1. A method of compensating for phase noise added by a spectrum analyzer to measurements of phase noise of a signal under test (SUT) taken by the spectrum analyzer, the method comprising the step of: applying a correction to a measured phase noise L(f m ) value for the SUT to determine an actual phase noise L A (f m ) value for the SUT, wherein the correction comprises mathematically removing an added phase noise L SA (f m ) value contributed by the spectrum analyzer from the measured phase noise L(f m ) value of the SUT.