🚀 go-pugleaf

John Larkin wrote:
> On Thu, 25 Jun 2009 13:48:35 -0400, Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
> 
>> John Larkin wrote:
>>> On Wed, 24 Jun 2009 21:42:07 -0700, <castlebravo242@att.net> wrote:
>>>
>>>> "David L. Jones" <altzone@gmail.com> wrote in message 
>>>> news:h8B0m.53620$FI5.3564@newsfe12.iad...
>>>>> Check out a rather unusual phenomenon you can see on your oscilloscope in 
>>>>> my latest blog (#14):
>>>>> http://www.alternatezone.com/eevblog/
>>>>> See if you can get it to happen on your scope!
>>>>> I'll offer a free subscription to my blog for the best screen shot posted 
>>>>> :->
>>>>>
>>>>> Oh, and the probe doesn't have to be shorted either.
>>>>>
>>>>> Have fun.
>>>>> Dave.
>>>>> -- 
>>>>> ================================================
>>>>> Check out my Electronics Engineering Video Blog & Podcast:
>>>>> http://www.alternatezone.com/eevblog/
>>>>>
>>>>>
>>>> When I worked a Century Data we had a problem with our new reel to reel 
>>>> vaccuum column tape drive when ever someone would walk by the drive when it 
>>>> was reading it would get data errors. The problem was that the shield on the 
>>>> cable from the heads to the preamp was grounded on both ends.
>>>>
>>>> Bob 
>>>>
>>> That's usually the best way to ground shields.
>>>
>>> John
>>>
>>>
>> Depends.  With floating transducers and shielded twisted pair, it's best 
>> to ground the shield only at the transducer end.  That way any 
>> capacitive imbalance doesn't produce any currents, because the shield is 
>> at the transducer's idea of ground.
>>
> 
> At the receive (instrument) end, the ungrounded end of the coax shield
> becomes the hot side of a resonant "vertical" antenna. Any voltage
> there becomes 100% common-mode signal on the inner twisted pair. 
> 
> What's wrong with a little current in the shield? The signal pair
> isn't affected. But volts, or tens of volts, of common-mode RF can be
> nasty.
> 
> John
> 

It depends on the shield resistance.  Putting a 1-ohm resistor between 
the shield and ground on the Rx end is as good as an open circuit for 
ground loop purposes, and should get rid of the antenna problem.

Cheers

Phil Hobbs

-- 
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net