I've put together a summary block diagram of the FSS boxes (based on the schematics of our version). This summarizes all the stages of filter and amplification as they were in the original design.  There have been modifications since then which we are working to document.  Craig and I will look at the boards again tomorrow and stick a probe in to measure at various points to confirm.  Craig and I have also been looking at LISO files (and porting this to a python wrapper) and he has also almost got a full set of plotted liso TF for each board and each stage on each board: this will help us breakdown what is going on.  He will post soon.

We also need to follow through with improving the cross over characteristics of the loops. It looks like we should reexamine the choice of components for the boost that was installed (by Frank et. al) at U7 on the PZT path. Its not clear to me why they chose that op amp to apply boost, given that it is already handling a notch there. I guess its fine where it is, but the values should be looked at. The values of resistors and caps in the boost are also not the same between north and south path.  Maybe we could also look at lowering the second stage low pass (U9) in that path from 34 kHz down to maybe 15-18 kHz.   Evans, in his squeezer FSS modification, added a mini-boost to U8, which seems like a good place to do it: unless there is something like saturation and railing in the later stages of amplification there. I think they had their main boost on the RF board, which is a different make to ours; we have just one summing op amp in that path.

Then, in the EOM path we can increase the AC coupling (twice) by lowering C23 and C24 while raising the first stage pole up to a much higher frequency by lowering R19. Evans (post) did this by lowering R19  (also lowers gain), not sure why can't lower c15 but its already at 3.3 nF.

Another thing I don't understand is why the notch is so high in the PZT path. I would have thoughts the resonances and peaks in the laser PZT response would be much lower, on the order 10s of kHz.