Motor cable and noise

We're in the process of reworking the motor cable to a) make it long enough for flight and b) remove the twelve Lemo connectors for easier integration.

As part of this, Dave Black has kindly built a protoype for the new cable design, so that we can make sure we don't create any new noise vulnerabilities when we remove the Lemos. The Lemos have the characteristic of keeping the cable shield continuous from the CWR to the forward-plate area, and maintain a faraday-like enclosure right the way through. This is likely not important, since the shield is broken at the motor location and there is already ample opportunity for the motor wires to contaminate the signals coming back from the pots or the array cabling.

Still, it seemed worth checking. Jeff and I compared scope measurements at equivalent points on the existing and the prototype pot-sensing cables and found no difference, so I've asked Dave to fab the new cable to match the prototype.

The new thing learned in the course of this (and the reason for posting here) is that we see <2mV rms noise on the scope from the pot cable with everything switched off, and not much more as we switch on CWR components until we power up the (switching) power supply for the Cal Source blackbody. This PS puts a lot of hash on the line and raises the noise to ~12 mVrms as indicated on the scope. Enabling the motors is still the dominant source (~35 mVrms), but I hadn't realized how noisy the Cal source power supply is.

Whether the Cal Source PS noise actually couples in to our FITS data remains to be tested, but we should look into that at our next opportunity since we don't have a workaround for it in place....we can switch it off at the Pulizzi via software, but I don't know whether that would leave us with a useful cal source blackbody given stabilization times. I need to check with Bob as to what his latest noise numbers are and how best to acquire comparable data with the PS switched off.