Wednesday, July 7, 2010

What exactly is SSB?

Q This may seem like a silly question, but what exactly is SSB?

A First of all, there is no such thing as a silly question. In Amateur Radio we often kick these acronyms around without bothering to define them. We just assume that everyone knows what we’re talking about! SSB stands for “single sideband.” It is basically a form of amplitude modulation (AM).

Modulation is a mixing process. When RF and audio signals are mixed together you wind up with a total of four signals: (1) the original RF signal, or carrier, (2) the original audio signal, and (3-4) and two sidebands whose frequencies are the sum and difference of the original audio and RF signals, and whose amplitudes are proportional to the original audio signal. The sum is the upper sideband (USB) and the difference is the lower sideband (LSB).

All of the information is contained in the sidebands, but 2/3 of the RF power is in the carrier. The carrier serves only to demodulate the signal in the receiver. It is essential for AM reception, but for SSB we can simply generate a carrier signal in the receiver and use it to recover the information (the voice, for example). There is no need to send the carrier along with the sidebands. There is also no need to send two identical sidebands when one will do. Depending on the mode you’ve chosen to operate, the upper or lower sideband is eliminated from the transmitted signal. The final result of all this “paring down” is a solitary single sideband— SSB—signal.

There are several advantages to using SSB. SSB transmitters are very efficient, requiring less bulky power supplies. SSB signals are also narrower than standard AM signals, meaning that you can fit more SSB signals into a given band. With the transmitted power concentrated in a narrower spectrum, the effective communication range can also be increased somewhat compared to AM or FM.

The main disadvantage of SSB is its relative lack of audio quality. You must tune your receiver precisely to achieve a natural-sounding voice. Even then, the frequency range of the audio is limited. A properly tuned SSB signal is perfectly readable, but it does not have the audio quality of AM or FM.

From QST January 2000