Figure 1.  A very simple amplitude modulator

Figure 1 shows an example of a very simple amplitude modulator.  This circuit basically consists of a resistive mixer, a rectifier, and a tuned LC circuit.

In this circuit, the carrier signal and the modulating signal are linearly mixed or algebraically added through Rc, Rm, and Rsum, with the mixed signal emerging at their common node. This mixed signal is then fed into the diode D, which rectifies the mixed signal, i.e., only the forward-going current of the mixed signal is allowed to flow through the circuit. Note that the rectified signal already varies in amplitude according to the modulating signal.

The LC circuit is a band-pass filter that's tuned to the carrier frequency, so it only allows the AM output (the carrier signal and its sidebands) to pass through, and shunts all other signals to ground. This is because a parallel resonant LC circuit exhibits the highest impedance at the resonant frequency.

At the carrier (resonant) frequency, L and C repeatedly exchange energy with each other, resulting in an oscillation that produces a negative half-cycle pulse for every positive pulse coming out of the diode. The amplitudes of these negative pulses follow those of the positive cycles so, in effect, the AM waveform at the output of the modulator is complete with both positive and negative cycles. 

Read More Articles

HOME