🚀 go-pugleaf

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