Peter, Sudarshan, Gabriele

Today we installed the ISS second loop servo board and connected all the cables to the board itself and to the photodiodes. 
After some tweaks, we could get all the eight photodiodes showing some reasonable signals.

The attached plot shows the signals measured on the eight photodiodes, still calibrated in counts. PD1-4 shows similar signals, although the gains seem to be different up to a factor 2. The same is true for PD5-8: they are quite similar one to the other. However, it seems that the signals on PD1-4 are a bit different from the signals on PD5-8. 

We also checked that the ISS QPD is cabled and working. As shown in the attached file, the QPD signals show some resonances at 1, 3.4 and 16 Hz, which are likely coming from motion of some optics. It is interesting to note that the same peaks are very well visible in all eight photodiodes. We don't know yet if the beam entering the ISS box is already intensity modulated at those frequencies, or if we have some jitter to intensity coupling at the ISS level. It seems that all PDs see the jitter peaks at a similar level, so the coupling should be in the common path.

There is also very good coherence of all PDs with the QPD signals, at all frequencies below 10 Hz.

We started investigating the transfer function of the transimpedance and whitening, to properly calibrate the signals in terms of dP/P. To be continued.