To get around the problems between the pomona LPF and low CM board input impedance, I've placed the LPF at the CM board fast output. This won't work as a permanent solution, since we only want to lowpass the ALS signal, but it should be fine for a single arm test. 

However, I kept getting blown out of lock when turning up the AO gain, but well before I really expect any real action from the fast path. Looking at the OLTF, I was seeing some large spike at ~36kHz nearing 0dB loop gain with unstable phase. This prompted me to look at the ALS error signal out to higher bandwidth with the SR785; before I only ever looked at it through the digital system. 

So, with the X arm locked via POX11 I, and ITMY misaligned to use AS55 as an out of loop sensor, I measured the spectrum of the I ouput of the ALS X demod board (which was set to be near a zero crossing via the delay line), and the Q Mon of the AS55 demod board. 

Both ALS and AS55 show a sharp line at around 36.5kHz, so something is really happening in the IFO at this frequency. Koji might have seen an indication of this back in March.

What's going on here? And what would be different about PRFPMI that wouldn't have made this a problem for locking?