🚀 go-pugleaf

John Larkin wrote:
> On Tue, 30 Jun 2009 17:27:21 GMT, Rich Grise <richgrise@example.net>
> wrote:
> 
>> 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
> 
> Audio: if one end of a shield is disconnected, the delta-voltage (60
> Hz, RF, esd spikes) between chassis is greater. If the signal is
> single-ended, that increases noise directly. If differential, it
> increases common-mode noise.
> 
> Why would you want to do that?
> 
> A 60-Hz (and RF, and ESD) "ground loop" can result from different
> boxes being plugged into different outlets which have some 60 Hz
> difference voltage (or form a relative antenna for RF.) Connecting the
> shield on both ends shorts out some or most of this difference voltage
> and REDUCES the effect of the "ground loop" on the signal.
> 

Unless the shield slowly commences a faint red glow, followed by some 
"amperage scent" wafting through the room ...

Seriously, I've seen that happen.


> Whoever did the drumming was passing on folklore.
> 

Yup. OTOH that provides a good business opportunity 8-D

-- 
Regards, Joerg

http://www.analogconsultants.com/

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