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VNA vs TDR: Measurement uncertainty (Part 2)

17 Mar 2016  | Paul Pino

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Best-Case Performance Analysis: Total measurement uncertainty was broken down into two components: test uncertainty (attributed to test fixtures, test method, and operator) and instrument uncertainty (attributed to the instrument itself). TDR instrument-related uncertainty accounted for 61 per cent of the total measurement uncertainty. VNA instrument-related uncertainty made up 22 per cent of the total uncertainty.

Measurement Repeatability in Best-Case Performance: Best-case performance testing examined measurement repeatability over 22 connect/disconnect cycles, indicating a downward trend in the test sample's measured time delay over 22 test cycles. Both the TDR and VNA recorded this trend, but with a significant difference: VNA measurements returned a range spanning 0.0983 ps as compared to the TDR's range of 0.275 ps.

TDR/VNA One-Port Measurement Parity: The VNA was reconfigured from a two-port to a one-port calibration and best-case performance testing was repeated. DUT time delay data was extracted from the resulting s11 reflection data. Findings indicated virtually no change in VNA instrument uncertainty as compared to two-port s21 data, and measurement uncertainty associated with connect/disconnect DUT testing decreased.

VSWR-Loss Product: A correlation existed between instrument measurement uncertainty and the DUT's VSWR and insertion loss. It appeared to follow the product of the DUT's VSWR and insertion loss. The VSWR-Loss product was a strong indicator of changes in measurement uncertainty across a variety of DUTs.

Understanding an Instrument or Test System's Capabilities
The topic of measurement is a popular one and fundamental to the test and measurement industry. Measurement uncertainty, however, is an often-ignored part of the measurement discussion. When we measure, we attempt to go from the unknown to the known. Addressing measurement uncertainty adds yet another dimension of unknown to our efforts, and this can be inconvenient. Once our trusted instrument of choice has produced a number, it is frequently taken as truthful, accurate, and good enough. In some cases, this may be sufficient, but when precision is required, knowledge of an instrument's or test system's capabilities is crucial. Without this information, the output of testing may be rendered useless or worse yet, create more questions than it answers.

Example: A specification calls for a passive device to have a time delay of 6.0 ps, ± 0.5 ps. Therefore, the device in question can have a time delay between 6.5 ps and 5.5 ps and still be within specification. If we agree beforehand that a measurement must be reproducible within limits to be considered legitimate, then the need to understand the measurement system's uncertainty becomes clear.

Any measurement system used to characterise this device must have an uncertainty of better than ± 0.5 ps to resolve the data adequately. A traditional rule of thumb states that measurement system precision should be approximately ten times greater than the tolerance it is being compared against. In many instances, this is neither practical nor possible, so concessions must be made to bound claims of measurement precision properly, calling yet again for an understanding of uncertainty associated with the measurement system.

In a production test scenario, specification compliance, especially during qualification, is often determined through a series of measurements over a period of time, as opposed to a single occurrence. If the measurement is not reproducible, compliance is unlikely. If the DUT has the stability and repeatability to deliver performance at a fraction of the stated ± 0.5 ps tolerance, measuring it with a system possessing an uncertainty of ± 1.0 ps will likely result in values ranging ± 1.0 ps about a nominal value.

Which is better, VNA or TDR?
The findings of these experiments suggest that, before making critical production measurements with either a TDR or VNA, it's necessary to understand the interaction of the DUT and the measurement system. Each has its strengths and weaknesses, but in the hands of a properly trained and experienced user, both are formidable tools. Data has been presented indicating that one instrument platform operates with a significantly lower level of measurement uncertainty under specific conditions, but no claims describing either instrument platform as superior to the other have been made. Ultimately, it is up to the individual to decide which best suits his or her needs given the application requirements.

About the author
Paul Pino received his BS degree in electrical engineering from the University of Delaware in 2000 after a long career in the automotive industry. He joined W. L. Gore & Associates Inc. in 1999 and has worked with various groups, including Gore's Signal Integrity Lab, the Planar Cable Team and the Fibre Optic Transceiver Team. For the past 10 years, he has worked within the Microwave Cable Team.

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