National Aerospace Laboratories

A Project Report on

REFLECTOMETRY AND ITS APPLICATION IN DETECTION OF FAULTS IN CABLE WIRE

submitted by

Aditya Yadav (12EC05)

in partial fulfillment of the requirements

for the award of the degree of

BACHELOR OF TECHNOLOGY

DEPARTMENT OF ELECTRONICS AND COMMUNICATION

ENGINEERING

NATIONAL INSTITUTE OF TECHNOLOGY KARNATAKA

SURATHKAL, MANGALORE - 575025

30 May, 2014 - 16 July, 2014

About NAL

National Aerospace Laboratories (NAL), is India's second

largest aerospace firm after Hindustan Aeronautics (HAL). It was

established by the Council of Scientific and Industrial

Research (CSIR) at Delhi in 1959 and its headquarters was later

moved to Bangalore in 1960. The firm closely operates with

HAL, DRDO and ISRO and has the prime responsibility of

developing civilian aircraft in India.CSIR-NAL mandate is to develop

aerospace technologies with strong science content, design and build

small and medium-sized civil aircraft, and support all national

aerospace programmes.

NAL is a high technology oriented institution concentrating on

advanced topics in the aerospace and related disciplines. Originally

started as National Aeronautical Laboratory, it was renamed National

Aerospace Laboratories to reflect its major involvement in the Indian

space programme, its multidisciplinary activities and global

positioning. It is India’s only civilian aerospace laboratory with a high

level of competence and the expertise of its scientists is globally

acknowledged

Motivation

The motivation behind doing this research intern is applying the

theory learnt in classroom to practical use. During the 4 year course of

electronics we learnt many transformation techniques of signals to

simplify the process. So to find how it really simplifies the complex

processes was fascinating and amazing.

During lectures in classroom I was only able to see limited scope of

signal processing applications. I just knew about those applications

which were taught but I wanted to experience firsthand the practical

applications of signal processing in industrial area. That’s why I

applied in NAL, Bangalore to do a research intern in application of

signal processing and gain better insights of the topic.

Training Experience

I worked in the National Aerospace Laboratories (NAL), a

constituent of the Council of Scientific and Industrial Research

(CSIR).

I worked in a project on reflectometry which is a noninvasive

technique that allows the analysis of properties of a medium.

This technique is based on the reflection of waves at the

interface of interest. Waves propagate into a medium (according to

the laws of propagation into the medium) and when it encounters a

discontinuity (impedance break), part of its energy is reflected back to

injection point. The analysis of the reflected signal can infer

information about the system or the medium under consideration.

Many techniques are based on the principle of reflectometry and are

distinguished by the type of waves used and the analysis of the

reflected signal. Among all these techniques, we can classify the main

but not limited to:

1. Time Domain Reflectometry (TDR)

2. Frequency Domain Reflectometry (FDR)

3. Neutron Reflectometry

4. X-ray Reflectometry

5. Ultrasonic Reflectometry

Time Domain Reflectometry or TDR is a measurement technique used

to determine the characteristics of electrical lines by observing reflected

waveforms.

The TDR works on the same principle as radar. A pulse of energy is

transmitted down a cable. When that pulse reaches the end of the

cable, or a fault along the cable, part or all of the pulse energy is

reflected back to the instrument.

The TDR measures the time it takes for the signal to travel down

the cable, see the problem, and reflect back. The TDR then converts

this time to distance and displays the information as a waveform

and/or distance reading.

Generally, a train of fast pulses is appropriately generated and applied

to a transmission line. The waveform in the line is observed at some

point (by a sampling oscilloscope or other Data Acquisition System).

By analyzing the magnitude, duration and shape of the reflected

waveform, the nature of the impedance variation in the transmission

system can be determined.

Frequency Domain Reflectometry or FDR technique is based on the

transmission of a set of stepped-frequency sine waves from the

sample.

A Frequency domain reflectometer is a tuned reflectometer used for

measuring reflection coefficients and impedance of waveguides over

a wide frequency range, by sweeping a band of frequencies and

analyzing reflected returns.

Several types of FDR exist and are commonly used in radar

applications or characterization of cables/wires. The signal analysis is

focused rather on the changes in frequency between the incident

signal and the reflected signal.

In this technique RF signal (3Khz-300Ghz) is used instead of a DC

pulse.

Some of its advantages over Time Domain reflectometry (TDR) are:

Much higher signal power is used and thereby longer cables can

be measured.

No blind spot or dead zone problem exists in FDR.

TDR is likely to miss RF characteristics such as corrosion, slight

pin gaps and damaged RF components.

The RF sweep used by FDR is sensitive to RF problems and

accurately identifies the return loss of the antenna. A TDR's

pulsed dc stimulus reflects little energy at RF faults.

Suggestions for Improvement

JTFDR (Joint Time Frequency Domain Reflectometry) can be

used to obtain a most effective wiring diagnostic solution and the

distance to the fault.

To further explore the merit of JTFDR, the next step is to

perform the “time to breakdown” accelerated aging test to predict the

remaining lifetime of the cables, however, this will require significant

upgrades to experimental equipment. Time to breakdown tests are

used to predict the remaining life of a cable by testing the

performance of sample equipment in accelerated aging conditions

until breakdown.

Conclusion and Future Work

This project was carried out successfully. In this report the theory and

limitations of classical cable diagnostic techniques Time Domain

Reflectometry (TDR) and Frequency Domain Reflectometry (FDR)

were discussed. In addition to that, LabVIEW simulation was

performed to achieve the overall waveform from a short circuit or

open circuit faults.

Yes, I would like to continue my work in reflectometry. The demand

in this field is very high as new and more advanced techniques keep

coming up. So it’s a constant challenge to keep yourself updated with

the latest techniques.