Following up yesterday's alog 24062.

We think that the ~650 Hz peak was caused by the ALS COMM VCO beating against the DIFF VCO. At times when the COMM VCO is parked at the lower value (as discussed in the earlier alog) the two VCOs are < 700 Hz apart and this beat shows up in darm.  To get rid of this peak, we added a line in guardian so that H1:ALS-C_COMM_VCO_TUNEOFS is set to 2 in full lock. This way, the COMM VCO gets parked at a high value far from the DIFF VCO. See figure 1 if you want to see the actual values of the two VCOs;  the top plot is from yesterday (peak present; the two VCOs are within 700 Hz of each other) and the bottom plot is from today (the two VCOs are >10 kHz apart, peak is not present).

Also attached (figure 2) is a darm spectrum comparing yesterday, when this noise was present, to the current lock. It is clear that the peak between 600 Hz and 700 Hz (and the higher harmonic) is now gone. No other peaks have been observed to appear due to this change. We think this has solved the issue, but we should keep an eye out to make sure it does not return. Also, it will be good to keep beating VCOs in mind if similar wandering peaks show up again.

Jordan, Robert, Sheila