40m QIL Cryo_Lab CTN SUS_Lab TCS_Lab OMC_Lab CRIME_Lab FEA ENG_Labs OptContFac Mariner WBEEShop
  PSL  Not logged in ELOG logo
Entry  Thu Nov 18 00:28:28 2010, koji, tara, DailyProgress, NoiseBudget, Noise from scattering light near the cavity's foam cap opening beat.pngVCO_RCNOISE.pngcompare.pngplot_code.zipSCRN0130.zip
    Reply  Thu Nov 18 03:10:03 2010, Koji, DailyProgress, NoiseBudget, Noise from scattering light near the cavity's foam cap opening IMG_3716.jpg
       Reply  Thu Nov 18 14:17:45 2010, tara, DailyProgress, NoiseBudget, Noise from scattering light near the cavity's foam cap opening 
          Reply  Thu Nov 18 15:09:22 2010, Koji, DailyProgress, NoiseBudget, Noise from scattering light near the cavity's foam cap opening 
             Reply  Thu Nov 18 21:42:33 2010, tara, DailyProgress, NoiseBudget, Noise from scattering light near the cavity's foam cap opening Beat_VCO.png
                Reply  Thu Nov 18 22:23:20 2010, Koji, DailyProgress, NoiseBudget, Noise from scattering light near the cavity's foam cap opening 
                   Reply  Fri Nov 19 01:21:53 2010, tara, DailyProgress, NoiseBudget, Noise from scattering light near the cavity's foam cap opening VCO_TF.png
Message ID: 402     Entry time: Fri Nov 19 01:21:53 2010     In reply to: 401
Author: tara 
Type: DailyProgress 
Category: NoiseBudget 
Subject: Noise from scattering light near the cavity's foam cap opening 

Here is the TF of LIGO's VCO. I measured the TF at 3 different RF output voltage levels (C3:PSL-FSS_VCOMODLEVEL) and plotted them together.

 

To measure the TF of the VCO box, I reduced the VCO RF before disconnecting any cables.

Source out from SR785 is split by a splitter. One goes to Ref ch A on SR785, another goes to VCO wide band input.

RF out to AOM is connected to Response ch B on SR785. 

The TF looks bad when I turned the RF output V to 5 which is normally used during the lock. I'm not sure if there will be

reflection of signal or not, so I decide to lower the RF output V.

The magnitude from each measurements on the plot are offset for comparison purpose.

 

As we can see, VCO does not have flat freq response. This should be able to explain the different shape between

Beat note freq and VCO feedback signal.

Quote:

TRANSFER FUNCTION!

Quote:

I measured the calibration df/dV for VCO to be ~48 kHz/V. Then I convert the Vnoise fro VCO to frequency noise.

 

I injected the signal at 90 Hz and 1 kHz and measure the corresponding peaks from beat noise and VCO.

Then I rescale both data to match the peaks to get the conversion factor.

                               

                        f=90 Hz, pk/floor               f =1kHz pk/floor

VCO                   1.43mV/20uV                   2.95 mV/20 uV

 

BEAT                 2.18mV/ 25uV                     4.11 mV / 25uV

conversion fac  

 beat =  VCO x K  =  1.53                                   1.37

let's use 1.4 for average

Since df/dV for beat is 71kHz/ V, df/dV for VCO is 71/1.4 kHz/V =  51kHz/ V.

The plot below shows RC noise as measured by VCO and beat note.

Quote:

What about the frequency response. Why did we see the different shapes between the spectra even with the coherence ~1.
You can measure the transfer function between those two VCO feedback and should try to explain it.

Is there any transfer function in the AOM VCO, for example?
If I can believe D980401-B.pdf, the VCO freq control path seems to have pole-zero pair at 1.5Hz and 41Hz.

Quote:

The Voltage vs Frequency conversion for VCO is here.

It's not linear over the range. I have to check what is the average value of the VCO during our measurement.

Then, if it's small enough, linear approximation can be used to convert the data from VCO to Hz/rt Hz.

I think It's better to use linear fit because the psd of the voltage does not contain information about the sign of the signal.

Quote:
  • Even after the removal of the Al tip, the scattered light noise looks still exist.
  • Particularly I still could see apparent fringe wrapping when I shook the table or touch the foam construction.
     
  • I still could not reject the possibility of the backscattering towards the PMC.
  • We had the fringe wrapping up to ~200Hz. This corresponds to the motion of the scattered body or the optical
    path for the scattered light by ~100um. Is that possible?
  • The VCO feedback and the beat note PLL feedback seemed to have the same information so far.
  • The lollipops at the trans mission ports are terrible. They are mechanically incorrect.
  • What is the correct conversion between the VCO feedback and the beat note PLL feedback??
    Both are VCO feedback signals but the slope looks different. Need precise investigation.

 

 

 

 

 

Attachment 1: VCO_TF.png  20 kB  | Hide | Hide all
VCO_TF.png
ELOG V3.1.3-