I'm implementing the suggested cap and resistor updates in the last post (PSL:2301).
I couldn't find any 100 nF or 33 nF caps in polycarbonate in WB EEshop. These haven't been manufactured since 2000 and I couldn't find stock at the 40 m either. Instead I used polypropylene as they have comparable specs (as far as I can tell) and should be good up to ~1 MHz (I think). If not, its easy enough to fix.
Replacements on D040105-C FSS servo board:
I also checked the boost that hacked onto U7 of the FSS servo board. This is a capacitor in series with R29 (5.6 kΩ). I checked the actual value it is 6.8 nF in the North FSS box. This combination (5.6 kΩ + 6.8 nF) in the feed back path of U7 gives a pole at 4.18 kHz. This should be enough to give an increase of gain of 10 dB by 418 Hz and 16.2 dB by 100 Hz. This is probably more than we need. It could be shifted down to 1 kHz (28 nF) if we do eventually reach 1 MHz UGF.
The loss of gain from changing R19 means that we are getting a factor 12.45 less gain in the EOM path. I turned the common gain slider up to max of 10V (+32 dB) and see a UGF 224 kHz. The loop is stable with boost on and fast gain set to -1.07 V and also with the boost off; the cross over for this configuration of gains is 11.3 kHz. The RMS noise on north PDH error monitor is 19.4 Vrms (just looking on the oscilloscope). It seems pretty stable for now, I'll take a TF later after a few more tweaks of values. Also its late and I want to go home.
Now we have a problem with gain, we need more of it. It looks like that if we could find gain of +10 dB somewhere we could push the UGF out to 593 kHz and have a healthy phase margin of 42 deg. If we can find more gain we can probably go further. The phase margin gets to 30 deg at 806 kHz so that might be a logical point to stop. However, there appears to be a little peaking at 750 kHz that we need to watch out for. Not sure what it is, don't think I've seen it before. We'll have a closer look at it when it becomes a problem, but we might need to reexamine tuning of notches or putting another notch in if the combination of OLG UGF and and boost mean we need to push out this far.
We could collect error signals and actuator signal PSDs and model the optimal ballance and crossover between PZT and EOM, but this will take weeks to build an actuate model. For now its probably faster and easier to see empirically if the EOM can handle the higher gains without running out of actuation range.
In terms of finding more gain, we can get a bit by boosting do any of the following: