Reports until 12:22, Tuesday 08 November 2016
H1 PSL
daniel.sigg@LIGO.ORG - posted 12:22, Tuesday 08 November 2016 - last comment - 13:52, Tuesday 08 November 2016(31319)
PMC gain change

With the higher sideband power we were limited to ~2 kHz ugf in the PMC with the lowest gain settings. Today, we changed resistor R19 to 4.75K from 1.21K which reduced the electronics gain by about 2.

While having the servo board in the shop we check R16/R17 which are 9.09K/1.00K rather than the 19.6K/1.00K indicated in the schematics. This explains the gain mystery described in alog 31120. The calibration of the slider has been changed is now accurately going from 0dB to 40dB.

We measured the transfer function with 0 dB, 12 dB and 22 dB of gain. The servo goes unstable at 25 dB, so 22 dB should be the maximum used. The ugf was around 1.1 kHz, 3.2 kHz and 9.4 kHz, respectively.

The reason we are not able to go below 1 kHz ugf is todays PSL change which improved the mode matching into the PMC and increased the optical gain (maybe by a factor of 2).

Comments related to this report
keita.kawabe@LIGO.ORG - 12:23, Tuesday 08 November 2016 (31320)
Link

OLTF pictures.

Images attached to this comment
daniel.sigg@LIGO.ORG - 13:52, Tuesday 08 November 2016 (31323)
Link

For the first plot we compare the ILS and PMC error and control spectra before and after today's change.The ILS gain is unchanged, whereas the PMC gain slider was set to 6 dB to make the ugf similar to before. We did not apply the 770 Hz pole that is formed by the PZT and the output resistor. The following observations can be made:

  • The ILS mixer spectrum is a factor of up to 10 lower than before. This is due to the SR560 gain boost, see alog 31223.
  • The peaks are greatly reduced in the PMC spectra. Looking at the control signal the 1 kHz peak was reduced by about a factor of 5.
  • At 100 Hz the PMC mixer output is about twice what it was before indicating that the optical gain increased by about the same.

For the second plot we set the PMC gain to 0 dB:

  • The PMC control signal seems to be limited by ADC noise at around 100 nV/√Hz.
  • The PMC mixer signal at 1 kHz and above look almost identical despite the fact that the optical gain increased by 2 and we are above the ugf.
    The power on the locking diode has been held constant—even so the mode matching improved by ~2. This indicates that the peaks scale with the total power on the locking PD and are not related to frequency or length noise. 
Non-image files attached to this comment