Quick & dirty cable length & impedance measurement

-July 11, 2014

Most analog folks are familiar with the use of a pulse generator and a scope to measure cable reflections and impedance. For those who are not, this lashup is called a reflectometer. Available as complete instruments, the portable versions are used often by cable installers; high-end versions are used for shorter pcb transmission lines in the lab. This trick will not replace a vector network analyzer, but for basic cable measurement, here is how to do it:

  • Connect a pulse generator with a 50O output to a few feet of 50O coaxial cable (e.g., RG58, RG174), just long enough to reach to a BNC Tee connector mounted on a scope vertical input. Set the scope input impedance to 1MO, not 50O.

  • Connect a long length of your cable under test to the other side of the Tee connector. Leave the far end of the cable open.

  • Set the pulse generator to a narrow pulse of about 1 to 100ns; the width depends on the cable length and can be fine tuned later. The duty cycle should be as small as possible and again depends on the cable length. Pulse voltage is not critical; 1V peak will do.

  • Set the scope trigger level to just under the peak voltage of the 1V pulse and set the time base long enough to display a pair of 1V generator pulses. Now you get to play.

What happens is the pulse launched into the cable whips by the scope and triggers the sweep. That pulse continues to the far end of the cable where it encounters the open end. Energy cannot be destroyed, so the pulse bounces back towards the generator. As it passes the scope, it displays again, this time a bit lower in amplitude due to the cable attenuation, and with a bit more slope to the rise/fall time because the high-frequency harmonics are being attenuated more. You might see some additional echoes if the pulse reflects off the scope input capacitance or the generator. The only echo of interest is the first one after the launched pulse.

If you know the cable length, you can calculate the round trip time. In most cables, the pulse travels at a velocity factor of about 66% of the speed of light in a vacuum (300 meters/µs), resulting in a cable speed of roughly 200 m/µs (check the cable data for a more precise number). Figure out the time it will take for the pulse to arrive back at the scope, to get an idea of where to set the generator period between launched pulses (duty cycle), and the corresponding scope time base, to see the reflected pulse. This pulse will probably not trigger the scope with an amplitude below the trigger level unless the cable has very low attenuation. If you prefer, you can trigger the scope from the pulse generator sync output.

If you do not know the cable length, you now know how to measure it. Play with the pulse period and time base until you can see both the launched pulse and the reflected pulse. The time between them is the round trip time of 0.1 m/ns, or 10ns per metre of cable length. If the cable round trip time is too short for the minimum pulse width of the generator, you can still see the reflected pulse as a secondary step on the generator pulse. See the sketch below:

Impedance measurement is simply a matter of placing a small trimpot of about 200O across the far end of the cable and adjusting it to minimize the amplitude of the reflected pulse, which now gets fully absorbed by the trimpot load (less a small reflection from the trimpot stray capacitance). If the trimpot is set higher than the cable impedance, the reflected pulse is the same polarity as launched, but if the trimpot is below the cable impedance, the reflection polarity is opposite. Take the trimpot off the cable, and without disturbing the setting, measure it with an ohmmeter. This is the cable characteristic impedance.

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