🚀 go-pugleaf

On Thu, 25 Jun 2009 12:39:08 -0700, 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:
>>> 
>>>> ... 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.
>>> 
>>> That's usually the best way to ground shields.
>>> 
>>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.
> 
I think half of us are talking about low-noise audio frequencies, and
half of us are talking about RF. The way I heard it, you leave one end
unshielded for audio preamps and stuff, but ground both ends for RF;
I think it has to do with wavelength and, well, propagation
characteristics. Of course, the thing that was drummed into me while
they were teaching me this was "GROUND LOOPS!". :-)

Cheers!
Rich