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...for instance, if you are talking about oscillators, then you might want to measure the phase noise at a spot frequency offset from the carrier. But this is just the phase noise on one side of the carrier, hence Single Sideband phase noise. The number would be something like -90dBc/Hz@100kHz away from the carrier. The measurement bandwidth is assumed to be 1Hz. However, if you want the real effect, you need to integrate the phase noise over a certain offset. So lets say you integrate the phase noise offset from the carrier from 100Hz to 10KHz. This is the integrated phase noise power on only one side of the carrier. There is an equal amount of noise on the other side of the carrier. So after the integration, you add an extra 3dB and you now have Double Side Band integrated phase noise.
Mixers, well... that's a little different. Steve Mass has the best explanation in his mixer book or we can continue the discussion here if you like.