🚀 go-pugleaf

On Tue, 30 Jun 2009 11:38:10 -0700, Joerg <invalid@invalid.invalid>
wrote:

>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.

Yup. I've seen some ghastly wiring, some done by licensed
electricians.

The most 60 Hz I've experienced (between outlet ground pins, in an
office building) was a few volts RMS, and plenty stiff. Messed up some
printer signals pretty good.

The real way to pipe signals around is fiberoptics. A kilovolts of
common-mode is no problem.

Hey, time for our engineering meeting; Japanese food today!

John