stefan.ballmer@LIGO.ORG - posted 19:51, Monday 01 July 2013 - last comment - 12:45, Tuesday 02 July 2013(6949)
5h VCO drift plot
Here are plots of the long term drift of the arm offset frequency (as measured my H1:ASC-Y_TR_A_NSUM_OUT), and the 3 relevant VCO frequencies. All curves are calibrated in Hz IR. Between 1600sec and 2000sec we manually moved the ALS-C_COMM_VCO_FREQUENCY to the next fringe and back. The alignment was tweaked up between 2600sec and 2800sec. Traces, top: - Blue: IR transmitted light, calibrated in Hz using zpk([81.9545+i*81.9529;81.9545-i*81.9529],[850;850],0.656,"n") - Red: ALS-C_COMM_VCO - Bright green: Green light transmitted (arb. units) Traces, bottom (note that the scale is 1000x bigger): - Green: ALS-Y_VCO - Gold: IMC-VCO Conclusions - For 3h the IR frequency offset was well predicted by the ALS-C_COMM_VCO - I don't what happened at about 11000sec. - To epics channel precision the IMC-VCO cancels out ALS-Y_VCO exactly (actually, I'd expect the difference to be equal to ALS-C_COMM_VCO, but they are too noisy to tell) - The alignment tweak at 1600sec to 2000sec changed the offset by about 50Hz. - A typical RMS over 10min is about 12Hz, dominated by VCO motion.
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I should have mentioned the calibration required to get IR Hz: ALS-C_COMM_VCO: x0.5 (green light conversion) ALS-Y_VCO: x0.5 (green light conversion) IMC-VCO: x2.0 (double-pass) All calibrations also include a large offset to get the numbers down to ~100Hz to be plot-able.