As seen previously, there are many different ways of transmitting information via voice on a radio transmission. This article is concerned with the more complex component of an Amplitude Modulated transmitter, called SSB or single sideband. This mode is most commonly used for HF communications and is very popular with radio amateurs wishing to talk long distances (DX)

 

SSB (Single Sideband)

Single Sideband was developed in the 1920’s, following a patent registered in 1915, but didn’t really make an impression on the amateur communications arena until the 1950’s. This was mainly due to the level of technology catching up with the requirements to implement the mode.

Single Sideband is 1/3 of a modulated AM signal envelope. When audio is applied to a radio transmission, there are 3 components to the resultant signal. These are: the carrier, which contains no information but is the reference for the signal, and the upper and lower sidebands, which contain the same information as each other.

So, if we were to remove the carrier (which contains no information) and one sideband (seeing as the sidebands are identical), we could have a transmitter that uses considerably less power to produce the same output power, or for the same input power, could have up to 3 times the final transmitted output power.

This is a very simplistic view on SSB. In reality, it is relatively complex to create a single sideband transmission. There are two reasonably common methods of creating an SSB signal, with this first method being the most common. The simplest method is to feed the audio signal and the oscillator into a “balanced modulator” (enhanced mixer) which is configured to remove the carrier part of the signal. The resultant “double sideband” signal is then fed through a crystal filter with a very narrow (approx 2.4Khz @ 6db) bandwidth and very steep skirt response to remove the unwanted sideband.

Generally (although not exclusively) there is only 1 filter, and the frequency of the generated signal is shifted up or down to allow the filter to remove the unwanted sideband. Certain high grade transmitter manufacturers have included 2 filters, but these transmitters often have the facility to switch between variations of the SSB mode, such as: SSB with no carrier (the most common), DSB (double side band) with no carrier (quite popular on the HF Amateur Radio bands, SSB with reduced carrier (used by some HF stations to allow easier reception by having a percentage of carrier in the signal as reference) and DSB with reduced carrier.

The second, and possibly less common method of SSB generation is to use 90degree phase separation modulators. These use a network of amplifiers and capacitive/resistive filters to alter the phases of the audio information component and the generated carrier until the carrier and the unwanted sideband have cancelled each other out and the only component left is the wanted sideband. In years gone by, this method wasn’t popular due to the high component count, and the inability to get cheap close tolerance capacitors to create the filtering networks. However, with the advancement of digital signal processing (DSP) electronics, this method is finding more popularity due to the reduced cost of implementation. Common forms of this type of modulator are the Hartley and Weaver modulators.

Reception and recovery is also more difficult than for an AM signal. Because there is no carrier for the receiver to reference against to recover the audio, a carrier insertion method has to be employed. This is usually called a BFO or Beat Frequency Oscillator, and inserts the carrier normally at the second IF (intermediate frequency) in a super-heterodyne receiver, which is often at 455khz. On most communication receivers, there is a control known as “clarifier” or “fine tune”. This moves the received signal around the 455khz BFO and allows the detector to “de-modulate” the audio by making it more intelligible. After the carrier reinsertion, the detection process follows the same path as the AM detection. This method is known a synchronous detection and is highly reliant on oscillator stability.

There are other methods to de-modulate an SSB signal. One way used by high grade communications receivers is the envelope detection method. As above, this method is now starting to become more popular following the advancement of DSP

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Filed under: "How To" Radio

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