In this section we describe the circuits used for generation and demodulation of amplitude modulated signals. An analog multiplier IC AD633 (Analog Devices) has been used to generate the AM signal. The AD633 is a functionally complete, four-quadrant, analog multiplier. It includes high impedance, differential X and Y inputs, and a high impedance summing input (Z). The low impedance output voltage is a nominal 10 V full scale provided by a buried Zener. The functional diagram of the AD633 is shown in Figure 4.
Figure 4: Functional block diagram
From Figure 4, we find that,
Details of AD633 is available in the data sheet.
Amplitude modulator Circuit with AD633
The AD633 can be used as a linear amplitude modulator with no external components. Figure 5 shows the circuit. The carrier and modulation inputs to the AD633 are multiplied to produce a double sideband signal. The carrier signal is fed forward to the Z input of the AD633 where it is summed with the double sideband signal to produce a double sideband with the carrier output.
For single tone modulation, EM=Am Sin(ωmt) is used. The index of modulation can be varied by changing Am.
Demodulation of AM signal
As stated earlier, an envelope detector has been used here for demodulation. An envelope detector (Figure 6(a)) is an electronic circuit that takes a high-frequency modulated signal as input and provides an output which is the "envelope" of the original signal. The capacitor in the circuit stores charge on the rising edge, and releases it slowly through the resistor when the signal falls. The diode in series rectifies the incoming signal, allowing current flow only when the positive input terminal is at a higher potential than the negative input terminal (Figure 6(b)).
Figure 6(b): Envelope detection process
For a sinusoidally modulated signal, if the time constant of the detector is chosen such that
|,the detector can always follow the message envelope.
The complete experimental setup is shown in Figure 7.
Figure7. Setup for the Real Time Amplitude Modulation and Demodulation Experiment
Figure 8. Actual Circuit connection