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AdV-SVS (Pre-commissioning)
garaventa, sorrentino, vardaro - 16:55 Wednesday 10 February 2021 (50735) Print this report
Rough attempt for a Coherent Control loop

Today the aim was to improve the LO, BAB superposition exploiting the dithers line applied to the two AA mirrors. We realized soon that the RF magnitude of the HD detector will be affected by the phase variations between BAB and LO. Thus we tried to lock a sort of CC loop between LO and CC but sending simultaneously CC, BAB and LO. We proceeded in the following way:

  • At 10:42 UTC we stopped the acquisition of the SC PLL correction signal
  • We removed the HD RF Channel from the mDEMOD and we demodulated it analogically with a mixer at 4MHz
  • We observed the beatnote between the BAB and CC by shuttering the LO path. Fig 1 shows the beat note fring amplitude obtained by offset the demodulation by 2 Hz
  • With the demodulation phase we centered the CC BAB beat note at -0.015. Fig 2
  • We switched on the LO and we observed the CC fringes
  • We implemented in EQB1_HD_AA a coherent control loop that used a normal DAC and the actuator along the beam path of the HD_M4 mirror. Fig 3 shows the only BAB yellow, BAB + LO  with open loop magenta and BAB + CC with closed loop blue. Up to now we are able to correct up few hertz by reintroducing noise above 10 Hz. The used gain was 100 and we used only a simple integrator without roll off.

The code of the loop is between line 107 and 116 of the EQB1_HD_AA loop.

Before trying some conclusions we need to check if the homodyne is well balanced, if there is too much noise due to the bab and if the two beam are still well superposed because trying to close the loop sent some kicks to the actuators and maybe we misaligned the LO.

 

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