Last week we injected some noise on the SSFS at recombined (entry 41411). This allowed us to tune several noises to try to characterize better the loop. I took a look today to these measurements, but the coherence is not very good, since when we are in recombined, the corrections sent are very high and we soon saturate.

However, I also took a look to the measurements took by Antonino and Annalisa the same day in the afternoon, this time in Low Noise 3 (entry 41417). As in Low Noise 3 we have much less correction being sent the noise injections are very good, and I tried to make a first estimation of the status of the loop by comparing them with the modelled OLTF (FIgure 1). Notice that at this point the third order boost was already engaged (with some extra digital noise which should be lower now).

• The UGF at the moment of the measure is 8.5 kHz. The phase margin is not veyr big (~20deg) and the high gain stabiilty margin is ~15kHz. We should be able to increase the gain up to 10-12 kHz.
• To match the measurement it was necessary to add a delay of 11 us to the SSFS loop (in the past it was 10 us).
• I also tried to fit the crossing frequency between both loops (IMC and SSFS) by fitting the structure we see in the phase between 800 and 2 kHz. I had to decrease the crossing point by a factor almost 2 to fit this feature, which would correspond to a crossing frequency of ~70 Hz. However, this is not in agreement with the adjustment we had to do to get something stable (we have increased the gain of the IMC loop by a factor 2).
• Notice also that there are two new structures around 1kHz!

I would like to understand better the changement on the crossing point. For this I think we need to make an extra noise injection at lower frequencies (this would make more clear the position of the crossing gain, since we would see it on the amplitude) and also a measurement of the loop of the IMC/RFC to cross-check the model.