Measure the Length of Cable by an Oscilloscope

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Purpose

Use an oscilloscope to measure the length of wire.

Principle

Time-domain reflectometer (TDR) is an electronic instrument that measures the wire by observing the reflected waveform in the cable.

One use of the time-domain reflectometer is that it can measure the length of a cable (like twisted pair cable and coaxial cable).

The time-domain reflectometer looks like this

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If the speed factor of a certain cable is unknown, you can calculate the VF by a cable of a known length.

VF = cable speed factor, constant, without unit.

L = cable length, unit: meter.

Δt = the time difference between the transmitted signal and reflected signal, unit: second.

C = speed of light, unit: meters per second.

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As the above screenshot shows, the time difference between the transmitted signal and the reflected

the signal is 61.2 nanoseconds, which is calculated using the formula as below.

The calculated cable length is 5.504 meters, and the actual length of the cable is 5.5 meters.

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Measure the length of 0.75 square wire

1 Change a connector to measure the length of the 0.75 square wire.

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As the above screenshot shows, the time difference between the transmitted signal and the

the reflected signal is 16 nanoseconds, which is calculated by using the formula:

The calculated cable length is 1.51 meters, and the actual length of the tested cable is 1.5 meters.

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Use the oscilloscope’s cursor to measure the time difference between the transmitted signal and

the reflected signal.

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2 Connect the TDR pulse generator to the oscilloscope and insert the to-be-tested BNC extension

cord into the BNC tee adapter as below. Turn on the oscilloscope and 5V DC power supply.

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Experimental procedures

Measure the length of the BNC extension cord

1 Connect the TDR pulse generator and the BNC adapter as follows

sophiehandrix

For all control cable manufacturers this device is very helpful. Because it helps them to measure the length of cable to provide 100% authentic cable to their customers. I will tell about this device to my cable manufacturers friends, this device will definitely help them.