biff44
you cannot use spectrum analyzer for Phase noise measurement of 160 dBc, it's limited by its dynamic range not more than 155 dB in high end Agilent or R&S.
There is a possibility to do that with spectrum + notch + LNA setup (filter the carrier and amplify the noise with LNA), but it's a complex setup.
Latest VCO/PLL meter has this capability, but it's at the limit of its performance, can obtain it only with corrections (already implemented).
It's important ot note if you're interested in phase noise effects only or also to am/fm effects: with spectrum you cannot distinguish phase noise from am/fm conversion, you measure both; with dedicated phase noise testers, as they are based on a completely different way of measurements, you measure only phase noise.
I hope it can help.
Mazz