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HEWLETT-PACKARD
JOURNAL
T E C H N I C A L I N F O R M A T I O N F R O M T H E - h p . L A B O R A T O R I E S
Vol. 15, No. '
LISHED CALIFORNIA THE HEWLETT-PACKARD COMPANY, 1501 PAGE MILL ROAD, PALO ALTO, CALIFORNIA FEBRUARY, 1964
Time Domain Reflectometry
J. HE MEASUREMENTS we make are limited by the tools
we have at our disposal. Often measurements must be
made by roundabout methods because no instruments
exist with which to make them directly. Such difficul
ties are typical of pioneering work in any field. It is
then the business of the instrument maker to devise
tools to simplify frequent measuring needs.
Sometimes the cumbersome methods first required SYSTEM UNDER TEST
become so habitual, so ingrained, that the development
of improved techniques is overlooked. This seems to Fig. 2. A Typical Time Domain Reflectometer.
be the case with respect to transmission and reflection
measurement in the UHF and microwave range. Meth
ods long in use here are now obsolete but are slow to pletely new set of data must be taken and re-inter
preted. The process is very time-consuming.
give way to more modern direct methods.
The time-honored method of measuring reflections The swept-frequency reflectometer speeds the meas
on a transmission line is to measure, as a function of urement considerably but does nothing to simplify the
task of interpretation. Since no phase information is
frequency, the standing wave ratio (SWR) produced
obtained, the results from even a single simple discon
by the reflections. The resulting curves of magnitude
tinuity are ambiguous.
and phase of the SWR can, in simple cases, be un
scrambled to give the location and nature of the reflec The direct method of measuring reflections is to
tions, but the interpretation is difficult at best. With send out a pulse and listen for the echoes. This is what
each alteration or attempted improvement, a corn- we do in radar, this is what the dolphin does, and the
blind man who taps his cane. If the pulse is short
enough each reflection produces a characteristic echo
distinct from all others. The interpretation is extremely
simple and the effect of changes can be seen instantly.
The pulse echo method has been used for many years
for the location of faults in wide-band transmission
systems such as coaxial cables. Here the time scale is
such that microsecond pulses and megacycle band-
widths suffice. But in the laboratory setup, where re
flections may be separated only an inch or less, nano
second pulses and gigacycle bandwidths are needed.
Thus pulse echo reflectometry as a laboratory tool has
had to await the development of fast pulse generators
and oscilloscopes. With these the era of time domain
reflectometry has arrived and it is time for engineers to
become familiar with this new measurement technique.
Fig. 1. Time domain reflectomder setup using ◦ Jabse Service Manual Search 2026 ◦ Jabse Pravopis ◦ onTap.bg ◦ Other service manual resources online : Fixya ◦ eServiceinfo