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Re: FDC cableing



Hi guys,
	Ok it seems this is generating enough discussions or opinions that we 
(I, since I have the similar cable sample) should just make some 
measurements.
	But in the meanwhile I will toss in more opinions, they are nearly free 
(unlike measurements :) )
	The DC resistance of the shield is irrelevant. It has no DC function. 
In fact, we specifically do not want to carry currents of any kind 
through the shield, DC or AC. Equipment ground on both ends is connected 
to the experiment clean ground. The shield is connected on one end only 
(probably the ADC/TDC board end). The ADC boards (and TDC boards) must 
have input receivers that are capable of extracting the differential 
signals despite a reasonable amount (maybe 1 Vpp) of common mode ground 
difference noise. Any current the shield carries will (obviously) be 
carried on the preamp-to-detector ground pins and therefore impose a 
voltage on the preamp inputs, aka an interfering signal which the 
preamps will respond to, which we do not want.
	Neither braid nor the foil will have much impact on 60,180, etc. line 
frequency pickup. Because of the random lay loose pair construction, 
this pickup (in differential mode) should be small anyway. But any 
trouble we have, the shield is not going to help on this. Obviously 
should not run the cable parallel to power trays carrying AC line 
(unless that's in rigid steel conduit).
	Very low frequency interference is not a problem, as Joe says it is 
easily taken out by any pedestal subtraction algorithm. There will be 
some pedestal subtraction no doubt.
	We want to cut down on interference pickup in the range 100kHz up to 50 
MHz or so. We want to reduce radiated fields from the cable in the same 
range. We want to have the cable propagation be independent of the 
location of other nearby cables and metal objects. We want to not have a 
signal dependent current injected into the experiment ground (as Elton 
points out). These are the goals of the shield. I'm rather sure the foil 
shield suffices. I'll try to justify this with some measurements.
	I certainly agree with Joe, it is a lot of copper in the shield so it 
is probably good/necessary to eliminate it. At least, it would seem 
crazy to jump through many other hoops to minimize mass but ignore this 
item. It is possible however (if safety people allow it) to strip back 
the entire jacket and shields from the last 1m or so of cable at the 
detector end. Possibly would have to add a woven plastic braid to keep 
the pairs bundled nicely. Once it is (nearly) outside the magnet bore 
the jacket and shield must be present. I think this would probably work 
ok from an electrical performance point of view. Just another option to 
consider, perhaps.

	Gerard


Elton Smith wrote:
> Hall D Electronics:
> 
> 
> Hi Joe,
> 
> If at all possible we want to avoid capacitively couple all signals to
> ground. Therefore, I do not think you want to eliminate the copper. Also,
> realize that the DC resistance of the thin Al may be quite high.
> 
> Elton.
> 
> 
> Elton Smith
> Jefferson Lab MS 12H5
> 12000 Jefferson Ave
> Suite # 16
> Newport News, VA 23606
> elton@jlab.org
> (757) 269-7625
> 
> On Thu, 7 Jun 2007, Joe Beaufait wrote:
> 
>> Hall D Electronics:
>>
>>  To Elton:  I am more worried about the high frequency than the low. Any
>> low frequencies can be filtered out without affecting data, so removing
>> the braid still sounds good to me.
>>
>>  To Fernando: The large amount of copper in the braid is much denser
>> than any of the plastic and is a radiator. If we can get half to 3/4 of
>> it out of the cable, that has to be a huge improvement on the multiple
>> scattering front. With hundreds of cables there is over a thousand ft of
>> cable in the bore, it would probably remove > 25lbs of copper from the
>> active area. It will also decrease the diameter over all.
>> Joe
>>
>>
>>
>>
>> Fernando J. Barbosa wrote:
>>> Hi Joe, Gerard, Dave and Elton,
>>>
>>> One more suggestion?
>>>
>>> I suggest we get a full picture from the manufacturer regarding their
>>> design criteria for the electrical and non-electrical specs before we
>>> start designing a new version of the cable. I am not convinced that
>>> removing the braid will improve things. After all, we have space and
>>> the metal in the braid is a small fraction of the total material, right?.
>>>
>>> I also concur that whatever we order must be in the final design
>>> configuration.
>>>
>>> Regards,
>>> Fernando
>>>
>>> Joe Beaufait wrote:
>>>> I am glad we have decided to go ahead and purchase a sample of the 25
>>>> pr. cable. This cable is going to be the standard for much of the
>>>> data acquisition system and it would be very nice to see how hard it
>>>> is to work with.
>>>>
>>>> On that note, there were some comments made at the Wednesday FDC
>>>> meeting  concerning the possible redesign of the cable. Daniel
>>>> mentioned that we might ask to make the outer jacket thiner and I
>>>> promised to look into it but, while looking at the cable diagram I
>>>> also noticed that this cable is double shielded. It has an aluminized
>>>> mylar layer with drain wire, and a copper braid. I believe we could
>>>> do away with the copper braid and reduce the copper in the cable by
>>>> at least half.
>>>>
>>>> I would like comments from the group on whether or not you would like
>>>> me  to pursue this.
>>>>
>>>> Thanks
>>>> Joe
>>>>
>>
>>
>>