The CC1 PD is used to detect RF signal at 14MHz. While checking this 14MHz signal peak on the spectrum analyzer, I found another peak that is moving in the frequency domain, and its peak height also changes.
This moving peak can reach a level of almost about -63dBm (before amplification). The frequency of this moving peak seems to be +/- 10MHz around 14MHz. Besides, it seems to become even larger when it approaches the 14MHz peak. More importantly, the RF signal from CC1 PD is only -51.3dBm (before amplification), which means that this moving peak may introduce non-negligible noise into the CC1 loop. There is a movie in this link shows the situation.
I also found this peak still exists even when all optical cavities are unlocked. This situation is in the movie in this link (after 20dB amplification). At around 8 second, I switched off CC PLL, the moving peak disappeares. So it seems this moving peak is related with PLL loop.
I also confirmed that this peak is not RF signal cross talk. Because when I block the light incident on CC1 PD, this moving peak disappears.
Today, we checked again this moving peak. At the beginning, this moving peak appears to be very close to 14MHz again. But after 10min, on spectrum analyzer, it goes away from 14MHz peak and stay there until almost 30min.