New Preamp Tests

["Fernando J. Barbosa" <barbosa@jlab.org>]

Hi Yves,

    I looked at your most recent tests and the pictures really helped. 
It seems to me that all point to a ground problem - not easy to diagnose 
remotely. In particular, connecting the shield to GND or the bypassed HV 
causes oscillations is suggestive of noise being coupled from GND to the 
preamp input. I also noticed that there may be a long path from the 
end-plate (straw tube ground) to the preamp GND, although this is 
something to consider later and not the cause of the oscillations at 
this time.

    I have a bit of great news from here. Simon and I got the new 
preamps installed and running on the FDC prototype and the system is 
very quite (~20 mV p-p noise floor) and with huge pulses (some >1.3V). 
We looked at both anodes and cathodes with 18 m cables connected to 
Gerard's shaper cards (these should not be a factor in the oscillations 
you get, I think). Aside from a few things to check with Gerard 
(polarity and gain), the system looks good.

    Simon will send an email tomorrow to a link where all the pictures 
and notes will be placed. I suggest looking at the overall ground 
connections - a simple diagram illustrating every connection (shields, 
cables, braids, power supplies) would be very helpful. Per your 
suggestion, it would be great if Gerard could visit with you after the 
collaboration meeting. If you are here for the collaboration, we could 
also discuss this issue and go over the FDC set-up as a reference.

Best regards,
Fernando
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