NAOJ GW Elog Logbook 3.2
I have compared the transfer function measured in entry #693 and the error signals measured in the entry #699 with the Matlab model of the servo.
#1 plot: TF measured vs model. Eleonora's thesis model (PZT pole of 30 kHz).
#2 plot: TF measured vs model, where I changed the high frequency part of the TF in order ot fit the measurements. In particular, I have moved to higher frequency the frequency of the PTZ pole. This can be explained due to the fact that the PZT transfer fucntion is not really known, even if this is strange that we have to change the model vs Eleonora's thesis measurements.
#3 plot: TF measured vs model. I changed the high frequency part and also changed the frequency of the zeros at 1540 Hz to 1000 Hz. This is strange, since the frequency of these poles is given by the electronics, but maybe the coherence of the TF measuremnt around 1 kHz is not very high.
#4 plot: error signals measured vs model. In the model I have used a laser frequency noise of 7.5 kHz/f /sqrt(Hz), as measured during Eleonora's thesis. We remark that the error signals are higher than the model.
#5 plot: error signals divided by 2.5 vs model. The factor 2.5 is not explained. A wrong calibration factor? Some problems with data acquisition? An higher input noise?
As further measurement, I would suggest to save the correction signal of the PZT and maybe try to have a better measurement of the TF below 1 kHz.
We have measured the spectrum of the piezo correction, through the channel PZT mon.
In the plot we took into account the factor 100 of attuation of the channel PZT MON and we used the calibration 2 MHz/V.
The spectrum looks similar to that we measured in july. We fitted it with the curve 7.5 kHz/f, which is compatible with the expected free running laser noise.
I attach the .txt file with data not calibrated.