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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We can use a pulse generator and an oscilloscope to make a set of economic time-domain

reflectometer instruments to measure the length of  the cable.

The principle of using TDR to measure the cable length.

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Transmit a high-speed edge pulse signal to one end of an unknown length cable. The signal will

propagate along the cable to the other end. If the other end of the cable is disconnected, the signal

will reflect back to the starting point after reaching the end point. Use an oscilloscope to measure

the time difference between signal transmission and reflection, you can calculate the length of the

cable.

The wire or cable only transmits energy, and the wire itself does not consume energy or almost does

not lose energy. When the radio-frequency signal reaches the end of the cable, there is no way to

release the energy, and it will propagate back along the cable. Just like us yelling at the wall, the

sound hits the wall and returns back to produce an echo.

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The formula for calculating cable length is as follows

Length calculation formula

L = cable length, unit: meter.

Δt = the time difference between signal transmission and reflection, unit: second.

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

VF = cable speed factor, constant, no unit.

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The speed of the electrical signal in the cable cannot reach the theoretical speed of light. The ratio

of the speed of light to the actual speed of the electrical signal in the cable is called the velocity factor

or velocity of propagation). Abbreviation: VF or VOP.

For coaxial cables, the speed factor (VF) of the coaxial cable varies with different dielectric materials

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Experiment Equipment

l Fast edge pulse generator small board

l One oscilloscope supporting 10 nanoseconds or smaller time base

l 5V DC power supply

l To be tested cable

l BNC adapter

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                                TDR Pulse Generator

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BNC Tee Adapter ( one male to two female)

BNC Adapter (male to male)