Elli, Sheila, Alexa, Rana, Evan

Since we haven't been able to reliably transition ALS COMM to sqrt(TRX+TRY), we decided we'd back off a bit and try the transition with a single arm only (i.e., no DRMI).

The sequence that we found worked for us is as follows:

• Adjust the ALS COMM VCO offset to −100 Hz_green.
• Engage the sqrt(TRX) path with a 1 Hz pole and a 40 Hz zero, an offset of −0.7 ct in TR_CARM, and an overall TR_CARM/REFLBIAS/REFLDCBIAS gain of 80 ct/ct. The common-mode board has an input gain of 0 dB (ALS COMM has an input gain of −4 dB). Addtionally, we have the usual filters engaged in TR_CARM/REFLBIAS/REFLDCBIAS:
  • a 1 kHz pole and a pair of complex zeros at 35 Hz (with a Q of 1) which are meant to make sqrt(TRX+TRY) fall off as 1/f out to high frequency
  • a 1.6 Hz pole and a 40 Hz zero which imitates the response of the ALS COMM VCO.
• Turn off the COMM VCO frequency servo. Tune the VCO voltage so that the sqrt(TRX) output (as monitored via REFLDCBIAS_OUT) is close to zero.
• Engage a 0 Hz pole and 1 Hz zero in the sqrt(TRX) path, so that sqrt(TRX) has a true integrator.
• Turn off the ALS COMM PLL boost (pole at 1.6 Hz, zero at 40 Hz).
• Ramp down the analog gain of ALS_COMM on the summing node board to −32 dB. Then switch off the input.

For most of this process we had a GPIB-controlled SR785 hooked up to the common mode and summing node boards in order to monitor the relative strengths of the ALS COMM and sqrt(TRX) signals. For the excitation we drove EXC A on the common-mode board; for ALS COMM we monitored TEST1 on SUM A, and for sqrt(TRX) we monitored TEST2 on SUM A. This was incredibly helpful for sorting out how to properly shape sqrt(TRX) and how to engineer a stable crossover during the transition.