Question

a) Describe how to detect an amplitude modulated wave.
b) Sketch a neat diagram for a detector for AM signal.

Answer 1

(a)

The attached diagram in part (a) shows the block diagram for detection of an amplitude modulated wave. It consists of the following stages:

1. Detection of modulated wave using a received antenna.

2. The received signal is passed through a low noise amplifier to increase the amplitude of the signal before demodulation.

3. The amplified signal is passed through the RF stage where it is brought down to an intermediate frequency which is much lower than the carrier frequency but also much higher than the modulating signal frequency. Demodulation is done on the signal at intermediate frequency as the demodulation circuit performance is better at this frequency.

4. The above signal is passed through a amplitude demodulator/detector (discussed in part (b)) where the desired modulating signal is retrieved.

5. A further amplification of the demodulated signal is done for the finaloutput signal.

(b)

The attached diagram in part (b) shows the block diagram for detector stage of AM demodulation. It consists of the following stages:

1. Received signal at IF frequency is passed through a half wave-rectifier (a diode) so that only positive values of the received signal are filtered out.

2. It is then passed through an envelope detector which retains the envelope of the rectified wave and is closely similar to the modulating wave for the correct choice of envelope detector circuit components. An envelope detector consists of a resistor and a capacitor in parallel. across which the output signal is obtained.