We added the Thorlabs HV Driver in between the FSS and the NPRO today. The FSS is locking with it, but we haven't taken any loop gain measurements.
This box takes 0-10 V and puts out 0-150 V. I set up the FSS SLOW loop so that it now servos the output of FAST ot be at +5V instead of 0V. This is an OK
temporary solution. In the future, we should add an offset into the output of the FSS board so that the natural output is 0-10 V.
I am suspicious that the Thorlabs box has not got enough zip to give us a nice crossover and so we should make sure to measure its frequency response with a capacitive load.
We measured the Thorlabs HV Driver's TF today. It is quite flat from 1k to 10k before going up to 25 dB at 100k,
and the response does not change with the DC offset input.
The driver is used for driving the NPRO's PZT which requires higher voltage than that of the previous setup.
We need to understand how the driver might effect the FSS loop TF, and we want to make sure that the driver
will have the same response with DC input offset.
We used SR785 to measure the TF. Source ch was split by a T, one connected to Driver's input, another one connected to the reference (ch A). See fig2.
The driver output was split by another T. One output was connected to NPRO,
another was connected to a 1nF capacitor in a Pomona box, as a high pass filer (for high voltage), then to the response (ch B)
The source input is DC offset by 2V which corresponds to 38 V DC offset on the driver's output.
The capacitance of the PZT on the NPRO is 2.36 nF, as measured by LC meter.
The result shows that the driver's TF is flat from 1k to 10k, and goes up at higher frequency, see fig1.
The next step is trying to roll of the gain at high frequency for PZT. A capacitor connected to ground might be used to roll off the frequency of the driver's output.
We will inspect the TF at higher frequency (above 100 kHz) as well.