Related log(s): 21131
Summary. - Today, I have updated the first anti-whitening filters of the OMC DCPDs in the OMC digital front end model in order to make them more accurately match the actual analog circuits. The attached show screenshots of the foton filters before and after the update.
Impact. - The new OMC DCPD responses are different from what they used to be by 1% at 10 Hz in magnitude and almost no change at 100Hz and above. The O2 DARM model must take this update into account. Note that even though the qualitative behavior of the mismatched anti-whitenings is similar to the large bias we have been seeing in the DARM response (for example 24569), they do not explain the bias (which is about 10% at 10 Hz in magnitude).
DCPD balancing. - According to my calculation, the balancing should be good at a 0.1% level without introducing an artificial imbalance in the OMC front end. So I removed the existing artificitial imbalace (-0.3%) and update the OBSERVE and DOWN SDFs. However, the imbalance should be experimentally double-checked and possibly re-adjusted.
Today, while Jenne and Sheila were re-tuning the OMC dither loops, I looked at the balance of two OMC DCPDs. The attached shows the null and sum spectra, and ratio between DCPD A and B.
The isolation between null and sum is as good as 66 dB, according to a injected line at 12 Hz (OM3 pitch by Jenne and Sheila). The two PDs match each other with a 0.1% level accuracy in magnitude and 0.1 deg level for the phase. This seems good enough for the moment. Though, we should check the responses above 100 Hz at some point.
