http://literature.agilent.com/litweb/pdf/5966-4855E.pdfThe most general approach to evaluating the time domain response

of any electromagnetic system is to solve Maxwellâ??s equations in the

time domain. Such a procedure would take into account all the

effects of the system geometry and electrical properties, including

transmission line effects. However, this would be rather involved for

even a simple connector and even more complicated for a structure

such as a multilayer high-speed backplane. For this reason, various

test and measurement methods have been used to assist the

electrical engineer in analyzing signal integrity.

The most common method for evaluating a transmission line and its

load has traditionally involved applying a sine wave to a system and

measuring waves resulting from discontinuities on the line. From

these measurements, the standing wave ratio (σ) is calculated and

used as a figure of merit for the transmission system. When the

system includes several discontinuities, however, the standing wave

ratio (SWR) measurement fails to isolate them. In addition, when

the broadband quality of a transmission system is to be determined,

SWR measurements must be made at many frequencies. This method

soon becomes very time consuming and tedious.

Another common instrument for evaluating a transmission line is the

network analyzer. In this case, a signal generator produces a sinusoid

whose frequency is swept to stimulate the device under test (DUT).

The network analyzer measures the reflected and transmitted signals

from the DUT. The reflected waveform can be displayed in various

formats, including SWR and reflection coefficient. An equivalent TDR

format can be displayed only if the network analyzer is equipped

with the proper software to perform an Inverse Fast Fourier

Transform (IFFT). This method works well if the user is comfortable

working with s-parameters in the frequency domain. However, if

the user is not familiar with these microwave-oriented tools, the

learning curve is quite steep. Furthermore, most digital designers

prefer working in the time domain with logic analyzers and

high-speed oscilloscopes.

When compared to other measurement techniques, time domain

reflectometry provides a more intuitive and direct look at the DUTâ??s

characteristics. Using a step generator and an oscilloscope, a fast

edge is launched into the transmission line under investigation.

The incident and reflected voltage waves are monitored by the

oscilloscope at a particular point on the line.

Soil Water Content with Time Domain Reflectometry (TDR)EVS