There is a large wandering peak in the IMC error signal. (measured at Imon). A movie is attached. We tried several things to try to pin down where this comes from. This wandering peak also shows up in the end station PLL and goes away when the IMC is unlocked.
This afternoon I checked out some signals from the PSL to see if there were any similar wandering peaks in them and didn't see anything. I looked on the TTFSS Field box mixr(in2) fastm test2, on the ISS Photodiode A output Photodiode B output. PMC mixer out. Unless there are better signals to look at, I would conclude this isn't from any of the PSL loops, but the IMC itself.
Alexa and I also tried a few things to see if the wandering peak would go away under different conditions. It does sometimes go away, but this seems unrelated to the things we tried. We tried:
- turning off the ISS,
- turning off the slow feedback from the COMM PLL to the IMC VCO,
- turning off the COMM PLL,
- changing the gain on the FSS,
- unlocking the arm cavity,
- misalinging ETMX.
We saw the wandering peak in each of these situations. We also had a look at Imon with the IMC unlocked, it wasn't clear what we were looking at when the mode cleaner was flashing. We misalinged MC1 to look at the dark noise, and didn't see any wandering peak there. We also looked at the IMC open loop gain, there are no resonances or notches there, I remeasured several times and got the same result each time. (53kHz ugf 45 deg phase margin).
Alexa noticed that the UGF is higher than it used to be: she measured it in alog 10069 to be 28 kHz. The neither the settings on the MC board nor the transmitted power have changed.
We have not been able to do the COMM handoff tonight, although the arm cavity and COMM PLL seem to be quite stable.
We also attempted to measure the COMM PLL spectrum using the single shot green beam, to get a measurement of the laser frequency noise. the beat note was about 10mV peak to peak (measured with 1MOhm impedance on the scope) which was not enough for us to lock the PLL.
