RadioReference Wiki:Ka3jjz

What is an AM Signal?

A regular am broadcast signal consists of a lower sideband a carrier and upper side band. An AM signal is vulnerable to several problems. Fading can cause distortion of the signal; noise on either sideband would be heard on the signal itself. Too a signal that is too close to the desired signal will cause a hetrodyne which is heard as a whistle or tone. Are there any techniques to help resolve this?

Enter Synchronous AM

Using synchronous AM (sometimes referred to as AM-Sync, Sync AM, S-AM or AM-S), the carrier is replaced using a circuit known as a synchronous detector. As this signal is very stable, it is not subject to distortion because of fading. Another side benefit is that the audio is generally more pleasant to hear, as any distortion is virtually wiped out by the stability of the inserted carrier.

You can accomplish something similar by using a Beat Frequency Oscillator (BFO) which is nominally used for hearing sideband (such as from hams). You tune to zero beat the carrier (you will hear a whistle as you tune; the tone will decrease as you get closer and closer until the tone is gone). Unfortunately on some receivers, turning on the BFO also turns on a narrower filter which makes the sound more muffled (depending on the receiver).

Using Exalted Carrier Single Sideband

This technique (often abbreviated ECSS) depends on the receiver's selectivity as well as being able to select one sideband or the other. Either using the BFO or a true synchronous detector, you tune away from the interference by selecting one sideband or the other. You would then use other controls (like the passband control found on the Drake radios) along with notches to remove any remaining hetrodynes. Properly doing this can really clean up a signal. Tuning like this can be reported as ECSS-U if you were listening on the upper sideband, or ECSS-L if listening on the lower.