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May 1998

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Subject:
From:
Paul Terranova <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 6 May 1998 15:31:35 -0400
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About TDR,

The good news is that the error is supposed to get worse the further you
are from 50 ohms impedance. I won't claim to know too much about this
since I'm a Materials Engineer, not an EE. The guy to talk to is Mike
Trangmontage (spelling?) at Tektronics. He's the TDR guru who happens to
sell a PC based program to measure TDR. He has a good paper on it to. I
haven't spoken to him in about 3 years but I have his number as
617-861-5846 or 1-800-756-7000 box #863-6001.

Yes you can calibrate the unit for whatever value but usually it gets
calibrated to 50 ohms. If you usually measure impedances of 100 ohms,
then you might want to calibrate it with a standard of 100 ohms. We use
a 50 ohm airline standard that doesn't need to be certified yearly.
However, when I was looking about 4 years ago, no one was making 100 ohm
airlines. Check with Mike on using resistors. I think its ok to use if I
remember. I assume you already have the IPC test method on performing
TDR. There's some useful info in it.

Regards,

Paul Terranova
Analytical and Environmental Test Services Lab

 Digital Equipment Corporation
 200 Forest Street
 Mail Stop: MRO3-1/D2
 Marlboro,  MA  01752-3085

'    Phone: (508)467-3109
* Fax: (508)467-6796
* Email: [log in to unmask]
WebSite: http://www.digital.com/lab-services

                -----Original Message-----
                From:   Glynn Shaw [mailto:[log in to unmask]]
                Sent:   Wednesday, May 06, 1998 12:27 PM
                To:     [log in to unmask]
                Subject:        [TN] Quick Check of TDR Impedance
Accuracy

                Oh Guru's:
                A circuit designer has suggested that we can double
check the accuracy of
                our TDR for measuring specific impedances by connecting
the TDR's probes
                directly to a precision resistor of the desired
impedance. When I tried it,
                I got a nice flat line on my scope, but alas, it was not
exactly where my
                instrument said it should be. In addition, different
resistors yielded
                differing amounts of error (4 ohm discrepancy at 50 ohm,
14 ohm discrepancy
                at 100 ohm). My unit was calibrated by Tektronix only 2
months ago.
                Therefore, my questions: Is the Electrical Engineer
correct; is his
                procedure accurate? There is no reference to it in any
of our TDR manuals,
                and I think that if such a thing were true, then mention
should have been
                made of it.
                As Bartels and James would say: "Thanks for your
support."


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