J. Kissel, T. Vo, R. McCarthy

After installing the new QPD.mdl library part and getting the channels reflected onto the the QUAD MEDM screens, we found nonsense coming out after the normalisation step of the signal processing. Here, "nonsense" is defined as 
Input to the normalization:
Pin = -2400
Yin = -2900
SUM = -10700
Output of the normalization:
Pout = Pin / SUM = 2.4e+9 = nonsense
Yout = Yin / SUM = 2.9e+9 = nonsense

Given that this library part is working quite well for the rest of the LIGO world that's using it, it implies that there's something screwy going on with our signal chain.

We then tried turning off the signal input and inserting offsets with small (off order 1), known values to provide simple tests to see if the normalization was functional. With just offsets, the signals make sense. DAH! After a little more head scratching and a few more rounds of guess and check, I realized the difference between the input signals and the offsets I was trying: the raw input signals are negative (which Thomas informs me is expected from the electronics). *ACHOO!*

Inverting the sign in the SEG (input filters), all works great. My guess is that the RCG Saturation Part, which has its limits set to [1e-6,1e12] treats the negative numbers as the lower limit, and divides the signal by 1e-6. (Hence 2400 / 2.4e9 = 1e-6).

An analog signal chain red-herring that led to some interesting discoveries:
We'd suspected initially that there was some high-frequency content saturating the ADC (that made it not visible in EPICS variables). A spectra of the raw segment channels, up to 7 kHz, later revealed no such features.

However, while we still suspected this was true, we dove into the whitening chassis just to better understand the signal chain (since it's the only thing between the QPD and the AA chassis). We discovered that, though originally designed to have one, the ISC whitening board (D1001530), which is internal to the Oplev Whitening Chassis (D1100013), did NOT have its daughter board that fixes the state of the typically-digitally-controlled, switchable-state filter board. Mohana had some extra, and will ship some over night. Once those are in place, we can play around with the whitening and gain to find a good state in which we can leave it.