The cross correlation technique in combination with external harmonic mixers enables to measure low phase noise signals much easier, faster and more accurate than ever before.
https://www.youtube.com/watch?v=1tMiAMOjYK0
Phase noise measurements beyond 50GHz
The R&S FSWP phase noise and VCO tester from Rohde & Schwarz measures phase noise with high accuracy at frequencies beyond 50GHz.