We have several Tektronix TDR's (11801's), and I would not recommend
checking calibration using resistors.  The use of a precision 50-ohm
coaxial air line would be much better.  If you don't have one, then a
short length of semi-rigid coax would be OK - a few inches to a foot
(just so it's long compared to the TDR risetime) - if you can stand a
little less accuracy.  Air line accuracy would be within tenths of an
ohm, semi-rigid would be on the order of an ohm or so.  In either case,
I would not provide a DC termination because it will affect the voltage
level the TDR calls 50 ohms.  TDR accuracy also decreases the further
you get from 50 ohms, higher or lower.

We have measured cable & board impedances with different quality SMA
terminations on the far end and it causes variations in the readings of
the impedances ahead of it.  In theory a TDR doesn't "know" what lies
down the path before the wavefront gets there.  But the TDR is not
really a transient (single-shot) measurement.  It gathers its info by
sampling repetitively a series of voltage steps over a time much longer
than the wavefront propagation occurs.  And the DC conditions will
affect the measured impedance values.  You can check this condition by
putting different resistors on the end of the test cable and seeing its
effect on the reported impedance of the cable.  (One little disclaimer
here - I do not have experience with TDR's designed for field testing of
cables, just general lab use TDR's, so they may behave differently.)
Even the series resistance of a long cable will affect the reported
value of a terminating resistor at the far end.  This can be verified by
measuring the resistor close to the TDR and then farther away at the end
of a long enough cable.

I hope I didn't drown you in the details.

Now about that Bartles & James ...

> -----Original Message-----
> From: Glynn Shaw [SMTP:[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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