To investigate the origin of bumps at 50 and 100 Hz in FDS measurement, I removed a phase shifter for CCFC LO and directly connected the DDS output for CCFC LO to the mixer with brown+green LEMO cables.

I tuned the CCFC demodulation phase by changing the LEMO cable length between DDS and mixer for CCFC so that time difference between center and 0 crossing point of the CCFC error signal becomes 44.4 ms, which corresponds to 44.4 ms*1.2 kHz/s = 53.3 Hz detuning. By using brown+green LEMO cables, I could realize the time difference of 44.4 ms. The sign of CCFC error signal is opposite compared with the one with the phase shifter.

The attached figure shows CCFC FDS without the phase shifter. The 100Hz bump becomes better without the phase shifter, but the 50Hz bump is still present. Also the 50Hz harmonics become larger without the phase shifter.

The detuning drift is 36-52 Hz, but this will be better with mcmc fit. The detuning is a bit smaller than the optimal value, so I will change the LEMO cable length for CCFC LO.