What is TDR?

PUBLISHED: September 25, 2014

Time domain reflectometers, or TDRs, have been among the most common tools in the field of test and measurement for 50+ years. Widely available and easy to use, TDRs are indispensable for design and diagnostic applications of all kinds.

But what is a TDR anyway? And why is it so useful?

How TDR Works

Time domain reflectometry is a measurement technique that provides information about electrical transmission lines by analyzing the reflections of short electrical pulses.

When electrical system has known impedance properties, any inductive or capacitive discontinuities produce reflections that can be picked up by a TDR. 

The magnitude and position of these discontinuties provides information about what the discontinuity is (e.g. broken cable, loose connector, corroded circuit board). Perhaps more importantly, TDR can also tell you the location of the discontinuity to within a few centimeters.

An example is shown in Figure 1 below. A HYPERLABS HL1101 Ruggedized TDR is connected to an unterminated 1 meter length of 50 ohm coaxial cable. Graphical output is shown in our ZTDR™ software. 

Because the coax cable is in open at its far end (5.5 m), the full signal is reflected back to the TDR sampler. The incident step (2.5 m) shows the initial pulse generated by the TDR.

Figure 1: Time domain waveform on 1 meter of coax (click for full size)

Some people like to think of TDR as a sort of "radar" for cables and circuit boards. The general concept is the same; by seeing what is (or isn't) reflected, you can learn a lot about what's out there.

We won't delve too deeply into the math of time domain reflectometry here (you can learn that in the Wikipedia article). For now, suffice it to say that this technology has been reliably used by scientists and engineers for over 50 years.

Where TDR is Used

TDRs are indispensable for design and diagnostic applications of all kinds.

These instruments are commonly used for such applications as cable fault detection, impedance characterization, and delay/time-of-flight measurements.

One of the first, and probably the most widespread, application of TDR is in the telecommunications industry. TDR is used to analyze long lengths of cables which are often either overhead or underground. If a fault in the cable is detected, a technician can pinpoint the place where repair is required.

Electrical impedance, an important property of cables and circuit boards, can also be derived from TDR readings. This application is called "impedance characterization", and the instruments are sometimes referred to as "controlled impedance analyzers".

Virtually all electronics manufacturers use some sort of impedance characterization as part of the their design and quality control processes.

Although less common, TDR is also used in a variety of geotechnical and civil engineering applications. These include detecting rock and concrete fractures, testing building integrity, measuring water levels and turbidity, and soil moisture analysis.

Future Application Guides will cover these and other TDR applications in much greater detail.

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