Yesterday afternoon we reconnected the ADC to the whitened signals for PD5-8. Now the status is the following:

• PD1-4: we are acquiring the direct output of the transimpedance, before the whitening. We can use those signals to check the DC level, but we can't see the AC noise
• PD5-8: we are acquiring the whitened output of the boards. We can see the AC noise, but no DC signal

I wanted to compare the RIN meaured by the first loop photodiodes PDA and PDB with was is measured by the second loop photodiodes. However, looking into the way PDA_REL and PDB_REL are reconstructed, I found a couple of strange things, and I'm quite convinced that the estimated RIN was wrong. Peter King computed the whitening transfer function of the PDA and PDB box. Comparing this with what was implemented in the PD?_CALI_AC, I found a quite large discrepancy I could not understand. Therefore I implemented an inverse of the expected whitening, which is good between 1 Hz and few kHz, as shown in the first plot. I also found that the low pass filter used in the PD?_CALI_DC was giving me a strange notch at about 10 Hz in the PD?_REL signals, so I changed it into a simple low pass, as visible in the second plot. For both the AC and DC parts, the old filters are still there.

The second loop photodiode PD5-8 outputs are dewhitened as explained in a previous entry. The output is calibrated in V, and the DC level that we measured before changing the ADc connection is about 3 V.

The third plot shows the RIN as visible in both the first and second loop photodiodes when the first ISS loop is open. In particular, we see that the second loop photodiodes are in good agreement with the first loop photodiodes, at least above 100 Hz. This is a cross check that the calibration in terms of RIN is, if not correct, at least self consistent.

The fourth plot shows what happens when the first loop is closed. As expected the out of loop PDB shows a flat RIN that I guess should correspond to the shot noise level, even though I don't know right now how much pwoer is getting into the diode, so I can't teel if the absolute level is correct. Looking at the second loop photodiodes, it is clear that we have a huge excess of noise with respect to the level obtained at the output of the first loop. At this point we can't tell if this is real intensity noise, or just jitter noise that couples to the second loop PDs. Recall that we didn't optimize at all the coupling of jitter to RIN for the second loop photodiodes. For sure, there's still a lot of work to do...