We had some fun with PDH box #1437 (CCW) on Friday. First, we realized that it had 2 poles and 2 zeros, so that it went as 1/f² up to the zeros for the cavity pole. We thought this was unintentional, so we went about changing it. We removed the feedback capacitor in the first filter stage of the box, rendering that stage a unity gain buffer, and then moved the zero of the remaining stage up to where our cavity pole actually is (see previous entry). The new transfer function is below.

It turns out that Rana knew very well that there were two sets of poles and zeros, which shouldn't matter as long as the gain was rolled back to 1/f by the cavity pole before the UGF. We may want to go back to the 2-pole/2-zero topology to increase our low-frequency gain, though since band between the poles and zeros is larger now (due to our shifting the zeros up), the phase will have room to get closer to -180 degrees before the zeros kick in; I will do some LISO modeling.

Here is a plot of gyro OLTF with the above box.

The PZT resonance at ~25 kHz is limiting our bandwidth at the moment. Here is a closeup (forgive the arcane frequency ticks).

This TF was taken with the PDH box gain set just below the point where the system oscillates (@ 25 kHz). We put in Alastair's artisan twin-T notch filter to try and notch it out. This worked reasonably well, but we then ran into problems with a lesser resonance around 17 kHz or so. We may have to make another filter stage for this one (and perhaps fine-tune the original to better match the true 25-kHz resonance). Below is the OLTF with the notch.

The UGF is up slightly, but turning it up any higher than this gives us a 17-kHz oscillation. I am a bit confused about the gain-phase relationship, as it seems---that just below the gain setting that gives us an oscillation---the peaks that are just below unity gain have a reasonable phase margin, while once they hit unity gain the phase jumps all over the place. Need to do more futzing.