🚀 go-pugleaf

Nico Coesel wrote:
> 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 :-)
> 

Try differential transfer, shielded twisted pair or Twinax soldered to 
the burden resistor, from there to your measurement box, place a 2nd 
signal transformer there. Make sure the twisted pair is source- and/or 
end-terminated properly, usually 100-120ohms. And never, never use this 
connection as the burden resistor even if there is a fire extinguisher 
at hand ...

-- 
Regards, Joerg

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