadcasted over long distances no information is lost.
Therefore, the amplitude of the carrier wave is modulated with respect to the amplitude of the modulating signal and is transmitted as an AM wave.
Now, we need to have a receiver system that fetches the incoming AM wave. And a part of this receiver system is the AM wave detector, whose block diagram is shown in the figure. Before we understand the detector blocks, let us first establish the stages the AM wave goes through in the receiver before reaching the detector.
The AM wave, upon being received by the antenna goes through two stages before reaching the detector:
Radio frequency (RF) amplifier: The AM wave gets weakened as it propagates through the channel from the transmitter to the receiving antenna. To strengthen this wave, an RF amplifier is connected to the receiving antenna.
Intermediate frequency stage (IF) stage: The AM wave is changed to a lower frequency and is amplified at this frequency. This is done to improve frequency selectivity and to resolve the signals that are close together in terms of frequency.
The AM wave from the IF stage is now transmitted to the detector that we shall now look at.
The AM wave received by the detector block is still a modulated wave with both carrier and modulating signals.

The detector consists of two blocks:
A rectifier: Here, the AM wave undergoes the process of rectification where one half of the waveform of the AM wave, either the positive half or the negative half, is clipped completely. This is done by a single diode and the process is called half-wave rectification. Once this is done, the rectified wave is transmitted to the next block, which is, the envelope detector.
Envelope detector: Also known as the demodulator circuit, it is an electronic circuit consisting of a diode, a capacitor and a resistor that extracts out the modulating signal from the modulated wave and gives the carrier information as the output. It takes the AM signal from the rectifier and provides the envelope of the modulated signal as output. The envelope is nothing but the smooth cure outlining the extremities of the modulated signal. It is a function over time and thus envelopes all the instantaneous amplitudes (all original information is coded as change in amplitude with time) together. Thus, this envelope is finally the original information carried by the carrier wave, which is what we ultimately require.

Note:
Remember that the detector is a part of the AM receiving system. The output of the detector is then transmitted to an Audio frequency (AF) amplifier which amplifies the message signal at the output which can be used for the intended purpose of its transmission.
The modulation index is also an important parameter to be considered as it determines the strength and quality of the transmitted signal. It is given by:
$\mu = \dfrac{A_m}{A_c}$, where $A_m$ and $A_c$ are the amplitudes of the modulating and carrier signals respectively. The greater the modulation index, the stronger and clearer the audio signal will be. The maximum value of $\mu = 1$ where $A_m = A_c$. Any further increase in $\mu$ or $A_m$ will result in overmodulation, which will lead to excessive distortion of the transmitted modulated wave, which may lead to loss of carrier information, and hence should be avoided.