The 56.8406 Hz comb that Keith pointed to in recent data shows up in End-X magnetometers, and seems to be an isolated spike repeating at that rate.

Keith's recent alog 29783 points to a comb with a spacing of 56.8406 Hz that has been seen before in ER9 and in the one-arm tests. TJ's Bruco results in alog 29828 show that the 56 Hz region of DARM is coherent with the SEIRACK and FLOOR magnetometers. The first plot shows the coherence just at the fundamental frequency.

The line seems strongest in SEIRACK_Y, so I tried folding the channel. The frequency doesn't seem to be an exact multiple of anything digital. But taking 1153 samples at 8192 Hz comes very close to 8 times the comb spacing, so I used that. The result is in plot two, which is four hours of mag data from today folded over 1153 samples. Eight spikes show up very clearly, so there does seem to be some kind of spike happening every 0.01759 seconds.

There are some other channels showing up in Bruco that may not be connected to anything - PEM-EX_ADC_{8,12,13}. I think these are spares. At EY there are some magnetometers on these channels but these look too dead for that. They seem to see the same line, so maybe this really is some kind of DAC problem, or maybe there's just some electronic pickup.

We should get an independent magnetometer out to EX and look around for this comb, and also just confirm that it's not native to the DAC.