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Re: An unusual Oscilloscope phenomenon



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That's usually the best way to ground shields.

John



Re: An unusual Oscilloscope phenomenon


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IIRC head signals mostly went via twisted pair. If they had a shield as well we
would certainly not ground them on both sides.

Tony

Re: An unusual Oscilloscope phenomenon



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You *want* large amounts of high-frequency common-mode noise?

John


Re: An unusual Oscilloscope phenomenon


John Larkin a écrit :
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Probably. Lots of people don't understand this and I've seen this kind
of mistake (opened shield) recommended in lots of highly regarded EMC books.

But that's really nice for us. (Joerg?)


--
Thanks,
Fred.

Re: An unusual Oscilloscope phenomenon


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Yeah! But shhht, don't spill the beans here, at least not until I am
retired :-))

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: An unusual Oscilloscope phenomenon


Joerg a écrit :
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I think I'll retired after you, but this kind of misconception is
sufficiently widely spread that we don't have to worry :-)

--
Thanks,
Fred.

Re: An unusual Oscilloscope phenomenon


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Oh, I am not worried at all :-)

But I am concerned about what happens when guys like you and I are in
their 90's, the hands are shaky, the mind ain't what it used to be and
the senior living place doesn't allow a Tektronix mainframe in the
rooms. The next generation doesn't have anough analog engineers.

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: An unusual Oscilloscope phenomenon



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[snip]
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Won't matter.  We'll all be deep into communal despair :-(

                                        ...Jim Thompson
--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Re: An unusual Oscilloscope phenomenon


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All the more money for me.  ;-)

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms



Re: An unusual Oscilloscope phenomenon



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Yup. Once people get fixated on "ground loops" they are forever lost
to reason.

Ditto high-speed "return currents."

John


Re: An unusual Oscilloscope phenomenon


On Thu, 25 Jun 2009 13:33:59 -0700, the renowned John Larkin

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Two coaxial shields with one end of each grounded at opposite ends is
pretty nice.



Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
snipped-for-privacy@interlog.com             Info for manufacturers: http://www.trexon.com
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Re: An unusual Oscilloscope phenomenon


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OK, how about "image charge" then?

Jeroen Belleman

Re: An unusual Oscilloscope phenomenon



John Larkin wrote:
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   There is a fairly mindless thread about ground loops on
news:sci.electronics.repair right now.


--
You can't have a sense of humor, if you have no sense!

Re: An unusual Oscilloscope phenomenon


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3D%
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Is that open end shield rather than opened shield, or am I missing
something here. Grounding one side is pretty common knowledge, though
you get arguments over which side to ground.

Re: An unusual Oscilloscope phenomenon


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No, which is precisely the reason to ground only one end of the shield.

Cheers!
Rich



Re: An unusual Oscilloscope phenomenon


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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.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
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Re: An unusual Oscilloscope phenomenon


On Thu, 25 Jun 2009 13:48:35 -0400, Phil Hobbs

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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.

John


Re: An unusual Oscilloscope phenomenon


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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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Re: An unusual Oscilloscope phenomenon



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

 If
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Right. That's easy to add as needed.

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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


Re: An unusual Oscilloscope phenomenon


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[...]

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Once I took off on a Europe flight out of SFO and the antenna top stuck
out of the clouds while the rest of the tower was in the white. Picture
perfect, and I had my camera in the overhead <aargh>. I could still bite
myself, that opportunity never came again.


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Yesterday I had my comeuppance WRT to the Instek GDS-2204 scope that I
thought is so much better than Tektronix. In many ways it is, and also
doesn't have the nasty conducted emissions on the probe cables. But when
debugging 74MHz gear I could not figure out where a weird buzz was
coming from. Until I turned off the scope ...

--
Regards, Joerg

http://www.analogconsultants.com /

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