There's a huge 364Hz line in ALS TRY.
I'm not sure if this was already there for a while. Though we probably changed the loop gain of the ALSY PDH loop when we realigned the green path on ISCTEX today.
alexa, nicolas
In order to determine whether the line we were seeing on ALS TRY was a result of aliasing of something higher frequency in the ADC, we checked the analog spectrum of the ALS TRY photodiode.
Attached photo shows the line at 364 Hz in analog as in digital, so no evidence for aliasing.
Sheila, Alexa, Nic, Evan
Sheila and I went down to EY to see if we could track this down.
This 365 Hz line appears to be caused by oscillation in the 2" PZT mirror on ISCTEY. If the green refl beam clips on the RFPD, this produces amplitude modulation that leaks into the PDH error signal.
Line is obvious on ALSY QPDs on the transmon.
It seems the oscillation is mostly yaw (364Hz), though there is a pitch line about 10x smaller (420Hz), as well as lines in ALSX QPDs (pit: 389Hz yaw: 363Hz).