Takase, Takano

As reported in klog 29446 a common mode servo for GrPDHY seemed to generate a sharp line noise. Today we replaced this servo with a spare one.

Before the replacement we investigated the relation between the 28MHz peak and the cables connected to the servo. As a result, it was found that when a BNC cable which connects between the demodulator and the servo the 28MHz peak went away. When we unpluged the other cables the frequency of the peak changed, but the peak itself existed as almost the same level. After that, we replaced the BNC cable (1m) to a new one (2m), and the 28Mz peak dissapeared.

After the cable inspecton, we replaced the servo to the spare one. Then, we did the same measurement with the old and the new BNC cable, respectively, and confirmed that the peak existed with the old cable even with the new servo.

Finally we connected the demodulator and the spare servo with the new BNC and brought the old servo and the problematic BNC cable back to the office.