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Re: CDC upstream end config



Hi Curtis,

Another important observation has been provided to me by Gerard. Under 
option #2, it is preferable to have the shield at HV because under 
normal operation there is no stored charge (zero potential) on the 
cable. If the shield were to be grounded, under normal operating 
conditions noise would be induced as a result of microphonics and the 
stored charge on the cable.

Another advantage of having the shield at HV is that there is no 
additional noise contribution to the preamp input due to the reference 
connection - the straw and the shield are connected to the same supply 
and experience the same conditions. This is not the case if the shield 
were to be connected to ground.

Therefore, the best approach under option #2 is to have the shield at HV 
and have an insulating jacket on each cable.

Regards,
Fernando



Fernando J. Barbosa wrote:
> Hi Curtis,
>
> I believe I didn't fully address the first part of your question on 
> your earlier email. This has to do with the use of the shielded cable 
> connecting the straws to the connectors.
>
> On option #1, a voltage trip  turns off the HV supply for a HVB card 
> (24 straws). In this case, the cable does not need to have a spec 
> meeting the operating voltage of the straws because the potential 
> difference between the inner conductor and the shield (connected to 
> the same HV supply channel) is always about zero. The cable shields 
> need to be insulated to prevent contact with adjacent groups of 24 
> straws. The cable can be very thin.
>
> On option #2, there is a requirement to disable individual straws 
> within the same group of 24 straws. In this case, the shielded cable 
> needs to meet the operating HV rating at all times. The shield also 
> needs to be connected to a fixed, low impedance reference, either HV 
> or ground (attach to the connector accordingly). Connecting to ground 
> has the advantage that the shield does not require an insulation 
> jacket. The cable cannot be as thin as the one in option #1.
>
> Please refer to my earlier email regarding the cable specs.
>
> Regards,
> Fernando
>
>
>
> Fernando J. Barbosa wrote:
>> Hi Curtis,
>>
>> Coax cable manufacturers specify the operating voltage between the  
>> inner conductor and the shield and where the shield is normally at 
>> ground - most applications. This spec refers to the dielectric 
>> between the inner conductor and the shield, among other variables. 
>> The manufacturers also specify the type and dimensions of the jacket 
>> insulating material which can then be used to determine their usage 
>> or application.
>>
>> Regards,
>> Fernando
>>
>>
>>
>> Curtis A. Meyer wrote:
>>> Hi Fernando -
>>>
>>>    I know that you have stated that the shield needs to be at HV, 
>>> but I originally thought this was because the cable you had was not 
>>> spec'd for high voltage. Are we sure that any cable is really spec'd 
>>> to have high voltage on the shield? Most manufacturers spec the 
>>> inner wire relative the the shield and the outside world, but I 
>>> don't recall seeing
>>> a spec from the shield to the outside world?
>>>
>>>     Curtis
>>>
>>> On Mon December 8 2008, Tim Whitlatch wrote:
>>>  
>>>> Thank you Fernando for your reply. This would require someone to 
>>>> install the FEP jacket on all the wire? Is this shrink wrap?
>>>>
>>>> Thanks,
>>>>
>>>> Tim
>>>>
>>>> Fernando J. Barbosa wrote:
>>>>   
>>>>> Hi Tim,
>>>>>
>>>>> My comments follow:
>>>>>
>>>>> Tim Whitlatch wrote:
>>>>>     
>>>>>> Hello All,
>>>>>>
>>>>>> Per our video conference this morning, the following was discussed;
>>>>>>
>>>>>> Gary will continue to manufacture a 24 place upstream plate with 
>>>>>> proper geometries from "geometry C". this will give room for 
>>>>>> simulating worst case spacial considerations for assy. He will 
>>>>>> pursue the original type setup with 5.5 inch long cables max.
>>>>>>
>>>>>> As a backup, Gary will manufacture an offset plate with the 24 
>>>>>> holes to take the stress off the conductive rubber sleeve (per 
>>>>>> SLAVA)
>>>>>>
>>>>>> Fernando, Gary followed up on the coax cables you suggested. They 
>>>>>> all seem to come without an outside insulating jacket. Is it 
>>>>>> possible to use these cables by keeping the braided shield at 
>>>>>> ground? Also, is it possible to use cable that is not rated for 
>>>>>> our high voltage by doing some tests?
>>>>>>         
>>>>> Of the two suggestions to check the cable routing, one of the 
>>>>> coaxes should have a jacket but the semi-rigid cable should have 
>>>>> an aluminized jacket. In this case, an insulating sleeve/jacket 
>>>>> needs to be installed and there are various types offered by 
>>>>> multiple companies. I suggest FEP, if available in the required 
>>>>> diameter. The shield needs to be at HV and in normal operation the 
>>>>> potential difference between the shield and the inner conductor is 
>>>>> zero. However, if a straw wire is disabled, the full HV potential 
>>>>> will be present and the cable needs to conform to these requirements.
>>>>>
>>>>> Cables can certainly be tested at higher voltages than the rated 
>>>>> operating voltage specified by the manufacturer but that's a 
>>>>> safety issue. I strongly recommend against overlooking the 
>>>>> manufacturer's safety specifications.
>>>>>
>>>>> Regards,
>>>>> Fernando
>>>>>
>>>>>
>>>>>     
>>>>>> I will pursue with Tom Carstens from Hall B about connector leaks 
>>>>>> on their drift chambers.
>>>>>>
>>>>>> Yves will look at getting some extension pcbs made for testing 
>>>>>> Slava's ides per layout on 9-29-08. Can we get a connector made 
>>>>>> directly onto the HV board that would plug into this?
>>>>>>
>>>>>> Thanks for your time,
>>>>>>
>>>>>> Tim
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>         
>>>
>>>
>>>
>>>   
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