Observation: It has been reported on the DetChar calls that there are lines observed in the Pcal spectrograms that are wandering in the 10-50 Hz range and some higher harmonics. The worry would be that this is actually on the light and causing an extra force on the optic, albeit at several orders of magnitude less than the nominal Pcal injection lines.

Investigation: Taking a DTT spectrum of the calibrated PD readback channels, H1:CAL-PCAL(X|Y)_(R|T)X_PD_OUT_DQ (apparently calibrated in Newtons of force applied to the optic--helpful!), we can easily see peaks corresponding to the injected lines (see attached, H1pcalYpd.pdf). Conveniently, since the channels are calibrated, the force applied to the mirror is within ~1% at the injection frequencies. So, if the observed wandering lines were actually on the light, then we would expect to see the lines at the the same amplitude in the two channels. Instead, there is ~25% difference in peak values for an example wandering line near 15 Hz.

This would suggest the line is not on the Pcal laser light. Robert S. suspects these Pcal wandering lines are more likely a DAQ issue. He says that he has observed other wandering lines within a factor of a few of the DAQ noise floor in many PEM channels. Recall that for the Pcal channels, all of them are being read into the DAQ on the same ADC card, so it's not surprising that these wandering lines are seen in many channels (see attached, H1pcalYall.pdf).

Additionally, due to the large amount of change in the observed line frequencies, this would eliminate mechanical resonances as a culprit.

I checked for coherence between Pcal PD readback and the nearby magnetometer, H1:PEM-EY_MAG_VEA_FLOOR_(X|Y|Z)_DQ, but did not find any significant coherence at the fundamental wandering line frequency seen recently, ~15 Hz (see attached, H1pcalYpdMag.pdf). Also, the nearby magnetometer shows no peak in the spectrum at ~15 Hz.

Conclusion: It would seem then that the wandering lines in Pcal are more likely a DAQ issue than a Pcal issue.