Re: Preamp tests

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

Hall D Electronics:

Hi Yves,

    Did you say the shields are not connected at all? That's it! The 
cause of the oscillations is due to the fact that the cables connecting 
the sense wires to the HV board are just antennas - their shields, to be 
effective, need to be connected to some low impedance reference point. 
Because these cables are RG174, I believe from a previous picture, the 
maximum potential they can tolerate is about 900 V. Therefore, their 
shields cannot be connected to GND.

    Looking at the layout of the HV board, I notice there is a resistor 
(R50) connecting a trace to the HV and with a decoupling capacitor (at 
the bottom of the board). Was this trace meant to connect the shields of 
the cables to the HV reference? In this case, the potential between the 
inner conductors and the shields is about zero. The shield has then a 
low AC impedance to GND through the capacitor. You have to be careful 
how you route the cables for safety but try connecting the shields to 
this trace. You may want to the decrease R50 to 10 kOhm or so.

    Alternatives? Use thicker cables that can handle 3 KV (RG58?) - this 
may not be an option; bring the sense wire pins out of the gas volume 
and design a HV board with short traces and good shielding. There may be 
others that we need to look at carefully.

    Also, I noticed that the decoupling capacitors are 33 pF - a bit on 
the low side. I would change the capacitors to 220 pF, 3KV. This value 
is not critical - just a commonly available capacitor.

Best regards,
Fernando

Yves Van Haarlem wrote:
> Hi Fernando,
>
> I made a report of all the tests I did today, you can find it on the 
> wiki:
>
> http://www.jlab.org/Hall-D/software/wiki/index.php/CDC_19_12_2007
>
>
> It should include answers of your questions and measurements you 
> suggested.
>
>
> Cheers,
>     Yves
>                           /--------------------------------
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> On Wed, 12 Dec 2007, Fernando J. Barbosa wrote:
>
>> Hi Yves,
>>
>>   I have attached the results I obtained from the tests with the 
>> interposer + Preamp Card + cable + termination. As you can see, the 
>> pulse response is quite nice and the noise is also quite low.
>>
>>   As I mentioned before, the oscillations you observe with your set 
>> up are definitely a result of the connections to the preamp card. I 
>> believe your tests over the last few days showed that ground loops 
>> are not the cause.
>>
>> Best regards,
>> Fernando
>>