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Single-sideband modulation

Single-sideband modulation (SSB) is a refinement of the technique of amplitude modulation designed to be more efficient in its use of power and bandwidth. It is closely related to vestigial-sideband modulation (VSB) (see below).

Amplitude modulation typically produces a modulated output signal that has twice the bandwidth of the modulating signal, with a significant power component at the original carrier frequency. Single-sideband modulation improves this, at the cost of extra complexity.

The best way of thinking of SSB modulation is to first consider an amplitude modulated signal. This will have two frequency-shifted copies of the modulated signal (the lower one is frequency-inverted) on either side of the remaining carrier signal. These are known as sidebands.

To produce an SSB signal, apply a filter that will filter out one of the sidebands, and remove the carrier signal. What remains still contains the entire information content of the AM signal, using substantially less bandwidth and power, but cannot now be demodulated by a simple envelope detector.

An alternate method of signal generation has been gaining popularity recently in part due to the availability of low-cost DSP systems. To generate an SSB signal with this method, you first generate two versions of the original signal which are mutually 90° out of phase, usually by implementing a Hilbert transformer in a DSP. Each one of these signals are then mixed with carrier waves that are also 90° out of phase with each other. By either adding or subtracting the resulting signals, you can generate a lower or upper sideband signal.

When the 'wrong' sideband is only partially suppressed, the resulting modulation technique is known as vestigial-sideband modulation (VSB).

The front end of an SSB receiver is the same as that of an AM or FM receiver, consisting of a superheterodyne RF front end that produces a frequency-shifted version of the RF signal within a standard IF band.

To recover the original signal from the IF SSB signal, the single sideband must be frequency-shifted down to its original range of baseband frequencies, by using a product detector which mixes it with the output from a "beat frequency oscillator" (BFO).

For this to work, the BFO frequency must be accurately adjusted. If the BFO is mis-adjusted, the output signal will be frequency-shifted, making speech sound strange and 'Donald Duck'-like.

Note: SSB and VSB can also be regarded mathematically as special cases of quadrature amplitude modulation.

SSB as a speech scrambling technique

SSB techniques can also be adapted to frequency shift and frequency invert baseband waveforms. These effects were used, in conjunction with other filtering techniques, during World War II as a method for speech encryption. Radio telephone conversations between the US and England were intercepted and 'decrypted' by the Germans; they included some early conversations between Roosevelt and Churchill.

Today (2001), such simple 'inverter'-based speech encryption techniques are easily decrypted using simple techniques and are no longer regarded as secure.

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