Product detector

A product detector is a type of demodulator used for AM and SSB signals. Rather than converting the envelope of the signal into the decoded waveform like an envelope detector, the product detector takes the product of the modulated signal and a local oscillator, hence the name. A product detector is a frequency mixer.

Product detectors can be designed to accept either IF or RF frequency inputs. A product detector which accepts an IF signal would be used as a demodulator block in a superheterodyne receiver, and a detector designed for RF can be combined with an RF amplifier and a low-pass filter into a direct-conversion receiver.

The simplest form of product detector mixes (or heterodynes) the RF or IF signal with a locally derived carrier (the Beat Frequency Oscillator, or BFO) to produce an audio frequency copy of the original audio signal and a mixer product at twice the original RF or IF frequency. This high-frequency component can then be filtered out, leaving the original audio frequency signal.

If m(t) is the original message, the AM signal can be shown to be

Multiplying the AM signal x(t) by an oscillator at the same frequency as and in phase with the carrier yields

which can be re-written as

After filtering out the high-frequency component based around cos(2ωt) and the DC component C, the original message will be recovered.

Although this simple detector works, it has two major drawbacks:

Frequency of an AM carrier can be accurately determined with a phase-locked loop, but for SSB, the only solution is to construct a highly stable oscillator.

There are many other kinds of product detectors as well, which are practical if one has access to digital signal processing equipment. For instance, it is possible to multiply the incoming signal by the carrier, times the square of another carrier 90° out of phase with it. This will produce a copy of the original message, and another AM signal at the fourth harmonic, by means of the trigonometric identity

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