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 Mon Oct 26 19:10:59 2015, gautam, Update, Green Locking, AUX PDH loop characterization Tue Dec 8 23:24:08 2015, gautam, Update, Green Locking, Y end laser (Lightwave) PZT calibration Sun Dec 13 21:55:28 2015, gautam, Update, Green Locking, Y end laser (Lightwave) PZT calibration Mon Dec 14 16:27:11 2015, gautam, Update, Green Locking, Y-end AUX PDH noise breakdown Mon Jan 4 16:45:11 2016, gautam, Update, Green Locking, Y-end AUX PDH noise breakdown Mon Jan 4 16:09:54 2016, gautam, Update, Green Locking, Y end laser (Lightwave) PZT calibration
Message ID: 11907     Entry time: Mon Jan 4 16:45:11 2016     In reply to: 11879
 Author: gautam Type: Update Category: Green Locking Subject: Y-end AUX PDH noise breakdown

Summary:

I've re-measured the noise breakdown for the Y-end AUX PDH system. Spectra are attached. I've also measured the OLTF of the PDH loop, from which the UGF appears to be ~8.5kHz.

Discussion:

As Eric and Koji pointed out, the spectra uploaded here were clearly wrong as there were breaks in the spectra between decades of frequency. I redid the measurements, this time being extra careful about impedance mismatch effects. All measurements were made from the monitor points on the PDH box, which according to the schematic found here, have an output impedance of 49.9 ohms. So for all measurements made using the SR785 which has an input impedance of 1Mohm, or those which had an SR560 in the measurement chain (also high input impedance), I terminated the input with a 50ohm terminator so as to be able to directly match up spectra measured using the two different analyzers. I'm also using my more recent measurement of the actuator gain of the AUX laser to convert the control signal from V/rtHz to Hz/rtHz in the plotted spectra.

As a further check, I locked the IR to the Y-arm by actuating on MC2, and took the spectrum of the Y-arm mirror motion using the C1CAL model. We expect this to match up well with the in-loop control signal at low frequencies. However, though the shapes seem consistent in Attachment #2 (light orange and brown curves), I seem to be off by a factor of 5- not sure why. In converting the Y-arm mirror motion spectrum from m/rtHz to Hz/rtHz, I multiplied the measured spectrum by $\frac{3.907*10^6}{0.5*532*10^{-9}}$, which I think is the correct conversion factor (FSR/(0.5*wavelength))?

 Attachment 1: ErrSigBreakdown.pdf  156 kB
 Attachment 2: controlSigBreakdown.pdf  190 kB
 Attachment 3: YEnd_PDH_OLTF.pdf  99 kB
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