Andy, Josh

We have found RF beatnotes in the lock from Sep 12 (alog 29606), from 100 Hz up to Nyquist in DARM. We have only been able to find them in DARM so far, not in any of the usual auxiliary channels that typically see them stronger (POP/ISS/REFL).

These are a beatnote of the PSL VCO frequency with an apparently fixed line at 79131700 Hz. TJ points out that because the AOM is double-passed, it would be good to check for something at double that frequency as well.

As a reminder, we find the beatnote frequency by doing a linear fit of IMC-F to the PSL VCO readback (which has the correct absolute frequency but is only sampled once a second). Then we plot abs(f_VCO - f_offset) over DARM and choose the offset to make them line up. In the past, we've seen various multiples of this frequency show up (i.e. harmonics like the second or fifth). This time there's a multiplier of one. Attached are two of these fits. Note that we sometimes see a mirror of the line around something like 4800 to 5000 Hz - maybe it's the OMC dither?