Re: News on the preAmp + CDC

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

Hi Gerard,


>   I'll try to send out a more descriptive email later, but here's a quick summary of some points:
>   - Still remains that I have not seen any oscillation with the new preamp board when the output signals are reasonably connected (either using a transformer coupling into scope or using the receiver/shaper board, it would also be possible to use a differential scope probe but I did not bring mine and I don't think they have one here). I agree, Fernando, it would be good to try to provoke the oscillation again just to be sure we understand that issue, but I'll save this to do under more controlled conditions in my lab at IUCF - or of course you could try it also. I'm pretty sure this can be done (provoke the oscillation).
>   
I have not observed any oscillations so far, no matter what the cable 
length, termination or bench or FDC set ups. It would be great if we 
could provoke the oscillations as documented by Yves in trying to 
understand where the problem was.

>   - The power supply, Curtis I was recommending on general terms HP bench test supplies (as Fernando acknowledges), that other supply you have is not so great. But, for the real system we'll probably have some low noise switching supply, for instance from Wiener. It will be important to test the system with the real supply soon if that is feasible. The supply certainly needs low noise, floating output, and remote controls (ethernet probably).
>   
Both Wiener and CAEN offer nice modular, multi-channel supplies with 
noise somewhere around 10 mVpp. The plan is to have distribution box(es) 
with fuses, etc. to decrease costs and for safety.
>   - The receiver/shaper board is now connected, we will post some waveforms through it today. I was only able to field-install the 3M MDR connector on 8 of the 16 channels, it is (as Joe has noted before) a rather difficult animal. I can do better with tools back at IUCF in my lab - but I thought also that Joe/Fernando were going to send Curtis a cable with 3M and ERNI connectors installed. [And if this has to be made up still, it should be done with accomodation of the channel mapping, 16 of the 24 channels, different 16 on the preamp than on the receiver/shaper board. Also probably should be 2m cable, with a longer (18m?) for use later after the signals have been extracted nicely from the short cable setup.]
>   
Installation of the 3M MDR connector is indeed painful. I have ordered 
the tool - not cheap but we will need it for repairs in the field. I 
will be sending CMU a set of cables with 24-to-16 mapping once we get 
the tool.
>   - The _real_ problem with the CDC remains one of noise pickup. We have reduced it considerably by rearrangement of the grounds and signal connections, but it is still a completely unacceptable situation. I'm sure that there will be a requirement to make the signal connections between the boards and straw anodes as short as possible. That requirement is no surprise of course - but I think it needs to be considered more fully, how this will be done in the real CDC. For the test setup, I hope to have some progress on this issue by the end of the day.
>   
I completely agree that we need to make the connections as short as 
possible for the real CDC. I recall that originally Michael and Curtis 
optimized the gas distribution but we need to re-consider the layout of 
the gas, HV and signal more carefully.
>   - Fernando I will send info on the shaper board mods, so far I just plan a few resistors to change. Indeed on the existing board the input impedance is not 100 Ohm but 130 Ohm, so you will be seeing reflections, especially if you are not terminated at the preamp board end. [I still recommend installing 59.0 Ohm resistors on the preamp board, and am working with that setup. Later we can explore how it would be without them. Also note this cuts the gain in terms of voltage output, which probably helps things, especially regarding the receiver/shaper saturation as it exists now.]
>   More later,
>
>     Gerard
>
>   
I can have the 59 Ohm resistors installed on the preamp cards and Mitch 
may also want to have these installed. Together with the changes to the 
shaper board, we may decrease any saturation considerably. We just need 
to implement all the changes to all the set ups to avoid any confusions 
in the future.

Regards,
Fernando

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