Synchronous Vs Envelope Detection Updated 10/05/01

Some demodulators provide a choice between envelope and synchronous detection modes. When running the FCC Proof of Performance tests, its important to use the proper detection mode. Especially when measuring differential phase. Let's take a look at the two modes and see what they have to offer.

Envelope Detectors

Envelope detectors are usually considered to be the simplest form of detector. In fact, they can be quite complicated or they can be as simple as a diode and a low-pass filter. The performance of an envelope detector, when used to demodulate a television signal, is limited by the relative amplitudes of the signal being demodulated (bigger is better), and an effect called quadrature distortion. Quadrature distortion is a result of the asymmetry of our television signal. It's vestigial sideband coming from the modulator or transmitter and nearly single sideband within the television's IF circuit. One nice thing about envelope detectors (in addition to their low cost) is that they are not sensitive to the phase of the visual carrier. This will become important when we look at differential phase tests.

Synchronous Detectors

Synchronous detectors are considerably more complex than simple envelope detectors. They consist of phase locked loop and multiplier circuits. Demodulation is performed by multiplying the modulated carrier by a sine wave that is phase locked to the incoming carrier. Synchronous detectors are a subset of "product" detectors. If you are an amateur radio operator, you may have listened to single sideband suppressed carrier signals using receivers with a BFO for re-inserting the carrier and envelope detection; and other receivers with real product detectors. SSB signals sound much better using product detectors.

The advantage of synchronous detection is that it causes less distortion than envelope detection and works well with single sideband signals. It is the preferred detection method for most tests. Another characteristic of synchronous detectors is that they are phase sensitive. The amplitude of the demodulated signal is a function of the relative phases of the incoming carrier and the carrier generated inside the receiver. In the extreme case, if the phase of the modulated carrier and the regenerated carrier in the demod is 90 degrees (they are in quadrature) the detector output would be zero!

 

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