Measure The Length Of Coax With An Oscilloscope

measuring the length of coax with an oscilloscope setup


In this post, I’ll show you how to measure the length or find a break in a coaxial cable with the help of your oscilloscope and waveform generator. We can do this using signal reflection that occurs when a transmission line is not terminated correctly.

The setup is very simple just connect the output of the waveform generator, oscilloscope input and coax cable together with a T connector(see the image above). I set the waveform generator to output a 1MHz square wave at 2V and a 50% duty cycle. The set frequency and voltage aren’t that important. What matters more is that we have a fairly fast rising edge.

As you can see there is no reflection with a proper 50Ω termination.
But when we remove the termination or add a break in the line a signal reflection will appear. The first “step” is the initial signal while the second “step” is the reflection of the initial signal coming back. The time from the first step to the second one is the time it took the signal to travel to the end of the cable and back.
We can use the following equation Length = c * v * dT / 2 to figure out how long the cable is. The c is the speed of light, v is the velocity factor(the signal travels a bit slower than light through the cable, this factor tells us how much slower, usually around 0.8 but it depends on your particular cable), dT time between the first and second step, finally we need to take into account that the signal traveled to the end of the cable and back so to get the actual coax length we divide it by 2.
If we put the numbers into the equation Length = 299 792 458 m/s * 0.8 * 18nS / 2 the length comes out to be 2.15m. The actual cable length is about 2m so the result is just a little off.

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