J. Kissel, S. Dwyer, L. Barsotti

At the advice of DetChar (LHO aLOG 29828) we've balanced the DCPDs, which indeed hasn't been done since the new breadboard and PDs were installed (LHO aLOG 28862). Unlike the previous pair of DCPDs which were out-of-the-box perfectly balanced (see LHO aLOG 17650), the current pair has an imbalance of 2.1106%. This value has been entered into H1:OMC-DCPD_BALANCE, and accepted into the SDF "down" state for the h1omc model.

Method:
For the DCPD sum, we take an average of the two: A/2 + B/2.
The h1omc front-end infrastructure is built such that the percentage imbalance, e, is applied to both equally: A*(1 + e)/2 + B(1 - e)/2
So, the transfer function measurement of DCPD A/B is (1+e)/(1-e) ~ 1 + 2e. Since the transfer function magnitude is 0.957788, then the imbalance we enter will be e = ((A/B) - 1) / 2 = 0.021106 = -2.1106%.

We also found this value gradually (because we weren't sure how the infrastructure worked), by confirming the imbalance that minimized the DCPD NULL stream (and the coherence drops to zero). Overshooting the imbalance (e.g. by entering in -4.2%) shows a clear sign flip from the reference trace where there is 0% imbalance compensation.