Time Domain Analyzer is useful for measuring impedance values along a transmission
line and for evaluating a device problem in time or distance. Time
domain display provides a more intuitive and direct look at the device under test characteristics. In addition, it gives more meaningful information concerning
the broadband response of a transmission system than other measuring techniques
by showing the effect of each discontinuity as a function of time or distance. This document
will focus on time domain analysis generated from vector
network analyzers. The intent is to provide engineers with frequency
domain background, an in-depth view of how a time domain display is created
from frequency domain data and how to apply the time domain
display to common problems in RF systems.
Agilent offers other documents that cover in detail the use of time domain displays. See
the bibliography for more details.
The measurement technique of time domain reflectometry was introduced in the
early 1960’s and 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. TDR measurements are made by launching an impulse
or a step into the test device and observing the response in time. Using a step generator
and a broadband oscilloscope, a fast edge is launched into the transmission line. The
incident and reflected voltage waves are monitored by the broadband oscilloscope at a
particular point on the line. By measuring the ratio of the input voltage to the reflected
voltage, the impedance of simple discontinuities can be calculated. The position of the
discontinuity can also be calculated as a function of time by applying the velocity of
propagation along the transmission line. The type of discontinuity can be identified by its response.
While the traditional TDR oscilloscope was useful as a qualitative tool, there were
limitations that affected its accuracy and usefulness; a) TDR output step rise time
– the spatial resolution of the measurement depends upon the step rise time; b) poor
signal-to-noise ratio due to the wideband receiver architecture.
Then, in the 70’s, it was shown that the relationship between the frequency domain and
the time domain could be described using the Fourier Transform. The Fourier Transform
of the network reflection coefficient as a function of frequency is the reflection coefficient
as a function of time; i.e., the distance along a transmission line. It was possible
to measure the response of a DUT in the frequency domain and then mathematically
calculate the inverse Fourier Transform of the data to give the time domain response.
A high performance VNA combined with fast computation power created unique measurement
capabilities. Using error-corrected data measured in the frequency domain, the
response of a network to step and impulse time stimuli can be calculated and displayed
as a function of time. This gives traditional time domain reflectometry capability in reflection
and transmission and adds measurement capability of band-limited networks. By
locating network elements in time and removing their effects from measured data, the
vector network analyzer makes more precise frequency domain measurements possible.
Figure 1 shows how both time domain and frequency domain displays
can be generated by either a time domain reflectometer oscilloscope or a vector
network analyzer. Data captured using either a TDR or VNA can be transformed
into both displays.