Regarding PN measurements, as mentioned by rf-loop the SA must have a significantly lower internal PN that the subject. Achieving this is difficult and expensive and why specialized instruments capable of accurate PN measurements are available (and very expensive).
An interesting story on PN measurements back in late 80s we were developing a new tiny low PN microwave oscillator based upon a newly developed "Over-moded Bulk Acoustic Wave Resonator" created by Dr Ken Lakin. We didn't have the proper PN measurement capability in-house and I had just returned from an IEEE MTT conference where MA-COM described a PN measurement technique with some custom developed circuits that could be preformed with in-house equipment generally available. We engage our university with a grant to fund the development of this new type PN instrument technique, I later hired the brilliant grad student that did the work
The concept basically works like this. You need two identical subject oscillators which are coupled into a high quality passive mixer at the RF and LO ports, the signal level needs to be sufficient to turn on the mixer diodes. The IF port is low passed filtered with a passive LC filer which has low DC resistance (resistance contributes additional noise) and has a BW of the desired PN measurement BW. We initially used a 4th order capacitive input Butterworth LC at 100KHz. The output of the filter then is measured with a low frequency high dynamic range analyzer. You may need to include a low noise buffer amplifier before the analyzer, we used 40dB gain custom designed ultra-low noise amplifier with very low 1/f noise.
Since the two oscillators will injection lock thru the mixer port to port leakage, the mixer IF port output will be the sum of the two oscillators PN and thus 3dB higher since F1-F2 is zero and F1+F2 is filtered by the low pass filter. So each individual oscillator has a PN that is 3dB below the measurement, plus the mixer loss must be added (~7dB).
The setup required battery power and the two oscillators placed in a thermally isolated box to get reliable measurements which were about -140dBm (~22nv/rtHz) at 10KHz offset. Later we gained access to a proper HP PN measurement instrument and confirmed this measurement.
Best,