[Yuta, Evan]

In addition to constructing the calibration chain for the Michelson readout (alog 10213), we have also begun to construct a noise budget for the readout using REFLAIR_A_RF45_Q_ERR.

First, we measured the RF noise of the resonant PD near f2 using the HP spectrum analyzer. To place this in the noise budget, we assumed a noiseless demodulation that combines the upper and lower frequency components into a single bin. We found that the carrier peak in the spectrum was 3 Hz lower than nominal (nominal is 5*9099471); we aren't sure why, but we shifted the spectrum over by 3 Hz nontheless.To convert this "demodulated" spectrum (in V/rtHz) into m/rtHz, we divided by the PD transimpedance and responsivity, and then by the Michelson gain (in W/m).

Second, we unplugged the PD from the input to the demodulator, and then terminated the demodulator input with 50 Ω. We tried to then take a spectrum of the Q monitor using the SR785, but found that the noise was dominated by the 50-Ω terminated input noise of the SR785. So we instead looked at the spectrum of REFLAIR_A_RF45_Q_ERR with all whitening filters off and with the whitening gain at 45 dB. We found that if the whitening gain was at 0 dB, the spectrum decreased by only a factor of 3 in amplitude (and maintained the same shape). This seems to indicate there is a non-negligible contribution from noise sources after the whitening VGA.

Anyway, so foar the PD noise and the demod + whitening + ADC noise appears to account fairly well for the total observed noise that we saw in alog 10127. In the white part of the spectrum above 10 Hz, we appear to be off by a factor of 1.5 or so.