> I have a customer who wants to measure the characteristic impedance of a
> length of coax. Actually, we know what it is supposed to be according to
> the
> manufacturer, but they want to verify it.
>
> Test equipment available: high accuracy multimeters (5-1/2 digit?), cheap
> analog oscilloscope, relatively low frequency signal generator (sine,
> square, triangle) (top end is maybe 1Mhz), frequency counter, bench
> supply.
>
> Willing to buy high-precision R's or C's if needed.
>
> Accuracy required: hopefully a few percent, 10% is some help.
>
> I am really a digital guy, so this is a bit out of what I do.
>
> Any ideas?

How about this?

Drive the coax with a square wave through a resistor that is the same as
the suspected characteristic impedance (the driving resistance is not
critical here). Watch the coax input with a scope. Vary the resistance at
the far end of the coax until the scope shows a square wave. With anything
other than the characteristic impedance terminating the coax, you should
see the reflection. If the driving impedance is the same as the
characteristic impedance, you should see only one reflection. In that
case, the square wave will either have a step up or down before settling
at the proper voltage depending on whether the terminating impedance is
low or high.

Good luck!

Harold



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