We measured beat noise from the frequency counter(FC) instead of the feedback from PLL. The result is plotted below
We switch to use the FC because we hope it will allow us to measure the signal down to lower frequency (~10 mHz) which is not quite possible
for PLL because of its small input frequency range for the acceptable phase noise level.
The FC can measure the relative frequency from the chosen center frequency and give a voltage output (from 0 to 8 V.)
For example if we set the center frequency to be 160 MHz, with 1MHz/V gain, signals at 156 MHz and 164 MHz will correspond to 0 V and 8 V respectively.
Since the LIGO VCO range is ~ 10 MHz, we tried measuring the signal with 500 kHz/V setup which is equal to 4 MHz range. However the noise is too high,
so we have to choose a new gain setup, and 20kHz/V setup is still acceptable for our signal compared to the signal from PLL.
(It turns out that data from 50kHz/V is too noisy)
Attachment 1:
Blue and Green show the beat noise measurements from PLL and FC. They agree well. The gain for FC is set to 2kHz/V.
When the temperature became more stable, the data could be acquired to down to ~ 3mHz (red).
We used AC coupling for FFT measurement, so the TF for AC coupling is measured. There is a 160 mHZ high pass, and the data is corrected accordingly.
Attachment2:
Time series for ACAV's temperature, RCAV's temperature, VCO mon, and Frequency count Vout during the FFT measurement.
Attachment3: show TF measurement for AC highpass and fit with 160mHz high pass.
The plot is flipped because chA is AC couple and ChB is DC couple during the measurement, and
The output is B/A.
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This morning I was able to measure the beat noise down to 12.5 mHz. So I plot it together with the noise budget here.
We also added channel for frequency counter,C3:PSL-FSS_FREQCOUNT which will allows longer data acquisition time for lower frequency.
Then we will be able to see the temperature effect at lower frequency(~ 10 mHz.)
Fig1: beat signal at 12.5 mHz to 10 Hz
We are also working on PID thermal control for refcav.
Perl script for PID thermal control won't work on Solaris because it doesnt have ezcaread/write command, we will get that from op40m machine at the 40m.
(We can't run Perl script on Linux because it complains that it fails to read / write data from C3:PSL-FSS_HEATER.)
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