Elli, Daniel, Evan
We tried adding a dc bias to the PSL EOM in order to reduce the RFAM, with little success.
At the top of ISC R1, we inserted a minicircuits bias tee into the 9 MHz drive that goes to the PSL EOM. With a DMM, we measured the input impedance of the 9 MHz drive to be an open, and the input impedance of the analogous point in the 45 MHz drive was 70 Ω. We take this to mean that the 9 MHz drive goes directly to the EOM, with no intervening amplifier.
To see the effect of dc bias, we first hooked up the dc port of the tee to ISC-EXTRA_C_AS_AO_4, which is a slow DAC channel on the ISC rack. We applied ±10 V dc offset while watching the time series of REFL_A 9I&Q, but saw no obvious changes.
So instead, we hooked up the dc port of the tee to LSC-EXTRA_AO_2 (a fast channel) and drove it with a 32700 ct, 444 Hz sine wave. Then we looked for this excitation in spectra of REFL_A RF9I&Q.
Previously (LHO#15681), the RFAM level has been pegged at 1.1×10−3 Wrms/Wdc. That means that the fractional RFAM change that can be effected by this bias tee configuration appears to be something like 1×10−6 / Vbias, which is way too small to be practical.