To boost your understanding synchronous detection, let’s examine the impact of selective fading, distortion and weak signals on the AM detector in your receiver.
AM transmissions are usually double sideband with carrier. Power is distributed between the carrier and sidebands according to the modulation index. AM signals have a bandwidth of twice the message signal duplicated in each sideband. Carriers have a frequency much higher than the message, and are usually sinusoidal, although this is not necessary, e.g. switching modulators.
To demodulate AM, we either use the original carrier, or create a new carrier that is phase and frequency synchronized. The latter is called synchronous or coherent modulation. Before we get into the how of understanding synchronous detection, lets understand the why. In short, what problems are we trying to solve with a synchronous detector?
Envelope detectors are prone to distortion and lack sensitivity for weak signals. So, we want to reduce distortion and improve sensitivity. Here are some examples of these effects on shortwave broadcasting.
Let’s start with the effects of selective fading, which is prominent on HF broadcasting. Everyone knows that a shortwave signal fades up and down as it is effected by changes in the ionosphere propagation. But there are some twists to this story. First, long distanced transmissions can take multiple paths on the way to your receiving antenna. You end up with a situation in which multi-path effects interfere with each other because of the delayed arrivals. Second, fading can occur differently within relatively wide signals like AM. Within the 10-20 kHz bandwidth, each of the carrier and two sidebands can fade at different rates.
The end result is that the AM detector in your receiver will need to cope with the complex effects of selective and overall fading, as well as phase distortion in the carrier wave. Simple envelope detectors are not equipped to manage these challenges.
Understanding Synchronous Detection – Boosting or Replacing the Carrier
Propagation effects may cause the carrier to fade by 10-20 dB relative to the sidebands. This can cause effective over-modulation and distortion. Conversely, it can make the carrier to weak to drive the envelope detector above its threshold.
Synchronous detectors create a local carrier of known strength inside your receiver. The effects of selective fading are reduced considerably. Listening becomes more pleasant and reliable. Moreover, synchronous detectors are product detectors, and these square-law devices do not have an effective signal-to-noise threshold. Unlike rectifiers, they work cleanly for very small signals. Finally, by demodulating directly down to baseband using a mixer, you can get better filtering not available from the more simple RC time-constant filters in envelope detectors.
As an additional benefit, once you are using a product detector, you can choose between sidebands for your listening. This gets rid of phasing and distortion effects on the upper and lower sidebands from selective fading or adjacent channel interference. AM envelope detectors only work with both sidebands.
Next, we will consider the “how it is done” part of understanding synchronous detectors.