So, how do you implement synchronous detection? Lots of ways, it turns out. Some work better than others across a spectrum of complexity.
The essence of synchronous detection is replacing the original carrier with a stronger, more stable version in your receiver. Would you be surprised that the simplest way to achieve this is just tuning in the AM signal with your receiver in SSB or DSB mode? Technically, this approach is really exalted carrier, as the replacement carrier is not really synchronized. In the old days, with less stable or accurate oscillators, tuning AM with SSB was difficult.
However, modern synthesized receivers provide a high degree of frequency accuracy and stability. The odds are quite good that your SSB receiver can tune onto an AM broadcast signal and decode the audio with high quality – no more Donald Duck. Many experienced SW listeners use this approach all the time. It’s great for eliminating co-channel interference which usually impacts one sideband more than the other. Some people call this approach quasi-synchronous. The advantage of using the product detector for AM has been discussed. The weakness of this approach is that most older analog receivers only have quite narrow filters, i.e. less than 3 kHz.
However, when you use the Synchronous AM mode on most modern receivers, the manufacturer chose to implement synchronous detection in one of three ways. Two of these achieve synchronous detection by just re-using the original carrier.
Firstly, some receivers just pass a copy of the AM signal through a separate narrow filter, amplifying it and then mixing it back with the original AM signal. This was probably the original method, but it required a very stable receiver. Tuning requirements were critical.
Secondly, some receivers took the filter method a step further and used a high-gain limiting amplifier on the carrier channel. Combined with a narrow filter, the limiting gain amplifier produced a strong, clean copy of the carrier for re-injection. The advantage of the limiting amplifier approach was simplicity and it works well. My attempt to implement synchronous detector used this approach with the old Plessey SL624 integrated circuit. I hooked this up to the 100 kHz IF output of my Racal RA-17 and it gave me some great audio through a separate audio amplifier.
Implement Synchronous Detection Using a Phase Locked Loop
Thirdly, and perhaps best, is choosing to implement synchronous detection using a PLL and a voltage controlled oscillator. Phase locked loops will lock onto and track a carrier using a narrow bandwidth loop filter. Then, they automatically tune the VCO to the carrier frequency for injection into the demodulation chain.
When you see a Synchronous AM mode on most modern receivers, including portables, it is likely using the PLL method. The advantage of PLL is that it can track the original carrier down to very low levels. The problems of fading and weak signals just evaporate. The loop filter is very narrow, say 200 Hz, and ignores audio. This type of detector also works well with AM broadcasts that have a reduced carrier.
If you would like to learn more about these methods, check out this video.