Single sideband generation, Part 2

The generation of single sideband (SSB) signals first came to my attention via ham radio back in the early 1960s. My call was then and still is WA2IBH. The best phonetic I had for that call sign was “WA2 I’ve Been Had” but that’s merely a side note.

Most voice communication through ham radio back then was done by amplitude modulation or AM signals. When you heard someone on the air with an AM signal, the voice quality was usually pretty good. As I recall, the E.F. Johnson Viking Ranger transmitter was thought of as having the very best audio quality. Of course, when you had many signals on the air at the same time with different carrier frequencies, heterodyne squeals were an unpleasant fact of life which often degraded the intelligibility of the person whom you wanted to hear.

Enter into service, SSB.

To demodulate an SSB signal, a receiver needs to reinsert a carrier signal to replace the carrier signal that the sender is NOT transmitting. The resultant sound is intelligible, but the idea of audio quality is a lost cause. A human voice in a demodulated SSB transmission is difficult to linguistically describe. Perhaps it might be thought of as listening to a cross between Donald Duck and Mickey Mouse. A big improvement, though, is that there are no heterodyne squeals. All you hear from multiple signals coming through at the same time are distorted but intelligible voices. This is a MAJOR improvement. However, the acceptance of SSB in ham radio was not universally enthusiastic.

Short-wave receivers produced up through the 1950s would have automatic gain control (AGC) built in, but the response times of the AGC function were not well suited to SSB service. Modern AGC designs have “fast attack and slow decay,” meaning that the receiver gain is reduced very quickly upon arrival of an overly strong signal and that receiver gain is subsequently restored slowly. Since SSB signals have amplitudes that are “spiky,” meaning high peak amplitude to average amplitude ratios, the AGC circuits of these older receivers could be “pumped” by SSB signals, even if the receiver were not tuned exactly to the SSB signal’s exact frequency. Reception of pretty much anything else could and often was very badly affected. Modern AGC control is much better.

Many non-SSB users confronted by AGC pumping incorrectly assumed that SSB users were guilty of “splatter,” the descriptive term for the spectral spread of an overmodulated (> 100%) AM transmission. Derogatory terms such as “splatter sideband” and “silly sideband” were in common use.

Today, ham radio voice communication is dominated by SSB.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

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