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Difference between absolute SSB/residual SSB phase noise?

Question asked by erikdiez Employee on Jan 21, 2010
In the data sheets for the E8257D, E8267D and E8663D PSG signal generators, I see that for option UNX (ultra low phase noise), there are separate specification tables for Absolute SSB phase noise and Residual SSB phase noise. What is the difference?

Absolute phase noise is the phase noise measured on the output signal of a one port device. Generally, when phase noise plots or tables are shown on the data sheet of an oscillator or signal generator, it is the absolute phase noise measurement data.
Residual phase noise is a measure of the noise added to an input signal by a 2-port device. Some signal generators specify the residual phase noise in their data sheets. Think of the signal generator as a two port device (in this case, a frequency multiplier) where the “input port” is the external frequency reference (generally 10 MHz) and the “output port” is the RF output. 
Having residual phase noise data enables users to calculate the absolute noise of their signal generator at the RF output when using an external 10 MHz signal as the reference input.  Remember that the noise of the external frequency reference will be multiplied up to the output frequency and increase by 20 log(N) where N is the ratio Fout/ Fref. This noise will appear at offsets less than the phase lock loop bandwidth of the narrowest reference loop in the signal generator and should be added to the residual noise at these offsets to determine the absolute phase noise of the signal.  For example, in the case of the E8257D PSG analog signal generator, the loop bandwidth in the 100 MHz reference loop can be set from 25 Hz to 650 Hz.
Because the phase noise of the reference oscillator is multiplied up, users should exercise care when using an external reference. Sharing a common 10 MHz reference signal among several instruments is a good way to ensure that they won’t drift in frequency relative to each other. However, depending on the quality of that reference, it may also degrade phase noise performance.