Dan, Kwiamu, Sheila

We noticed last night that we had almost no gain at low frequencies in the DHARD loop.  This is a little strange, and doesn't really make sense with the suspension model and our control filters.  The beofre measurement, which doesn't have great coherence at low frequencies is shown in the first attachment. We have a ugf around 3 Hz with about 27 degrees of phase, however we also probably have some lower ugfs and not much DC gain.  This doesn't make much sense given our control filter (alog 17006).  

We added a boost, which is two poles at 0.001 Hz, and a pair of complex zeros at 1.5 Hz with a Q of 3.  The situation for this boost is a little tricky, we can't afford to loose gain below 1 Hz and we can't afford to loose the phase that we would loose if we used real zeros. We ended up with the loop shown in the second attached screenshot. We have 3dB of gain  margin on the low side, 6 dB of gain margin on the high frequency side, and 17 degrees of phase margin.  We could probably make this more stable by moving up the roll off of our control filter, which currently has a pair of conplex poles at 12 Hz.  

We are leaving the loop as it was for lock acquisition, the unconditionally stable loop is usefull durring CARM offset reduction, but engaging this boost on resonance.