🚀 go-pugleaf

John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

>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.

I wish... I'm working on interfacing a current sensing coil from a
welding transformer. During welding there is a couple of hundred volts
everywhere. This project starts to give me a headache :-)

-- 
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
                     "If it doesn't fit, use a bigger hammer!"
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