Product detector

Lore & Background

A product detector, also called a VIC detector, is a demodulator for AM and SSB signals. Unlike an envelope detector, which recovers the signal from the envelope, a product detector multiplies the modulated signal with a local oscillator—hence the name. It is a type of frequency mixer.

These detectors can work with either intermediate frequency (IF) or radio frequency (RF) inputs. An IF-based product detector serves as the demodulator in a superheterodyne receiver, while an RF-based one, combined with an RF amplifier and a low-pass filter, forms a direct-conversion receiver.

The simplest version mixes the RF or IF signal with a locally generated carrier (the Beat Frequency Oscillator, or BFO). This produces an audio-frequency copy of the original signal plus a component at twice the original frequency. The high-frequency part is then filtered out, leaving the original audio.

Mathematically, if m(t) is the message, the AM signal is multiplied by an oscillator at the same frequency and phase as the carrier. The result can be rewritten, and after filtering out the high-frequency term (around cos(2ωt)) and the DC component C, the original message is recovered.

This simple design has two major drawbacks. First, the local oscillator must match the carrier frequency exactly; otherwise, the output fades in and out for AM or becomes frequency-shifted for SSB. Second, even with matched frequency, the carrier phase must be correct, or the demodulated message is attenuated while the noise is not. A phase-locked loop can synchronize the oscillator in a synchronous detector for AM. For SSB, only a highly stable oscillator works.

More advanced product detectors are feasible with digital signal processing. For example, multiplying the incoming signal by the carrier times the square of another carrier 90° out of phase produces a copy of the original message plus an AM signal at the fourth harmonic. Filtering out the high-frequency component (around cos(4ωt)) and the DC component recovers the original signal.

Another sophisticated design resembles a single-sideband modulator. Two copies of the modulated input are created. One is mixed with a local oscillator and low-pass filtered. The other is mixed with a 90° phase-shifted oscillator, then the output is also phase-shifted by 90° and low-pass filtered. These copies are combined to yield the original message, similar to a dual-phase lock-in amplifier (an I-Q demodulator).

Compared to an envelope detector for AM, the product demodulator has advantages: it can handle overmodulated AM and AM with suppressed carrier, and it provides a higher signal-to-noise ratio. However, the envelope detector is simpler, cheaper, and offers higher fidelity because there is no risk of mistuning the local oscillator. A product detector (or equivalent) is necessary for demodulating SSB signals.

Reader's Guide

The product demodulator has some advantages over an envelope detector for AM signal reception. It can decode overmodulated AM and AM with suppressed carrier. A signal demodulated with a product detector will have a higher signal-to-noise ratio than the same signal demodulated with an envelope detector. On the other hand, the envelope detector is a simple and relatively inexpensive circuit, and it can provide higher fidelity, since there is no possibility of mistuning the local oscillator. A product detector (or equivalent) is needed to demodulate SSB signals. The product detector's ability to handle suppressed-carrier and overmodulated AM, along with its superior signal-to-noise ratio, makes it significant for applications where these characteristics are required, though its complexity and need for precise oscillator synchronization are notable trade-offs.

Did You Know?

• A product detector is also known as a VIC detector.
• The simplest product detector mixes the signal with a Beat Frequency Oscillator (BFO).
• A product detector is needed to demodulate SSB signals.