RE: [SI-LIST] : What's your favorite Screwy SI Concept?

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From: Dave Hoover (dhoover@mccurdy.com)
Date: Thu Jan 06 2000 - 12:42:55 PST


Speaking from a fabrication stand point.....

The (art) process of taking TDR measurements has many
variables. Most...stand a lot to be desired. There is
a substantial difference in measurements between TDR
equipment. (Tek, HP, and Polar) An even larger delta
is found when measurements are taken and compared to
a network analyzer result.
When current standard industry cabling and probing
techniques are used, there is about a .7 Ohm std dev
found when G. R. and R. and reproducibility tests are
done. So one could anticipate ~2.1 Ohms at 3 sigma. :/
But there is some light at the end of the probing tunnel.
NIST has some probes being evaluated that are yielding
much less. Measurement error is down as low as .07-ohms!!!!
That's just in equipment......
Now fab variation is another story.
I agree with Bob's comments about testing a VW to Ferrari
specs (No pun intended Gary). I also agree to Arpad's comment.
The IPC has us fabricators taking a sample measurement and
averaging the Zo based on the undisturbed interval of the
waveform.

Dave Hoover
-----Original Message-----
From: Muranyi, Arpad [mailto:arpad.muranyi@intel.com]
Sent: Thursday, January 06, 2000 12:47 PM
To: 'si-list@silab.eng.sun.com'
Subject: RE: [SI-LIST] : What's your favorite Screwy SI Concept?

Along those lines I would add that a nominal X-Ohm trace can
have +/- several Ohms of variation due to even and odd mode
signaling. So a "50 Ohm" trace can easily show 48-52 Ohms of
impedance in such situations. So much for all the fuss about
matching the impedance of the driver and/or termination to the
line impedance precisely...

Arpad Muranyi
Intel Corporation
===================================================================

-----Original Message-----
From: Haller, Robert [mailto:Robert.Haller@compaq.com]
Sent: Thursday, January 06, 2000 11:35 AM
To: 'si-list@silab.eng.sun.com'
Subject: RE: [SI-LIST] : What's your favourite Screwy SI Concept?

Ok, I'll bite too.

Reporting the impedance of a PCB trace at 50.123 Ohms when measurement
accuracy is +/- .5 ohms at best.

Using a 40 ps TDR edge to measure propagation delay for a considerably
slower application.

Bob Haller
Compaq Computer Corporation

>Subject: [SI-LIST] : What's your favourite Screwy SI Concept?
>Cheers,
>Andrew Phillips
>Supercomputing Systems AG
>Zurich, Switzerland

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