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Re: Preamp tests
Hall D Electronics:
Hi Fernando,
First of all, we rechecked the capacitors we are using and it turned out
that the ones we use now are 330 pF NOT 33 pF. When we brought the HV
board outside we used a new one which is stuffed with 330 pF capacitors.
The one we sent to Gerard is the old one which probably has the 33pF ones
one it. Gerard, is this true? I updated the wiki page containing these
values and apologize for the confusion.
I soldered the cable shield to the HV board but unfortunately it did not
change a thing. More details can be found on:
http://www.jlab.org/Hall-D/software/wiki/index.php/CDC_19_12_2007
(You have to scroll down a while to find the added part, maybe I have to
rethink how to report these things in the future)
I made a small summary of proposals I could filter out of the emails:
- Change R50 resistor to 10 kOhm
- Change ground connection to end plate using braid and make it
as short as possible.
Cheers,
Yves
/--------------------------------
/ Yves Van Haarlem
_--~~--_ /
/~/_| |_\~\ / Carnegie Mellon University
|____________| / Department of Physics
|[][][][][][]|:= / Wean Hall room 8404
__| __ |__ \ Pittsburgh, PA 15213
| ||. | == | | \ USA
(| ||__| == | |) \
| |[] [] == | | \ Tel.: +1 412 268-6949
| |____________| | \ +1 412 641-9252
/__\ /__\ \ Fax.: +1 412 681-0648
~~ ~~ \-----------------------------------
On Thu, 13 Dec 2007, Fernando J. Barbosa wrote:
> Hello everyone,
>
> Yves has a good plan in trying to get the detector working under the
> present conditions. I would suggest he concentrates on his step 1 in trying
> to remove the oscillations with the new setup.
>
> For a great short summary and guidelines on HV filtering, component
> values, etc., I suggest GlueX-doc-753 by Dan Carman. Although this note was
> written for the FDC, it applies equally well to the CDC.
>
> I also recently placed a document on the portal (GlueX-doc-927) that
> outlines the HV distribution and grounding for the FDC. This also applies
> well to the CDC. I will circulate, hopefully before the holidays, a document
> outlining the global HV, LV and grounding distribution philosophy for Hall D
> - all detectors.
>
> Best regards,
> Fernando
>
>
>
>
> Mitch Newcomer wrote:
>> 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
>>
>