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We have a custom made data cable at my work, is there any good way to measure \$Z_0\$ of a cable, or must the formulas be used?

Trying to work out what value termination resistor to use.

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  • 4
    \$\begingroup\$ If you had it custom made, and the impedance is important, it should have been in the specification! \$\endgroup\$ – stevenvh May 18 '12 at 4:17
  • \$\begingroup\$ It wasn't custom made by me, all work inherited from the predecessors... as usual \$\endgroup\$ – fred basset May 18 '12 at 15:29
  • \$\begingroup\$ Is the cable designed to be driven with differential signals (like twisted pair) or single-ended (like coax)? \$\endgroup\$ – The Photon May 18 '12 at 15:32
  • \$\begingroup\$ Differential, RS-485 \$\endgroup\$ – fred basset May 19 '12 at 6:22
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The proper basic kit is called a TDR (time-domain reflectometer). A more advanced version is called a Two Port Network Analyser, both are usually expensive pieces of specialist test kit.

However, you can measure the impedance with normal lab kit in the following way;

Build your own TDR setup; You just need a fast oscilloscope and a pulse generator. See The Wiki page for a TDR This works by sending a short pulse down the cable and measuring the amplitude of the reflected pulse.

Unless you have a very fast oscilloscope and signal generator, work with a long cable (10's of m or more) to ensure you get a decent delay (or you won't able to tell the difference between the incident and reflected pulses) However if the cable is too long the attentation will make distinguishing the reflected pulse from noise very difficult.

Depending on what construction is used, signals travel down a cable at about 0.7 * the speed of light.

Do the same with an open circuit, a known resistance and a short circuit terminating the cable. The three values should be similar, take an average.

Method

Your kit setup should be as follows, though the picture is missing the termination resistor (or short) from the tail end of the cable.
enter image description here

Measure the height of the pulse out and back (incident and reflected) and divide them (rho), then solve the following equations:

$$ \rho = \frac{Vr}{Vi} $$

Vr is the reflected voltage
Vi is the incident voltage
The characteristic impedance is Zo
The termination impedance is Zt

$$ \rho = \frac{Z_{t} - Z{o}}{Z_{t} + Z{o}} $$

More about this is explained in this document.

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Take a length of cable, leave the end open, and measure the impedance \$Z_a\$. Then repeat with the cable end shorted to get \$Z_b\$. Both are complex impedances. Your cable's characteristic impedance is

\$ Z_0 = \sqrt{Z_a \times Z_b} \$.

Again, this is the complex square root.

The characteristic impedance is frequency independent, but you can't measure it at DC, so not with a common multimeter. Like Telaclavo says, at low frequencies the characteristic impedance may vary; it will only be constant above 100kHz to 1MHz. Despite being frequency independent whenever possible you'll usually measure at your working frequency.

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  • \$\begingroup\$ Just to point out that he has to measure that at the frequency of interest, using special equipment. He can't do that with an ohmmeter. \$\endgroup\$ – Telaclavo May 18 '12 at 13:42
  • \$\begingroup\$ @Telaclavo - obviously, but I'll add it to my answer. \$\endgroup\$ – stevenvh May 18 '12 at 13:44
  • \$\begingroup\$ It is (mostly) independent of frequency... above a certain frequency, which can be between 100 kHz and 1 MHz. Below that, it varies a lot. google.es/… \$\endgroup\$ – Telaclavo May 18 '12 at 14:18
  • \$\begingroup\$ @Telaclavo - Right again :-). BTW, feel free to edit my answer if you think it needs improving/adjusting/clarification. \$\endgroup\$ – stevenvh May 18 '12 at 14:38

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