Jeff K, Chris Whittle
We used the interometer uptime to test the ITM charge measurement script by taking data for ITMX. With the previously used excitation frequency of 20.1 Hz being used for other measurements, we opted for an excitation at 11.5 Hz. At this frequency, we found an excitation amplitude of 1k counts to be sufficient for a good SNR. We have taken data at bias voltages between -10k and 10k counts, but have yet to perform post-processing.
We additionally tested whether flipping the analog voltage out switches to the quadrants had any effect on the locked interferometer. No effects were observed for flipping on/off quadrant voltages in various orders. This test was performed on just ITMX and with no actual output voltage to the quadrants.
Plotting the PSD for each of the tested bias voltages (see here) shows almost identical peaks in DARM for each bias voltage. I confirmed that the voltages being driven at the time were as expected (see here).
Zooming in on the PSD (see here) shows a peak ordering that seems consistent with a zero crossing at a large negative bias voltage. This is inconsistent with previous measurements, which found a zero crossing at 3k for ITMX. I will repeat these measurements later, covering the full range of bias voltages.
The same measurement was repeated sweeping over a larger range of bias voltages (up to ±100k counts). Attached are plots of the DARM response as a function of bias voltage for both sets of measurements. Both are consistent with a zero crossing at a large negative bias voltage.