How to Demodulate AM Signal - SPIRO THE TECH GURU

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The amplitude modulation is the simplest modulation technique among the wide verity of modulation techniques in use. In this technique the amplitude of a high frequency signal is varied corresponding to the variation in the amplitude of the low frequency modulating signal. The amplitude modulation of a high frequency signal is easy to achieve and the demodulation is also less complex compared to other techniques. The high frequency signal which is modulated to carry the low frequency audio signals are called ‘carrier frequency’ since they are used to carry the message signal to distant places with the help of wireless transmission devices. The audio signals used for modulation is called ‘modulating signal’ or ‘message signal’ or ‘base band signal’.

The demodulation of an AM wave can be done with only few components and unlike most of the demodulation technique there is no synchronization required between the modulator and demodulator circuits. The message signal appears as an envelope over the amplitude of the carrier wave and the demodulator make use of this to extract the modulating signal from the carrier and hence the technique of AM modulation is called envelope detection.

This article demonstrates how to generate an Amplitude Modulation (AM) and demodulate the same wave to get the original modulating wave. The AM wave is generated based on the circuits explained in article on AM modulation.

The AM demodulation is done using a low pass filter which can filter out the high frequency carrier from the AM wave in such a way that only the envelope of the carrier wave appears at the output of the filter. The amplitude of the filtered wave has variations corresponding to the amplitude of the modulating low frequency signal.

To get a better filtering using the Low pass filter the carrier frequency must be as large as possible and hence the carrier frequency generator circuit explained in the article AM modulation has to be modified for a very high frequency carrier signal. The only change that is required is the value of the capacitors C1 and C2 which determines the carrier frequency generation. The circuit used for generating the sine wave frequency is given below:

The value of the resistance R, R1 and R2 are kept same as explained in the article AM modulation. Now the frequency can be calculated using the equation:

The value of R1 is kept the same as 1K but the value of the resistance R2 can be varied. As the value of the R2 decreases the output frequency of the circuit increases. As it is mentioned in the article AM modulation that the minimum value of the R2 which produces the highest stable frequency is around 130 ohms. In this project to increase the carrier frequency at least 10 times that achieved in the previous project the value of C1 and C2 is reduced to 10 times than that used in the previous AM modulation project. Hence the maximum carrier frequency can be calculated using the frequency equation as shown below:

The message frequency or modulating frequency generator circuit is kept the same as discussed in the article AM modulation. The following image shows the carrier wave and the message wave displayed in a dual-channel CRO.

The modulation is controlled in this project by some adjustment on the input potentiometer and also with the introduction of a 100K ohm resistor. The carrier signal is now fed through the N-channel from drain to source other than from the source to drain as in the previous project. The output can now be taken from the drain end of the FET where the internal channel modulation by the gate signal is more pronounced. The modified AM modulator circuit and the image of the circuit wired in the breadboard are shown below:

The AM wave is displayed along with the original modulating wave using a dual channel CRO as shown in the following image:

The low frequency wave at the center is the message wave and the less bright high frequency wave is the modulated carrier wave. The amplitude of the message wave has been reduced significantly using the potentiometer R2 before applying to the FET so that it won’t over modulate the entire amplitude of carrier wave. The following image shows the waveform that appears at the input and output of the FET modulator.