I switched the RFPD between RCAV and ACAV, now the gain for FSS loop is set to 26 dB, but the beat signal does not change.
From previous elog entry, RCAV's RFPD does not have a peak at 35.5 MHz, and ACAV's RFPD has a peak ~36 MHz. And the FSS loop did not have enough gain,
so I switched RFPDs between both cavities. Attached pictures below show the error signal from RFPD when the cavities are scanned, before and after I switched them.
The power into the cavities are ~ 1mW for both cavities.
The phase is adjusted to produce the best error signal.
Phase adj were 4.16 then changed to 5.67 V
I also inverted the phase on PDH box for ACAV.
FSS gain can be reduced from 30(max) to 26 dB.
Then I measured the beat noise, with 100 kHz input range, gain 5. There are no change compared to before.