On mm-wave heterodyne measurements and source/LO signal cleanliness

As VNA-esque mm-wave measurements proliferate, often with highly multiplied signal sources, the question often arises on the required signal source cleanliness for a given level of performance. As applications increase, there is also the desire to use less expensive base sources which gives further impetus to the question. By varying certain closed loop synthesizer and drive parameters, the conversion efficiency of WR-2.2 and higher frequency systems were investigated over a range of phase noises. A fairly rapid conversion response change was observed at some frequencies with the incidence increasing with multiplier as expected. The frequency structure of the phase noise change and the correlation between source and LO noise appear to be as important as the absolute phase noise levels.