Synchronous Detection: An Introduction - Tecsun PL-368 Manual

Synchronous Detection: An introduction

For demodulating AM signals, most radios use diode envelope detectors.
When the amplitude modulated signal passes through the inductor-
capacitor loop, it will produce distortion, and the envelope detector will
also produce distortion. Therefore, one of the best ways to demodulate
the AM signal is to use a synchronous detector.
The synchronous detector reconstructs the unmodulated carrier frequency
based on the input signal, and uses this as a reference to identify noise
and distortion. Synchronous detection technology is often used in FM
stereo left and right channel signal demodulation and color TV
chrominance signal demodulation.
Synchronous detection has a high detection efficiency and can realize
small signal detection. It can eliminate the distortion (transient
intermodulation distortion) generated in the IF filter due to local fading,
slight offset, modulation overshoot, as well as inter-channel interference
and cross-talk modulation, and can also reduce noise interference.
The chart (on the next page) shows some common amplitude modulation
phenomena:
Example 1: It is an undisturbed conventional amplitude modulation signal,
both detectors give distortion-free output.
Example 2: The carrier is reduced because of local fading or the directivity
of the transmitting antenna.
Example 3: Sideband asymmetry is caused by local fading or reception
detuning.
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