Friday night myself and Koji measured the Transfer function of the QPD circuit at MC2 side using a chopper . Following was our procedure :
We connected some wires at the input and output of the filter circuit to one of the segment of teh QPD. - seg 1.
A laser light was shined on to the QPD, it was pulsed using a chopper. The frequency of rotation of the chopper was varied.
These wires were then fed to the spectum analyser , and a transfer funstion was observed, It was nearly a low pass filter
The chopper frequency was then made variable by giving the chopper a signal from the spectrum analyser. This signal just swiped a large range of the rpm of the chopper.
Now the input signal looked like a sine wave of varying frequency. the transfer function looked like a perfect LPF, with a small SNR.
Attaching the plot of the TF in the next e-log (this one is on windows and can't access /cvs/cds)