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.
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TRANSFER FUNCTION!
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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.
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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.
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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.
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- 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.
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