Comparing to Den's LLO alog 12590, we have about 10 times more DIFF noise at 10Hz, PLL locked, DIFF not locked. This turns out to be crucial for actuation range. So we looked at our DIFF noise using a simulated beat note, generated using an FM modulated RF source.


The first two traces of the attached plot show the noise using a fixed-frequency (unmodulated) RF source. The first (red) uses the additional /10 frequency difference divider (FDD), the blue doesn't. note that the blue trace is pretty close to the noise seen at LLO.

Traces 3 and 4 show the noise using an additional 0.3Hz, ~45kHz amplitude FM modulation, again with and without FDD.
While the signal seems suggestively close the our in-lock signal (black), the interpretation is not straight forward.
 - We noticed that the hight frequency increase was at least part to an imperfect digital modulation drive of the RF source. switching the RF source to an IFR affected the noise level at high frequency.
 - Up to about 10Hz this seems to be an (unresolved) harmonic forest from up-conversion. Unfortunately I can't say for sure from which source - the IFR actually gave me worse noise at low frequencies.