That roll up in the HF range to 3 kHz hump looks suspect. 

PLL?

We need to check that we're actually implementing the PLL properly and unwrapping the BN PSD properly from the actuation signal.  You could end up from a result like this if the UGF was, say, ~ 1 kHz: then the PSD * (marconi slope) approximation will no longer be true and would look more like PSD * (marconi slope + 1/Mix*SR560).  As demodulation for PLL frequency gives a 1/f in closed loop (from freq->V_out), this would give a roll up directly proportional to frequency around and above UGF when inverting. We should go back and check again that the transfer function measured yesterday of the PLL had a UGF of >60 kHz. You should remeasure the PLL open loop gain and post it on the elog. Make sure you use a small excitation signal and play around with the cycle averaging settings to smooth out noise for a nice clean trace. Also a schematic of exactly where signals are injected, values of gains, slopes and mixer conversion efficiencies (spec sheet value will do) and RF power from beat note detector.  

You'll also way to check that you're not saturating the NF1811.  You don't want weird artifacts from not being linear in response at RF... slew rate limit etc. Details of internals of NF1811 can be found in the PSL Electronics Wiki.

Just Marconi noise?

Also see figure 4.8 of Tara's thesis.  I'm having trouble making out the colours but there is peaking there in the marconi frequency noise at around 3 kHz. This plot places the 10 kHz/V modulation slope as being clear by at least an order of magnitude.  Always be suspicious. Are our macnoni's still performing?  Add this to the moderate-to-low-priorities list of things to check.  

RF AM again?

The BN PSD doesn't extend high enough to compare fully to all the others but if we find that the PLL -> BN is correct then maybe this is an old problem related to excess AM from the 36 MHz and 37 MHz EOMs producting AM and PM.  See PSL:2083 and related backlinks.  In particular we want to know if the laser RIN has some coherence with the BN.  See what Evan did in PSL:1524.  Before screwing around with optimizing the AM (maybe a 1-2 day task) figure out how you can use the transmission signals to get a RIN of both cavities and make a coherence plot.  This would be your smoking gun.  After that maybe look into whether you should sink time into optimizing RF AM and ISS. 

Checking RIN and coherence with the beat note is maybe a 1/2 day task once everything is locked up and working.

Also, put those thermal hats back on those EOMs.  Its winter, have a heart.