I measured the OLTF of the SHG and IRMC using the old spectrum analyser. I used two methods:

1) In FFT instrument mode, apply white noise to Perturb IN, measure Ch2/Ch1 (EPS1/EPS2) frequency response and coherence using a range of noise injection levels. The noise -> FFT method is fast but less precise when it comes to measuring characteristics about peaks.
2) In swept sine instrument mode, apply swept sine to Perturb IN, measure Ch2/Ch1 frequency response and cross spectrum (swept sine doesn't have coherence option)

In the case of white noise injection, lock at 500 mVpk was a bit temperamental but the spectrum could still be measured. Swept sine worked for an initial injection level of 20 mVpk. For values higher than that, it would unlock when the measurement reached ~ 100 Hz (it starts at the max frequency and goes downward).

Magnitude, phase and coherence for the two cavities are plotted for various excitation levels (fig 1-6), and the best result is collected in a 3x1 image (fig 7, 8)

SHG has a somewhat low unity gain of 600 Hz, nearly 100x lower than previous (50 kHz). Phase margin is about 50 degrees. Maximum coherence is obtained for about 200 mVpk noise injection.

IRMC is very low. It seems a bit strange actually. UGF is around 10 Hz and coherence is quite bad for excitation < 500 mVpk, but it also unlocks really easily when excited at this level.