Matt, Terra
We have 2 modes which are very close in frequency (both around 15540kHz) and we have had some trouble damping them. To help with this, I modified the PLL filters for these modes to support a lower bandwidth loop, which I hope will be less easily "distracted" by the nearby mode. The low-pass in FREQ_FILT1 (usually a 1Hz pole) is a modified elliptic which provides significant attentuation at 0.5Hz (ELF0.5), and requires a UGF of about 100mHz. To support this, the integrator in FREQ_FILT2 is moved down to 30mHz and the gain is reduced to 0.3. (Note that ELF0.5 has a gain of 0.5 below the cutoff. This helps to move the UGF down when this filter is on.)
So far this configuration has been working, but should it prove problematic the old filters can be moved back from FM2 to FM3 (which is the FM operated by the guardian).
While working on these modes, we found evidence for coupling between the ETMX ESD drive and Trans QPD signal.
I injected a sine wave at 15540 Hz (where there is no known mechanical mode resonance) to each ETM ESD drive through the PI damping loop of Mode17 (ETMX) and Mode25 (ETMY) and watched the response in the QPDs (H1:SUS-ETM?_PI_DOWNCONV_DC1_INP_IN1). I turned off all other noise and injected 10 000 and 50 000 counts set amplitude. We find that the X-arm TransMon QPD sees greater signal even when excitation is to ETMY.
In spectrum below, dashed and solid lines of the same color are the X-arm and Y-arm QPD responses, respectively. Blue and green are with 10k count excitation and orange and red with 50k counts.
To this end, we found a more reliable response from our PLL damping scheme by bringing the error signal for both modes (17 and 25) from Y-arm QPD, despite Mode17 being an ETMX mechanical mode. We should have a look at the cabling in the end stations for ESD and QPD signals.
Terra - can you repeat these ESD -> QPD coupling measurements with no light (IFO not locked)? This would help disentangle electrical cross-coupling from optical.