Time Domain Reflectometer: 3 Important Facts

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Points of Discussion

Introduction to Time Domain Reflectometer

Before we start learning about the time domain reflectometer – TDR, let us know a reflectometer.

Reflectometer: A reflectometer is a type of circuit that isolates and samples the incident and reflected powers from a load using a directional coupler.

Reflectometers are prime applications of passive microwave components. A reflectometer is used in a vector network analyzer as it can measure various parameters like – reflection coefficient for the one-port network, scattering parameters for the two-port network. It can also be used in replacement of an SWR Meter or also as a power monitor.

Time Domain Reflectometer: A time-domain reflector or TDR is an electronic device based on a reflectometer’s property that finds out characteristics of electrical lines from the reflected waves.

TDRs are used for finding out faults in cables like twisted pairs of cables or coaxial cables. This article will learn more about the device, the uses of the time-domain reflector, and explanations about it.

Time Domain Reflectometer
A typical Time Domain Reflectometer, Image Credit: By Megger Ltd. – Megger Ltd, CC BY 3.0, Link

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Description of Time Domain Reflectometer

Working Principle

A TDR analyzes the reflected signals sent by itself. To analyze the reflections, it first transmits a signal along the cable and waits for the reflection. If there are some defects or mismatches in the transmission line or the cable, the part of the incident wave is reflected. TDR receives the reflected wave and then analyzes it to locate and measure the faults. But if there are no defects or everything is fine, then the signal reaches the far end without reflection, and the cable is considered acceptable. The working principle of a Time Domain Reflectometer is almost similar to the working principle of a RADR.


The TDR analyzes the reflected wave. It is interpreted that the amplitude of the reflected wave determines the impedance of discontinuity. The reflected pulses also determine the distance of the reflected wave, which further determines the fault’s location.

Time Domain Reflectometer
Readings from a TDR, Image Credit: Constant314TDR trace of cable with open terminationCC0 1.0


Time Domain Reflectometer starts its operation by sending impulse or step signals or energies. Then it observes the reflected energy or the signals subsequently. The discontinuity of impedance is measured and analyzed by the reflected pulses of energies as the amplitude, magnitude, and waveforms help in analyzing.

For example, suppose an impulse function is sent from TDR towards a connected load. In that case, the reflectometer shows an impulse signal on its display, and the amplitude indicates the impedance of discontinuity. The following expression gives the relation between the load impedance and the magnitude of the reflected wave.

P = (RL – Z0) / (RL + Z0)

Z0 is the characteristic impedance of the transmission line or the coaxial cable. RL is the connected load resistance.

Any impedance discontinuity is observed as the termination impedance, and the termination impedance replaces it. The process consists of rapid changes in the characteristic impedance of the transmission lines.

Transmitted signals of TDRs

Time-domain reflectometers use various kinds of signals as incident signals. Some of the transmitters use pulse signals. Some of them use fast rise time step signals. Some of them also use impulse functions of signals.

TDRs using pulse signals send the pulse through the cable. Their firmness depends on the width of the pulse sent by them. That is why narrow pulse signals are preferred. But there is a shortcoming for the narrow width pulses as they are of high frequencies. High-frequency signals get distorted inside large cables.

Reflected Signals of TDR

Typically, the waves reflected from the load impedance or due to the impedance of discontinuity are similar to the incident waves in their shapes. Still, the magnitude and other properties get varied. If there is some change in the load impedance, the reflected wave does the exact change in its parameters to indicate the changes. For example, if the load impedance gets a step increased, the reflected wave will also have an increased step in it.

This property of reflected wave finds applications in many fields for Time Domain Reflectometer. TDRs are used to ensure the cable’s characteristic impedances, other impedance parameters, no mismatch at connectors or joints.

Time Domain Reflectometer
Signal transmission and reflection from load discontinuity, Image Credit: Oleg AlexandrovPartial transmittance, marked as public domain, more details on Wikimedia Commons

Applications of Time Domain Reflector

Time Domain Reflectors are mainly used for testing purposes of the very long cables. If any fault arises in very long cables, it is practically impossible to locate the fault after digging up the kilometers-long cable. That is when a TD reflectometer comes into action. The time-domain reflectometer is capable of measuring the resistances on connectors and can sense (detects) the faults way before the catastrophic failures.

TDRs also find applications in communication lines as they can catch any minute change of line impedance due to the introduction of any tap or splice.

Time-domain reflectometer devices are crucial for PCBs. Printed circuit boards designed for high frequencies need TDRs for their fault analysis. Some of the major applications are listed below in detail.

> Analysis of Semiconductor Devices

TDRs are useful for locating defects in a semiconductor package. Using the property of domain reflectometry, a TDR provides marks for each conductive trace. It is beneficial for finding out the exact location of the opening and shorts.

> Level Measurement using TDR

As mentioned earlier, TDRs are beneficial and essential devices for finding out and locating faults for long wire cables. A more advanced device – a TDR-based level measurement device can find out the level of a fluid using that ancient and fundamental property.

For measurement purposes, the device sends a signal through the cable or the waveguide. A part of the signal gets reflected after the signal incident or hits the medium’s target surface. Now, the device calculates the period by calculating the difference between the send time and the reflected wave’s receive time. The period now helps to determine the level of the fluid. As the device measures the fluid level, that is why it is called the Level Measurement Device.

The internal sensors of the device process the analyzed output using analog signals. But there are also some difficulties while the propagation of the signal gets varied by the medium’s permittivity. The moisture content also varies the propagation greatly.

> Applications of TDRs in Geotechnical Engineering

TDRs are extensively involved in the Geotechnical Engineering domain. They are used to observe the slopes’ movements using various tools like highway cuts, rail beds, and open-pit mines.

TDRs are also used for stability observation. In the process of observation, a cable is set up close to the concerning region. Any mismatch of insulators between conductors affects the electrical impedance of the coaxial cable. A hardcover surrounds the coaxial cable. It helps to interpret the earth’s movement via a rapid cable distortion. The deformation causes a peak in the monitor of the reflectometer device. Nowadays, signal processing techniques are doing the same job more efficiently.

> Determination of Soil’s Moisture

Time-domain reflectometers are used for determining the moisture level of soils. The process of measurement is quite a simple one. A TDR is placed inside different soil layers, and then the start time of precipitation and the time when the soil moisture increased is noted. TDRs are useful to measure the speed of water infiltration.

> Applications in Agricultural Engineering

As mentioned earlier, TDRs can measure the soil content. It is beneficial and crucial for the study of agriculture engineering and science. Researches and advanced studies have made time domain reflectometers more technically advance to measure the moisture content for soil and grain, foodstuff, and sediments. However, the primary building block remained the same. TDRs are very much renowned because of their accuracy in measurements.

> Applications in Aviation maintenance

The property of reflectometers has found applications in aviation wiring maintenance. The more specific property is the “Spread Spectrum Time Domain Reflectometry,” which is used to locate the fault and preventive maintenance. There are two main reasons behind using the property. The first one is the precision in the measurement, as the device gives accurate measurements. The second one is the TDR’s ability to locate defects in an extensive range that’s too in live.

Some other types of Time Domain Reflectometers

Time Domain Reflectometer
Optical Time Domain Reflectometer, Image By:
Optical time-domain reflectometer” (CC BY-NC-SA 2.0) by sjeemz

Time Domain Reflectometers get modified and advanced with time. The optical time-domain Reflectometer is one of the advanced types of TDR. It is an equivalent device for optical fiber. There is also a device like Time Domain Transmissometry, which analyses transmissions of optical fibers. Two more variations are: “Spread Spectrum Time Domain Reflectometry (SSTDR)” and “Coherent Time Domain Reflectometry (COTDR)”.