[Halld-cpp] [New Logentry] Follow-up Re: Continued Noise Investigation

[davidl at jlab.org]

Logentry Text:
--
Ilya, Sergey F., Dave Mack, and I did more investigative work this morning on the noise issue. We focussed on the HV board grounding. We saw an improvement when copper tape was used to connect the jumper for the ground between two HV boards to the ground planes. The 2 photos below show the jumper discussed and the initial test where copper tape was temporarily attached.

[figure:2] [figure:3]

This showed at least a factor of 3 reduction of the noise amplitude on the scope for a wire near the center of the chamber. We then soldered the copper tape to the HV jumper and then connected it to the chamber side by placing 2 large washers on the bolt and sandwiching the copper strap in between. We repeated this for all of the HV ground jumpers (5 in all). The outside ends were left in their original configuration shown in log entry 3544983. 

With Sergey F.'s help a pedestal calibration was done for the fADC125 modules (and the f250 module) after the configuration change to reset the baseline to 100 fADC counts. The plot below shows the result (blue) compared to a run taken yesterday with no additional grounding (red). A couple of things to note about this plot:

1. The values plotted are the maximum value of the largest sample. This is not the mean. (See two additional figures below for more details.) 
2. It's possible the pedestals were set to 200 instead of 100 for earlier runs due to the larger noise fluctuations. I don't recall for certain at this point.
3. An additional copper braid was clamped directly to the SHV shielding and attached to the chamber ground plane near the top corner. This would correspond to larger wire numbers and may be why the blue points dip lower at the far right of the plot. 
4. The dip around wires 25-30 may be due to a better connection of that strap. (not sure exactly how at this point). Note that the boundaries where grounds were attached were at 24 channel boundaries (24, 48, 72, ...) However, the point at where the strap was attached to the Al ground plane was determined by the bolts and we always chose the bolts corresponding to slightly higher wire numbers.

[figure:1]

The following two images show the waveforms of the first 24 wires for an event taken before (run 259) and after (run 267) the additional grounding was added. The oscillations can still be seen, but at much reduced amplitude after adding the additional grounds.

[figure:5] [figure:4]

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This is a plain text email for clients that cannot display HTML.  The full logentry can be found online at https://logbooks.jlab.org/entry/3546343
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