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Title: Measure the Length of Cable by an Oscilloscope [Print this page]

Author: WisdomAugust Time: 2022-12-3 07:59
Title: Measure the Length of Cable by an Oscilloscope

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

[attach]5301[/attach]

Author: WisdomAugust Time: 2022-12-5 08:52

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.

[attach]5303[/attach]

Author: WisdomAugust Time: 2022-12-7 08:00

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.

Author: WisdomAugust Time: 2023-1-3 13:11

The formula for calculating cable length is as follows

[attach]5327[/attach]

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.

Author: WisdomAugust Time: 2023-1-4 08:41

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

Author: WisdomAugust Time: 2023-1-5 14:36
[attach]5334[/attach]

Author: WisdomAugust Time: 2023-1-6 13:04
[attach]5339[/attach]

Author: WisdomAugust Time: 2023-1-10 13:44

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

Author: WisdomAugust Time: 2023-1-11 16:27
[attach]5352[/attach]

TDR Pulse Generator

Author: WisdomAugust Time: 2023-1-12 08:18
[attach]5353[/attach]

BNC Tee Adapter ( one male to two female)

[attach]5354[/attach]

BNC Adapter (male to male)

Author: WisdomAugust Time: 2023-2-1 08:19

[attach]5367[/attach]

BNC Connector (female to dual-site of twisted pair cable)

Author: WisdomAugust Time: 2023-2-3 08:37

We will measure the length of the two cables separately.

[attach]5374[/attach]

BNC extension cord

Author: WisdomAugust Time: 2023-2-6 08:07

[attach]5377[/attach]

0.75 square 300V/500V wire

Author: sophiehandrix Time: 2023-2-6 20:13
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.

Author: WisdomAugust Time: 2023-2-7 08:12

Experimental procedures

Measure the length of the BNC extension cord

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

[attach]5380[/attach]

Author: WisdomAugust Time: 2023-2-8 08:30

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.

[attach]5388[/attach]

Author: WisdomAugust Time: 2023-2-9 08:35

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

the reflected signal.

[attach]5390[/attach]

Author: WisdomAugust Time: 2023-3-1 08:29

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:

[attach]5405[/attach]

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

Author: WisdomAugust Time: 2023-3-2 08:10

Measure the length of 0.75 square wire

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

[attach]5408[/attach]

Author: WisdomAugust Time: 2023-3-3 08:34
[attach]5410[/attach]

Author: WisdomAugust Time: 2023-3-6 10:59

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.

[attach]5412[/attach]

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

Author: WisdomAugust Time: 2023-3-7 08:34

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

[attach]5428[/attach]

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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