Dan, Keita, Kiwamu, Sheila
During a long, patient lock this evening I was able to measure the DHARD pitch loop down to 0.2Hz. This follows Keita and Sheila's filter modifications to get some additional phase around the 3Hz UGF. The attached plot is a record of the measurement (look at the RED trace), I have saved the xml file with the filename at the top of the plot.
The phase margin at the UGF is good (~40deg), and the loop does not cross unity gain at higher frequency. There is almost a unity gain crossing at lower frequency, we have about 3dB of gain margin at 0.9Hz.
We're fine without aggressive boost.
It's clear that the boost (FM6) was not on in this measurement.
Yet, the measured TF looks OK in that even if the dip at around 0.9Hz changes somewhat and crosses the unity gain, it will be very stable.The phase margin at around the dip is between 140 and 180 degrees.
Also the phase at UGF was improved by 10+ deg due to the new FM2 and by disabling redundant notches (FM7 and FM9).
With the boost, we'll get close to 50dB gain at 0.1Hz at the expense of 13 degree phase at UGF, about 7dB gain at 1.5Hz peak, and 2dB or so higher high frequency (f>7Hz or so) response. That sounds kind of excessive to me.
Since the second UGF at 0.9Hz will not be a problem I'd rather leave that guy off. If we need more DC gain we can make a milder boost without messing with the gain at 0.9Hz.