🚀 go-pugleaf

On Thu, 25 Jun 2009 12:56:30 -0700, Joerg <invalid@invalid.invalid>
wrote:

>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.

Or a nice wideband ESD spark. Then the receive-end differential signal
might see a kilovolt p-p common-mode ring.

 If 
>the shield current is of concern it can be broken by a wee ferrite.

Right. That's easy to add as needed.

>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.

It's terrible here. Scope traces are gross fuzz, and all sorts of
opamp front-ends like to rectify stuff.

You should see the spectrum of a 10" clip lead.

http://www.jimprice.com/sutro/tower1.jpg

We did do a useful simulation of a system that has a superconductive
magnet/front end on one side of a room and some ADCs on the other
side. We convinced ourselves that we need to go differential.

John