andrew.lundgren@LIGO.ORG - posted 04:34, Friday 27 March 2015 - last comment - 02:08, Saturday 28 March 2015(17506)
Whistles in DARM in current lock
The whistles in DARM are still quite prominent in this lock. As briefly summarized in this alog, they happen whenever the PSL VCO frequency crosses 79.2 MHz. This points to the likely cause. This is a reliable enough indicator that we can get some statistics. In the first hour of the current lock (Mar 27 9 UTC to 10 UTC), the whistles happen at a rate of 4 per minute. Below are four spectrograms. The first two show some whistles identified this way. A new feature is that there seems to be a second oscillator very close by, or maybe some harmonic of the beat note. The next two spectrograms show a minute with several whistles, and the next minute where none occurred. Even when none go through zero frequency, you can still see the beat note hovering up near Nyquist.
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The attached script will find all the crossings which indicate whistle / RF beat note glitches due to the 79.2 MHz crossing. Currently the PSL VCO readback only appears to be changing once a second, so the time accuracy of glitch finding is only one second. For vetoing purposes, this could be improved by doing a linear fit of IMC-F to the PSL VCO readback, then using the much faster sampled IMC-F to measure the VCO frequency.
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