NAOJ GW Elog Logbook 3.2
The SHG/IRMC EOM modulation is too low. The channel DDS1 DAC0 produces only -9 dBm RF power, which gets amplified to 5 dBm from the RF amplifier rack. The nominal value to be used for the EOM is 25 dBm.
I could see the IRMC PDH error signal with the familar shape from far-detuned RF sidebands, however, the signal strength of the PDH is extremely weak. I suspect this is also the reason behind the SHG/GRMC weak lock, however in the case of the SHG the input IR is 900 mW (vs 3.5 mW for IRMC) so maybe some signal can be seen despite weak modulation.
I think a temporary fix is to use DDS1 DAC1 (SHG/IRMC demod), which outputs 10 dBm, and amplify to send to the EOM. Then amplify DDS1 DAC0 + 14 dB and split into demodulation signal. This will give ~ 2.5 dBm for each demodulation signal.
I had noticed the weird electronics settings problem for the SHG and GRMC servo gain after Mitaka campus was struck by lightning in late July. Perhaps that was also connected to the RF signal generation issues.
The spec sheet for the 88 MHz EOM says to apply about 12-15 dBm to get near 0.2 rad modulation depth. I swapped DAC0 (-9 dBm) and DAC1 (8.2 dBm) and plugged DAC1 into +14.1 dB RF amp. I intended to use +14 -12 to achieved 12 dBm RF injection but somehow it only came out of theRF amp as 11 dBm without any attenuator. DAC0 is also amplified ~ 14 dB and sends 4 dBm to the splitter for SHG and IRMC LO.
The SHG has a reasonable error signal and locks, but has oscillation even near zero gain. The IRMC has some visible RF sidebands in the reflection PD (approximately 3% of carrier on each side) but the PDH error signal is quite small. Turning the lock switch on the IRMC servo does nothing, it doesn't even disrupt the spectrum.
I should check internal connections.