evan.goetz@LIGO.ORG - posted 11:51, Monday 06 February 2017 - last comment - 16:33, Wednesday 28 March 2018(33927)
Uncompensated pole in ESD output filter bank
Evan G., Joe B., Jeff K. Joe B. pointed out that there is an uncompensated ~5.215 kHz pole in the ESD output filter bank because our filter design zpk([3229.1150], [], -1, "n") results in Foton providing a pole to compensate the zero at high frequency. So the actual installed filter is zpk([3229.115],[5215.189175235227],0.9999999999999999,"n"). The ~3.229 kHz zero compensates for a pole in the analog electronics. Unfortunately, the ~5.215 kHz pole is uncompensated. Attached is a Matlab figure showing the effects from 1 Hz to 10 kHz. The band of most interest to us is <200 Hz since above 200 Hz the actuation is sufficiently rolled off that it does not contribute to the response of the interferometer. Below 200 Hz, there is very small residual magnitude error. Phase, however, is incorrect by ~1.1 degrees at 100 Hz. The equivalent delay is ~30 usec. This could explain part of the unknown actuation delay, which--for LHO--is 61 usec.
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Exporting the transfer function from Foton for zpk([], [5215.189175235227], 1, "n"), we obtain a slightly different functional form than Matlab due to IIR warping near the Nyquist. Instead of 30 usec delay that approximates the phase error, a delay of 20 usec fits the Foton version more precisely. Since this effect is already absorbed into the unknown actuation delay, and any changes would require regenerating GDS and DCS filters, so we will leave this as-is. In the future, we could account for the ~3.229 kHz zero in CAL-CS model and need to be accounted for in DCS.
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Associated with FRS Ticket 7351.