WP 6523
I was asked by Terra to check whether mode 23 (32761.5 Hz ETMY) is visible also in the TRY QPD.
So I excited mode 23 today during the commissioning window --- we see the mode in the TRY signals as well.
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The way the mode shows up is such that segments 2 and 4 don't see the mode in a 0.1 Hz resolution FFT at all while the mode shows up in segments 1 and 3 (diagonal pair) in a differential way. Note that the excitation was done by driving the UR and LL segments of the ETMY ESD in common. The attached shows comparison of the spectra with and without the excitation.
- Brown trace: OMC signal downconverted with a 32800 Hz oscillator when no excitation was applied.
- Green trace: TRY segment 1 downconverted with a 32800 Hz oscillator when no excitation was applied.
- Blue trace: OMC signal downconverted with a 32800 Hz oscillator when excitation was applied.
- Red trace: TRY signal downconverted with a 32800 Hz oscillator when excitation was applied.
Apparently, the OMC has a better signal-to-noise ratio than the TRY QPD by a factor of 100 or so.
Also, because of the aliasing effect, the mode is split into two components after the 32800 Hz mixing operation; one at 22.5 Hz (= 32800 Hz + 32761.5Hz + 2^16 Hz) and the other at 38.5 Hz (= 32800 Hz - 32761.5 Hz). Usually the PLL for this mode is locked to the higher frequency one, 39 Hz.
