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Entry  Tue Mar 13 07:04:55 2012, kiwamu, Update, LSC, evolution of the sensing matrix in PRMI as a function of time PRCL.pngMICH.png
    Reply  Tue Mar 13 16:40:06 2012, kiwamu, Update, LSC, evolution of the sensing matrix in PRMI as a function of time: details lockins_MICH.png
    Reply  Tue Mar 13 16:56:19 2012, kiwamu, Update, LSC, evolution of the sensing matrix in PRMI as a function of time 
    Reply  Wed Mar 14 21:01:36 2012, keiko, Update, LSC, evolution of the sensing matrix in PRMI as a function of time 
Message ID: 6405     Entry time: Tue Mar 13 16:40:06 2012     In reply to: 6403
Author: kiwamu 
Type: Update 
Category: LSC 
Subject: evolution of the sensing matrix in PRMI as a function of time: details 

Here I describe the measurement of the sensing matrix.

 

Motivations

  There were two reasons why I have been measuring the sensing matrix :

  1.  I wanted to know how much each element in the sensing matrix drifted as a function of time because the sensing matrix didn't agree with what Optickle predicted (#6283).
  2.  I needed to estimate the MICH responses in the 3f demodulated signals, so that I can decide which 3f signal I should use for holding MICH.

 I will report #2 later because it needs another careful noise estimation.

 

Measurement

 In order to measure the sensing matrix, the basic steps are something like this:

  1. Excite one of the DOF at a certain frequency, where a notch filter is applied in the LSC servos so that the servos won't suppress the excitation signal.
  2. Demodulate the LSC signals (e.g. C1:LSC-REFL11_I_ERR and etc.,) by the realtime LOCKINs (#6152) at the same frequency.
  3. Calibrate the obtained LOCKIN outputs to watts/meter.
In the actual measurement I choose the frequency of the excitation signal to be at 283.1 Hz,
at which any of the LSC servos don't have gains of more than 1 and there were no particular structures in the spectra.
For the amplitude of the excitation, I usually choose it to be 1000 - 2000 counts.
Because all the actuators have response functions of approximately 10-9 / f^2 meter/counts  (#5637), the actual displacement in the excited DOF should be about 10 pm level.
Therefore the excited displacements must be always in the linear ranges and also the amplitude in counts is reasonably smaller than the DAC range.
 

LOCKIN detection

The attached cartoon below shows how the LOCKIN system works for the MICH response measurement.
In the case of the PRCL response measurement, the setup is the same except that only PRM is shaken.
Here is some notes about the LOCKIN detection.
  • The LOCKIN oscillator excites ITMs differentially
    • In order to purely excites the MICH DOF, the actuation coefficients were precisely adjusted (#6398).
    • Currently ITMY has a gain of 1, and ITMX has a gain of -0.992 for the pure MICH excitation. Those numbers were put in the output matrix of the LOCKIN oscillator.
  • The demodulation phase of the LOCKIN system was adjusted to be -22 deg at the digital phase rotator.
    • This number maximizes the in-phase signals while the quadrature-phase signals give almost zero.
    • This number was adjusted when the simple MICH configuration was applied.
  • In the demodulations, the LO signals have amplitude of 100 counts to just make the demodulated signals readable numbers.

 

lockins_MICH.png

 

Calibration of the LOCKINs

  The calibration of the LOCKIN detectors is easy because all the processes takes place in the digital land, where we know all the parameters.
In this phase the goal is to calibrate the signals into counts / meter.
To calibrate the LOCKIN output signals, the following equation is used :
 
 [The obtained LOCKIN output in counts ] = H x ADOF x CLO x CEXC x 1/2  ,
 
 where H is the response of a sensor (e.g. AS55_I, AS55_Q and so on) against a particular DOF in unit of counts / m and this the quantity which we want to measure here,
ADOF is the actuator efficiency of the DOF at the excitation frequency in unit of m/counts,
CLO is the amplitude of the local oscillator signal for demodulating the sensor signals in unit of counts,
CEXC is the amplitude of the excitation signal in unit of counts,
the last 1/2 term comes from the fact there is a low pass filter in each demodulation path. 
Therefore once we measure the response of a sensor, dividing the obtained LOCKIN output by ADOF x CLO x CEXC x 1/2 gives the calibrated response in unit of counts/meter.
  ADOF are well known as they have been measured several times (#5637).
For the MICH actuator I assumed that AMICH = 2 x (ITMY response) since they are balanced through the actuation coefficients.
Note that a confirmation of this calibration has been done
when the configuration is in the simple Michelson, where we can easily estimate the response of a sensor by letting the MICH freely swing.
 

Calibration of the responses to watts/meter

  With the calibration process described above, we obtain the sensor responses in unit of counts/m.
 Then we need to do another calibration to make them into unit of W/m.
If we think about how the RFPD signal flows, we get the following gain chain.
 
[raw response in counts/m ] = Hopt x CADC x Ldemod x GWF x Ztrans x RPD
 
Hopt  is the optical gain at a sensor which we want to calibrate. It is in unit of W/m.
CADC  is the conversion factor of the ADCs and the value is CADC = 1638.4 counts/m because their resolution is 16 bit and the range is +/-20 V.
Ldemod is the conversion efficiency of the demodulation boards in unit of V/V. I used the values which Suresh measured yesterday (#6402).
GWF is the gain of the whitening filter in unit of V/V,
Ztrans is the transimpedance gain of an RFPD in unit of V/A and I used the values summarized in (the wiki),
and RPD is the responsivity of the photo diodes and I assumed RPD = 0.75 A/W for all the RFPDs.
 
Therefore the calibration can be done by dividing the raw response value by the entire gain chain of CADC x Ldemod x GWF x Ztrans x RPD.
 

Settings and parameters

  •  LSC RF demodulation phases
    •  AS55 = 17.05 deg (minimizing the PRCL sensitivity in the Q-phase)
    •  REFL11 = -41.05 deg (maximizing the PRCL sensitivity in the I-phase)
    • REFL33 = -25.85 deg (maximizing the PRCL sensitivity in the I-phase)
    • REFL55 = 4 deg (maximizing the PRCL sensitivity in the I-phase)
    • REFL165 = 39 deg (random number)
  •  Whitening filters
    • AS55 = 30 dB
    • REFL11 = 0 dB
    • REFL33 = 42 dB
    • REFL55 = 30 dB
    • REFL165 = 45 dB
  • MICH servo
    • AS55_Q for the sensor
    • G = -5 in the digital gain
    • FM2, FM3, FM5 and FM9 actiavted
    • UGF ~ 100 Hz
    • Feedback to ITMs differentially
  • PRCL servo
    • REFL33_I for the sensor
    • G = 1 in the digital gain
    • FM2, FM3, FM4, FM5 and FM9 activated
    • UGF ~ 100 Hz
    • Feedback to PRM

Quote from #6403

Tonight I measured the sensing matrix again but this time I recorded them as a function of time using the realtime LOCKINs in the LSC front end.

I will explain some more details about how I measured and calibrated the data in another elog entry.

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