SINGLE SIDE BAND SUPPRESSED CARRIER (SSB OR SSSB) BASIC INFORMATION

The advantages of single sideband suppressed carrier transmission over double sideband AM are:

• removal of the carrier saves 2/3 of the total power

• removal of one sideband saves 50% of the remaining power

• an SSBSC transmitter only produces power when modulation is present

• the occupied bandwidth is halved; a spectrum saving

• the received signal-to-noise ratio is improved by 9 dB for a 100% modulated carrier. Halving the bandwidth accounts for 3 dB, the remainder from the improved sideband power to total power ratio. The S/N ratio improves further with lower modulation levels

• reduced susceptibility to selective fading and consequent distortion.

Two methods of generating a single sideband wave are in general use. One filters out the unwanted sideband after removal of the carrier by a balanced modulator. The other is a phase shift method.

Here, the modulating signal is fed to two balanced modulators with a 90◦ phase difference. The output from both modulators contains only the sidebands but, while both upper sidebands lead the input carrier voltage by 90◦, one of the lower sidebands leads it by 90◦ and the other lags it by 90◦. When applied to the adder, the lower sidebands cancel each other while the upper sidebands add.


A single sideband AM wave modulated by a sinusoid consists of a constant amplitude signal whose frequency varies with the frequency of the modulating wave. Note that this is not the same as FM: the frequency in SSB does not swing to either side of the carrier.

It is higher than the carrier frequency if the upper sideband is transmitted, and lower if the lower sideband is selected. The single sideband waveform is sinusoidal and, although the frequency of the reintroduced carrier must be highly accurate (±2Hz), the phase is unimportant making a single sideband receiver less complex than one for DSBSC.

On some systems a pilot carrier is transmitted and the transmitter output power is then specified in terms of peak envelope power (pep), the power contained in a wave of amplitude equal to the pilot carrier and transmitted sideband power. Where no pilot carrier is transmitted, the power is specified as peak sideband power (psp).