What is the principle of cable length tester?

Simply put, time domain reflectometer (TDR) is one of the technologies to upgrade the traditional cable tester to a cable troubleshooting tool.

Most cable verification testers can only perform basic continuity tests, such as wiring diagram and length. The verification tester with built-in TDR is not only limited to continuity verification, but also can provide the function of locating basic wiring faults, such as short circuit and open circuit. The cable tester with built-in TDR will apply a series of voltage pulses to each pair of cables, and some of these pulses will return to the tester from the cable position, accompanied by certain impedance changes. Open circuit and short circuit will show very special impedance changes. An open circuit or an open circuit will respond to a positive pulse while a short circuit will respond to a negative pulse. The tester will calculate the cable length or the distance between the open circuit and the short circuit point by recording the time sequence of the response pulse, and will judge whether the response is open circuit (cable termination or open circuit) or short circuit by recording the polarity of the response pulse.

For cable technicians who need to solve faults quickly, the information provided by TDR test is very valuable. Because most cable failures are caused by bad terminal connectors or metal objects piercing the cable, knowing the distance between open circuit and short circuit will enable technicians to quickly find the faulty connector and replace the nailed cable. On average, 80% of the time needed to solve a defect or problem is spent on finding the problem, and only 20% of the time is spent on solving the problem, so the ability of TDR function to locate the problem will greatly accelerate the pace of solving the problem.

CableIQ qualification tester released by Fluke Network has created a new method, which combines TDR test with wiring diagram test, makes the graphic representation of wiring diagram very easy to read, and shows the correct distance of open circuit and short circuit in proportion. For example, if the total cable length is 100 feet, the 3rd/6th pair of cables is disconnected at 75 feet. In the graphic display of the tester, the 3rd/6th pair of cables is only 75% of the length of other cables. In addition, the tester can also detect the open circuit of a single cable, measure the length of a single cable and detect the short circuit between cable pairs. Compared with the traditional TDR tester, the improvement of display accuracy and readability greatly speeds up the troubleshooting.

There are some alternatives to TDR technology, the most common one is the measurement of capacitance length. This method relies on the measurement of cable capacitance and calculates the length by applying an appropriate scale factor. However, compared with the troubleshooting ability of TDR, the inherent characteristics of capacitance method bring obvious defects. For example, for short-circuited cables, this method will not work because it no longer acts as a capacitor. Therefore, if the fault happens to be a short circuit, it will be impossible to test the length of the cable or the distance to the fault point. In a word, TDR is the best fault solution for basic cable verification.