Re: Preamp tests

[Mitch Newcomer <mitch@hep.upenn.edu>]

Hall D Electronics:

I agree  the  33pf cap to shield is pretty small.   I'd be inclined to 
hook a 50ohm resistor Shield to the back of the preamp assembly and a 
capacitor on the other end in series with a  similar
resistor.  Hopefully this trades off current in the shield with large 
voltage excursions.   We also break the copper connection on the HV 
return line using a 1k resistor in series
with the return to the supply.  The filter cap on the HV should be on 
the detector side. The HV supply then has no copper connection to the 
detector HV lines or filtering direct to the supply.
  The potentially noisy HV cable is decoupled at its entrance to the 
detector, not inside the RF shield.  -- Mitch


Gerard Visser wrote:
> Hall D Electronics:
>
> Hi Yves,
>     I have here 20 capacitors, 330 pF 2.5 kV NP0-type ceramic in an 
> 1808 size surface mount package, Kemet P/N C1808C331JZGACTU. This (or 
> equivalent from AVX or Johanson) is the type I would strongly 
> recommend to use in any new design. It _may_ be possible to use 
> X7R-type ceramic but it is usually not a good idea from a noise point 
> of view.
>     The above capacitors can be ordered from Digi-Key, stock number is 
> 399-4832-1-ND for cut tape packaging. They have stock. The price is 
> rather high if you only buy a small quantity... So my mind was to try 
> them out when I come visit you and then you can buy a large quantity 
> on a reel, suitable for production of some new HV boards. But if you 
> want to try some real soon, order some from DigiKey you can have them 
> in ~2days usually.
>     The coupling capacitor being too small should (I expect) make 
> _absolutely_ no difference to any noise pickup or oscillation 
> problems. So, no need to change the coupling capacitor with a view to 
> fixing that.
>     On the other hand the bypass caps should also be larger, for these 
> I have ordered and received 3.3 nF 3 kV X7R-type ceramic in an 1812 
> size surface mount package, Johanson P/N 302S43W332KV4E, DigiKey stock 
> number 709-1056-1-ND. Again this is probably what I would recommend to 
> use in any new design. (But a larger value might be dictated by test 
> results.)
>     Surface mount high-voltage ceramic capacitors are much smaller for 
> the same capacitance, primarily because leaded high voltage ceramic 
> capacitors usually use a traditional single-layer construction, 
> whereas the surface mount capacitors are usually multilayer. They are 
> quite reliable and easy to work with when professionally reflow 
> soldered and properly cleaned, but you do have to use extreme care 
> when hand soldering. Certainly they cannot be hand soldered as a 
> production method.
>     About the signal connecting cable, I do believe still that coax is 
> unnecessary _if_ the connections are short. At present your 
> connections are not short. The coax shields should be AC grounded - 
> but of course it needs to be a low impedance ground or else this will 
> just be a source of massive crosstalk. [In particular I think just the 
> 33pF bypass cap on that line is not low enough impedance, but you can 
> try it. The line itself may be too skinny, we'll have to see. Either 
> can be corrected by some hacks to the existing HV board design.]
>     A 2-layer board should probably be just fine for the new HV 
> coupling board design.
>     I suggest as step 1.5 route the input signals directly away from 
> the preamp output cable. This is probably best done my modifications 
> to the way the preamp interposer board connects to your existing HV 
> board. Also improve the ground connection from HV board to the 
> endplate (it should be as short as the signal connections, both 
> running together, and should be as short as possible, and preferably 
> of somewhat low impedance at up to ~10MHz, i.e., use a copper strip, 
> braid, or at least wire gauge 16 or less.)
>     Thanks,
>
>     Gerard
>
> Yves Van Haarlem wrote:
>> Hi Gerard,
>>
>> I asked McCracken how we came to this design: the plan of CMU was to use
>> coax cable to bring the signal from the sense wire to the HV board, the
>> plan was to connect the cable shield to the HV board ground. There was a
>> electronics meeting in Indiana where Paul Smith suggested to connect the
>> cable shielding to the HV and not to the ground. Gerard opposed the idea
>> of using coax cable. The decision on which capacitors to use came 
>> after a
>> small calculation done at CMU.
>>
>> I decided not to connect the cable shielding to ground when bringing the
>> HV board outside the CDC end plenum because we saw no difference (using
>> the old preAMP) with/without connection.
>>
>> Plan:
>>         step 1: I just started (together with Mike) to collect the
>> material to connect the cable shielding to the HV line on the HV board.
>> After that we will test this.
>>
>>         step 2: Change the capacitors on the board with the ones Gerard
>> proposes and test -> Gerard, what type/value do you propose; i remember
>> you saying we should also using a different type.
>>
>>         -- My hope is that this can be done in a short period of time 
>> and
>> that we can take data afterwards --
>>
>>         step 3: first iteration in redisigning the board (shrink it, new
>> orientation preAMp, maybe two ground planes, ...)
>>
>>
>> Cheers,
>>         Yves