Summary:

Installing 10uF bypass capacitors on the High Voltage power supply line for the HV coil driver circuit doesn't improve the noise. The excess bump around a few hundred Hz is still present. How do we want to proceed? 

Details

• The setup is schematically shown in Attachment #1.
• Physically, the capacitors were packaged into a box, see Attachment #2.
• This box is installed between the HVPS and the 2U chassis in which the circuit is housed, see Attachment #3.
• I measured the noise, (using the same setup as shown here to avoid exposing the SR785 input to any high voltage), for a variety of drive currents. To make a direct comparison, I took two sets of measurements today, one with the decoupling box installed and one without.
• The results are shown in Attachment #4. You can see there is barely any difference between the two cases. I've also plotted the expected noise per a model, and the measured Johnson noise of one of the 25kohm resistors being used (Ohmite, wirewound). I just stuck the two legs of the resistor into the SR785 and measured the differential voltage noise. There is a slight excess in the measured Johnson noise compared to what we would expect from the Fluctuation Dissipation theorem, not sure if this is something to be worried about or if it's just some measurement artefact.

Discussion:

So what do we do about this circuit? For the production version, I can make room on the PCB to install two 10uF film capacitors on the board itself, though that's unlikely to help. I think we've established that 

1. The excess noise is not due to the Acromag or the input Acromag noise filtering stage of the circuit, since the excess is present even when the input to the HV stage is isolated and shorted to ground.
2. There was some evidence of coherence between the supply rails and the output of the HV stage (with input isolated and shorted to ground). The coherence had the "right shape" to explain the excess noise, but the maximum value was only ~0.1 (could have been because I was not measuring directly at the PA95's supply rail pins due to space constraints).
3. The impedance of 10uF at 100Hz is ~150 ohms. idk what the impedance of the supply pins of the PA95 are at this frequency (this will determine the coupling of ripples in the HVPS output to the PA95 itself.

Do we have any better bipolar HV supply that I can use to see if that makes any difference? I don't want to use the WFS supplies as it's not very convenient for testing.

Not really related directly to this work but since we have been talking about current requirements, I attach the output of the current determining script as Attachment #5. For the most part, having 220ohm resistances on the new HAM-A coil driver boards will lead to ~half the DAC range being eaten up for the slow alignment bias. For things like MC1/MC3, this is fine. But for PRM/SRM/BS, we may need to use 100ohms. Chub has ordered all manner of resistances so we should have plenty of choices to pick from.