joshua.freed@LIGO.ORG - posted 16:45, Tuesday 16 June 2026 - last comment - 09:05, Wednesday 17 June 2026(90642)
SPI, Pathfinder Install, Issues with Saturating Oscillating FBR_PWRIN_REF.
J. Freed,
WE HAVE DISCOVERED NEGATIVE POWER
The FBR_PWRIN_REF PD that monitors the Power coming from the REF beam of the interferometer has gains alot of noise during certain intervals. Sometimes the noise even goes negative power. The noise is corelated with the frequency difference between the ref beam and the meas beam we set. The lower the frequency difference the higher the noise
SPI_PW_REF_Noise_Pt3.png Shows the correlation between Set fequency and the noise
SPI_PW_REF_Noise_Pt1.png is a close up of the transition. Since we have a 1sec ramp time the noise starts one second after setting a new frequency difference.
SPI_PW_REF_Noise_Pt2.png Shows that the noise is sawtooth at a frequency double the set frequncy.
I currently have no evidence of what this could be.
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Here's a screenshot of the oscillations stopping at 09:08:08 PDT (16:08:08 UTC). There's something in the MEAS signal as well, but the amplitude is less (at least with this 32 kHz sampling rate). Of course, nothing changed in the signal readout chains at this time. But lots of things are happening surrounding it. Also, the oscillations are different frequency in the MEAS vs. REF path (again, with the "this is clearly aliasing" grain of salt). But, since we're not saturating when the oscillations are NOT present, I don't think it's an issue with the transimpedance *resistor*; it's probably just that we need to double the capacitance, dropping the pole frequency from 13.5 kHz to 6.7 kHz. We'll put an analog o-scope on the signal this morning to confirm.
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