Today we attempted to convert the beat-note frequency into an analog voltage using the SR620 frequency counter.
First an observation: the stability of the green beat was seen to be much better than the 100kHz fluctuation seen yesterday. Probably because Kiwamu noticed that one of the MC mirrors had a large variance in its motion and changed the gain and filter parameters to decrease this. The PSL was therefore more stable and the green peak fluctuation was less than 10kHz over time scales of a few seconds.
SR620 D/A output resolution given by the manufacturer is 5mV over the -10 to +10V range and this range corresponding to 300MHz. We, however saw fluctuations of 100mV on the screen which looked as if they corresponded to the least significant bit. This would imply a resolution of 1.5MHz at this range. Even if the manufacturer's claim was true it would lead to a resolution of 75kHz, far in excess of the required resolution a few hundred Hz.
We therefore require to set up the VCO-PLL to obtain a finer frequency resolution.
In the mean time the green beat drifted beyond the 100MHz detection band of the green-PD. So we changed the x-end-NPRO temperature by -0.05 to bring it back into the detection band.
We are also considering, Rana and Koji's suggestion of using a set of 14 flip-flops to divide the ~80MHz beat frequency so that it comes down to about 4kHz. This could then be sampled by the usual 16-bit, 64kSa/s ADC cards and brought into the digital domain where further digital processing would be needed to extract the the required frequency variations in the 0 to 10kHz band. Found a nice paper on this object