This measurement pertains to the BL2002 VCO PLL unit.
Our goal is to measure the frequency fluctuations introduced by the VCO.
First the VCO calibration was checked. It is -1.75 MHz per volt. The calibration data is below:
Next we measured the Transfer function between points A and B in the diagram below using the Stanford Research System's SR785. This measurement was done with loop opened just after the 1.9MHz LPF and with the loop closed.
The TF[open] / TF [closed ] gave the total gain in the loop. As shown below:
Green curve is the Transfer Function with the loop open and the red with that of the loop closed.
Gain Shown below is the quotient TF[open]/TF[closed]
c) As can be seen from the graph above the loop gain is >>1 over 0.1 to 300Hz. And hence the frequency noise of the VCO is just the product of the voltage noise and the VCO calibration factor over this range,
d) the noise power at the point B was measured and multiplied by the VCO calibration factor to yield dF(rms)/rtHz:
The green line with dots are the data
The blue line is the rms frequency fluctuation.
This corresponds to a arm length fluctuation of about 20pm.