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What is an AM Signal?

A regular AM broadcast signal consists of a lower sideband a carrier and upper sideband. 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. Additionally, a signal that is too close in frequency 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 original carrier is mixed with a very stable carrier generated by 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 radios, turning on the BFO also turns on a narrower filter which makes the sound more muffled (depending on the receiver).


For this to work, the radio must be very stable (not drift) and tuning must be precise; even a deviation of a few hertz might result in an odd sounding off tone signal.


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.


Are there any radios that can do this today?

2 very popular radios - the Icom R75 and the Tecsun PL-880 - had the ability to use Sync AM; in fact the PL-880 had the capability of using ECSS as part of its so-called hidden features. Unfortunately neither radio performs all that well using Sync AM. Most people that have reviewed the R75 has noted the Sync AM on the R75 as merely an afterthought. In the R75, it was actually removed in some versions and brought back later. There were some outboard boxes for the R75 such as those from Sherwood Engineering, but they are very pricey and it can be somewhat involved to wire them in properly.


However the Tecsun PL-660 is reported to be much better at ECSS. But to see radios that had this capability that worked well, we must look in the past.


The king of ECSS capable radios in the portable world is the long-discontinued Sony ICF-2010 (also known as the ICF-2001D in some parts of the world). The Sony ICF-7600G was also very good. For portatops (radios that marry the characteristics of a desktop with a portable) the Eton E1 (and its cousin the E1XM) also performed well. This is to be expected, as the E1 was a joint venture between Eton/Grundig and R.L.Drake. In fact the E1 was serviced by Drake engineers at one point. The later editions of the Drake SW8 and SW8A also had this capability.


In the desktop world, the Drake R8 series (particularly the R8 and R8B - the R8A's performance on ECSS was rated not as good as the older R8), the AOR-7030 and some of the Japan Radio Company radios (the NRD-535 and 545) also had this ability.


In the world of Software Defined Radios (SDRs) the following applications are known to support Sync AM and/or ECSS. If there are others not listed here please feel free to add them;