AM modulation and demodulation Circuit example

AM demodulation is the process of recovering message signal from incoming AM modulated wave. There are two ways to demodulate AM signal-  (1) square law detector and (2) envelope detector. The square law detector uses non-linear devices such as transistors to recover the message signal while the envelope detector uses diode followed by low pass filter to recover the message signal. The envelope detector was explained in details in the Envelope Detector Circuit for AM Demodulation tutorial which included online calculator to calculate the component value for effective demodulation. In this AM demodulation using envelope detector tutorial it is shown how to use the envelope detector for demodulation of the AM signal that was generated using the AD633 analog multiplier circuit in the tutorial Standard AM with AD633 Analog Multiplier IC. This envelope detector can also used to demodulate DSB-SC(Double Sideband Suppressed Carrier) AM signal which was illustrated in the tutorial DSB-SC AM Generation with AD633 Analog Multiplier IC.

 Envelope detector is one way of demodulating an AM signal. It is inexpensive and efficient method of AM demodulation if the AM signal is narrowband(the carrier signal frequency is much greater than the message bandwidth).

Following shows the circuit diagram of generating AM signal using AD633 analog multiplier IC and AM demodulation using envelope detector.

AM modulation and demodulation Circuit example

In the above circuit diagram, message signal and the carrier signal are generated using the function generators. The message signal is Vtxm and the carrier signal is Vc. These two signals are applied to the AD633 analog multiplier IC X1 and Y1 pins respectively. The AD633 IC is used here as the AM modulator. The pins X2 and Y2 pins are grounded. The summing input pin Z is grounded and the AM signal appears at the output pin W. The AM signal at the output pin W is the standard AM wave which is given by the following equation,

 \(s(t) = (1 + k_a m(t))A_c cos(2πf_ct)\)

where s(t) is the modulated standard AM signal, m(t) is the modulating signal, Ac and fc are the carrier amplitude and frequency, and ka is a constant that determines the depth of modulation.

The mathematical way of generating a standard AM signal is shown in the tutorial Standard AM signal in Matlab.

The following shows the AD633 based AM modulator and the input signals.

AM modulator AD633

The output AM signal magnitude is small and hence it is amplified using the bipolar transistor based amplifier which is biased using the voltage divider biasing method. This amplifier is shown below.

BJT amplifier

See how the BJT amplifier can be build for breadboard. The received AM signal is then demodulated using the envelope detector. The envelope detector circuit is shown below.

envelop detector

The following shows message signal, the carrier signal the transmitted AM signal and the recovered message signal on oscilloscope.

AM signal and recovered signal waveform

The message signal is of 1KHz and 1V amplitude. The carrier signal is of 100KHz and amplitude of 1V. 

The following video demonstrates how the AM modulation and demodulation works.

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