Comparison of Vna
and Tdr measuremenT
unCerTainTy using
Coaxial Cables
Individuals working in digital applications tend to prefer the Time Domain Reflec- tometer (TDR), while those involved in
traditional RF applications consider the Vector Network Analyzer (VNA) to be a laboratory
staple. The push for ever-faster data rates has
fueled an analytical re-thinking of high-speed
digital signaling. Contemporary wisdom views
high-speed digital systems as high-frequency
applications, where more traditional microwave analysis techniques apply. Once this concept is embraced, engineers often exploit the
strengths of both the TDR and VNA, combining time and frequency domain analysis to accelerate design and development cycles. Both
instruments can measure impedance, time
delay, phase delay and reflection coefficient so
they are often thought of as equals. This begs
the question: Is there a quantifiable difference
in measurement uncertainty between the TDR
and VNA?
Characterizing the time delay of a passive
device, such as coaxial cable assembly is a common use for the TDR and VNA. It is therefore
an ideal vehicle for a performance comparison.
How do the two compare under ideal test conditions, and the less-than-ideal environment of
production testing? Do both instruments possess similar levels of measurement precision?
This article answers these questions by examining the measurement uncertainty and repeatability of the TDR and VNA.
DESCRIPTION OF EXPERIMENT
To understand the capabilities of any measurement system, it is important to test the system’s response to a variety of inputs to avoid
erroneous conclusions. For this discussion, the
term “input” refers to a “Device Under Test”
(DUT), which in this experiment were different cable assemblies from a variety of manufacturers, having a range of insertion loss and
VSWR characteristics. In a manner consistent
with commonly used production test practices,
measurements of the time delay of the cable assemblies described above were measured with
a TDR and a VNA. The resulting measurement
uncertainty of the two instruments under these
conditions was then compared.
A sample of six new cable assemblies were
used in the experiment, each equipped with
SMA pin connectors. Table 1 details their loss,
VSWR and physical length characteristics. The
electrical data in Table 1 was acquired through
VNA analysis. The experiment consisted of
two rounds of testing. Within a round, each
sample was connected to the TDR or VNA
and measured five consecutive times, without
being disconnected or disturbed (“repeat testing”). After five measurements, the sample was
PAUL PINO
W.L. Gore & Associates Inc., Landenberg, PA
