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May 1998

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Subject:
From:
Don Vischulis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 7 May 1998 21:47:06 -0500
Content-Type:
text/plain
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Glynn:

A resistor is designed to provide an impedence of 50 ohms in a DC circuit.  The characteristic
impedence of a circuit using TDR is highly dependent on the physical design of the resistor.
I''m only a Chemical Engineer, but I believe that the resistor type (carbon comp., carbon film,
metal film, wire wound, etc.) will also have different effects when using frequencies of 1MHz and
greater.

An electrical engineer with radio frequency background could supply better information, but strip
line or co-axial circuits designed to provide the proper impedence at the specified frequencies
will provide better calibration standards.

Don Vischulis
[log in to unmask]

Glynn Shaw wrote:

> 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.

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