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Individuals working in digital applications tend to prefer the Time Domain Reflectometer (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 removed from the instrument and not reconnected until the next round of testing ("round testing"). The sample assemblies were labeled 1 through 6 and their test order within each round was randomized to reduce test bias. Repeat testing reflects instrument uncertainty, while round-to-round testing reflects measurement reproducibility or test uncertainty.