How to select offset frequency for phase noise measurement?
For many systems (such as GSM, where an integrated phaseNoise of 8 degrees may be the system bit_error budget for the LocalOscillator under Synthesizer feedback control), the entire phase noise curve, from DC out to Symbol Rate, may be documented.
So ---- it depends on your purpose, on your system.
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Additionally, the wideband noise, out past symbol rate, will FM/PM modulate the transmitter power, and cause de_sensing of close_in channels.