Before we explore synchronous detection, let’s make sure we understand how envelope detection works. After all, it’s been around for a century.
Amplitude transmission and reception has been around for more than a hundred years. Part of the reason it has lasted so long is that AM is just so darn simple. AM involves mixing a message with a carrier. Messages are usually audio at a much lower frequency than the carrier. In the time domain, the resulting signal looks like an audio envelope imposed on a higher frequency carrier (left above). In the frequency domain, you see a carrier with upper and lower sidebands which contain the message.
For a more detailed explanation, check out my video about Amplitude Modulation using GNURadio.
Demodulating AM is as simple as passing the radio signal through an rectifier and a filter. This is called envelope detection. An envelope detector works by doing two things. It works a lot like a toy crystal radio. First the diode converts the radio signal (AC) into a rectified DC signal. This DC signal fluctuates with both the carrier and message frequencies. Second, the rectified DC is applied to a low-pass filter. Often, this low pass filter is just a capacitor and a resister. Combinations of RC provide a time constant which gets rid of the faster fluctuations from the carrier.
What remains is a DC signal that dances around in tune with the audio message. Another capacitor removes the DC offset. The result is then an AC signal at audio frequencies, which is sent to the audio amplifier and speaker. Here is a short video tutorial showing how envelope detection works.
So simple. How could anything go wrong?
Envelope Detector Works Under Stable Conditions
Diodes are not perfect and are only approximately linear. Diodes need to be biased to conduct properly. This bias voltage creates a signal threshold below which rectification fails. So, very weak and very strong signals tend to result in distortion as they pass through the rectifier.
Also, the envelope detector works best with a proper and steady relationship between carrier and sideband power. Unfortunately, as AM radio signals travel through space, they are effected by selective fading. This puts the relationship between carrier and sideband voltages out of kilter.
As a result, with most envelope detectors, we have to live with some distortion. When signals are weak or fluctuating rapidly, this distortion increases.
The recent development of synchronous detection overcomes most of the problems with envelope detection. We will describe this next.