Single sideband modulation is basically double-sideband modulation with one of the sidebands removed. This means that a single sideband signal takes less bandwidth than a double sideband signal; all of the information in the message signal is contained within both of the sidebands, so it is completely acceptable to eliminate one. Practically, single sideband modulation is harder to implement than either DSB or AM. It is also more difficult to understand, and the math is more difficult. Its big advantage is the bandwidth savings it provides, and the fact that it is much more efficent than AM. SSB modulation has a 9 dB signal to noise advantage over AM, meaning that at -6 dB an AM signal becomes unusable, but a SSB signal can still get through.

Single sideband modulation can be implemented in at least two ways. The first way would be to multiply the message signal by a carrier, which gives you a DSB signal. A filter can then be applied to remove one of the sidebands. Practically, it is very difficult to implement a filter with a sharp cutoff, so distortion results. If this method is used, the filter is usually built using crystal filter technology. Mathematically, this modulation of a message signal can be express by using the Hilbert Transform.

Phase-shift modulation can also be used to produce a single sideband signal. This works by taking the two copied of the message signal, one of which is multiplied by a carrier cosine, while the other is transformed by the Hilbert Transform before being multiplied by a carrier sine. These two signals are then added to or subtracted from each other to yield lower sideband or upper sideband signals.

Several methods may be used to demodulate single sideband signals. The first method is very similar to that used for DSB signals; just multiplication by a demodulation carrier and lowpass filter the result. As for DSB, if the phases of the modulation carrier and the demodulation carrier aren't aligned with each other almost perfectly, serious distortion of the message signal can result.

Another method of SSB demodulation is called carrier reinsertion. This involves adding the output of an oscillator to the received signal, and then feeding that signal into an envelope detector, the output of which is

yd(t)~= (1/2)Acm(t) + K

where m(t) is the original message signal, and Ac and K are constants. The message signal can then be extracted from the envelope detector output with relative ease. Again, frequency and phase of the demodulation carrier and the modulation carrier must match up.


Credit goes to Rancid_Pickle for the signal-to-noise ratio information. Thanks!

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