Using a spectrum analyzer is the oldest, most straightforward, and most widely used means of measuring phase noise. The basic procedure starts with measuring the carrier power (Pc) of the device under test (DUT) as an absolute value in dBm. The next step is to move to a given frequency offset from the carrier—that is, to a point in the phase-noise sideband. Then, you’d measure the noise power (Pn) contained within a 1-Hz bandwidth at this offset.
Subtracting the carrier power (Pc) from the noise power (Pn) yields phase noise in units of dBc/Hz at the given offset. In almost all cases, this procedure is repeated at different offsets from the carrier with results presented graphically and/or as individual-spot noise values.