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Re: Preamp tests
Hall D Electronics:
Hi Yves,
A value of 330 pF is good enough. You have performed quite a few
tests and it is quite a lot of work doing all of that soldering and
de-soldering. It is also difficult to do this sort of work remotely.
However, it is great that Gerard will be visiting with you soon to help
with this. If necessary, I can also fly there so we can all look at the
issues up close.
I just ask for your patience for a bit longer because ... we have
quite a few more tests. Just kidding - a few more. I know we are on the
right track and all this work will be of great help when we look at the
FDC next week here at Jlab.
Let's try the following:
Keep the HV off for the next measurements, even disconnect the HV cable.
We are just going to look at the oscillations and noise.
1. Disconnect one of the CDC-HV board cables, including the shield. The
output of the preamp for this channel should be very quite compared to
the others around it.
2. Reconnect the cable but this time connect the shield (as short as
possible) to a good ground point on the HV board. Check the outputs as
in 1). The HV board should have a low impedance to the reference GND by now.
3. Disconnect the CDC-HV board cable at the CDC end. I presume there is
a socket connector. Check the outputs as in 1). How does the cable
actually connect to the sense wire? Is there a long and un-shielded
connection? Place a piece of foil contacting the shield completely
around the connector, a Faraday cage. Check the outputs as in 1). There
should be no noise. If there is, I am puzzled but you may go ahead and
remove the foil and now short the socket connector to the shield. There
should be no noise.
4. What does the other end of the straw look like? Please take a few
pictures where necessary.
As a check of the scope probes, connect them together (the grounds
should also be connected together) and apply the math difference on the
scope. There should be no noise.
Best regards,
Fernando
Try the following:
Yves Van Haarlem wrote:
> 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
>>>
>>