🚀 go-pugleaf

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:
>>>
>>>> "David L. Jones" <altzone@gmail.com> wrote in message 
>>>> news:h8B0m.53620$FI5.3564@newsfe12.iad...
>>>>> Check out a rather unusual phenomenon you can see on your oscilloscope in 
>>>>> my latest blog (#14):
>>>>> http://www.alternatezone.com/eevblog/
>>>>> See if you can get it to happen on your scope!
>>>>> I'll offer a free subscription to my blog for the best screen shot posted 
>>>>> :->
>>>>>
>>>>> Oh, and the probe doesn't have to be shorted either.
>>>>>
>>>>> Have fun.
>>>>> Dave.
>>>>> -- 
>>>>> ================================================
>>>>> Check out my Electronics Engineering Video Blog & Podcast:
>>>>> http://www.alternatezone.com/eevblog/
>>>>>
>>>>>
>>>> When I worked a Century Data we had a problem with our new reel to reel 
>>>> vaccuum column tape drive when ever someone would walk by the drive when it 
>>>> 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.
>>>>
>>>> Bob 
>>>>
>>> That's usually the best way to ground shields.
>>>
>>> John
>>>
>>>
>> 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.
> 

It could become really, really nasty if the cable length happens to be 
close to 1/4, 3/4 and so on wavelength of the offending RF signal. If 
the shield current is of concern it can be broken by a wee ferrite.

Probably the litmus test for RF stuff like this is when people schlepp 
it to Otis Street and see if it can survive Sutro Tower.

-- 
Regards, Joerg

http://www.analogconsultants.com/

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