Sudarshan, Darkhan, Rick, Sheila, Rana, Evan
Summary
For a little while we've been worried that excess RIN might be causing some of the difficulty in transitioning the CARM sensor from ALS COMM to sqrt(TRX+TRY). Based on the measurements here, it appears that RIN from the PSL is not an issue. However, we're continuing to investigate whether the RIN further downstream of the PSL will cause us issues.
From 1 Hz to 100 kHz, the rms RIN is about 5×10−4. For a power recyling gain of 30 W/W and a CARM offset of 300 pm, we expect sqrt(TRX+TRY) to have a value of 1.7 ct and a slope of 5.6×10−3 ct/pm. The RIN therefore appears as a 8.5×10−4 ct rms fluctuation, which propagates forward to a 0.15 pm rms fluctuation in CARM.
Details
Sudarshan, Darkhan, and Rick spent some time diagnosing the health of the first ISS loop (using ISS PDB as the in-loop sensor, and ISS PDA as the out-of-loop sensor). They turned down the gain of the OLTF by approximately 10 dB in order to reduce the UGF from 80 kHz or so to more like 25 kHz (so that it sits right in the middle of the loop's phase bubble, where there is about 45 degrees of phase). The OLTF is shown in an attachment. Next, we took spectra of the in- and out-of-loop sensors. The dc voltage on both was 2.2 V.
Initially there was some uncertainty (and perhaps there is still some uncertainty) about the frequency-dependent shaping of the ISS PD signals. Based on the schematic in D1001998, I claim the shape is a dc gain of 0.2 V/V, two zeros at 0.07 Hz, two poles at 3.1 Hz, and a pole at 135 Hz.
