[Washimi, Yokozawa, Mori, Kaihotsu, Sugimoto, Enomoto]
(Yesterday's work)

Background: 9860

We investigated 60 Hz noise in the ALS system, and successfully reduced it by properly dealing with different ground levels of ALS0/1 rack and LSC rack!

= What we did =
- Randomly did the connect and disconnect test around X/Y PDH servo, and got a feeling that the ground level difference might be the cause. Connecting BNCs from the X PDH servo (in LSC rack) to the VCO (in ALS0 rack) made 60 Hz noise worse.
- Replaced the DC power supplies for PDH servos from independent ones to common ones. Now PDH servos are powered by +/-18 V and +/-24V taps of LSC rack, instead of independent DC power supplies.
- Following the Washimi-san's suggestion, the feedback signal to the VCO is now received by a differential receiver utilizing the summing node.
The problem was that  the output of the PDH servo is single-ended and the input of the VCO is also single-ended even though they are in different racks and run on different ground levels.
The previous connection was: PDH OUT --> VCO IN -->(here split with a T connector) --> SUM IN (which is a differential receiver)
The current connection is : PDH OUT --> SUM IN (differential receiver) --> SUM OUT2 --> VCO IN

= Results =
See the below plot. 60 Hz and its harmonics were reduced.