•  
• EDN Europe MAGAZINE
•  
• Free SUBSCRIPTION
• Subscription Renewal
• View Media Kit

• EDN
• EDN Australia
• EDN Asia
• EDN China
• EDN Japan

Gore study contrasts VNAs and TDRs

W.L. Gore has released the results of a study in which its engineers compared the measurement uncertainty of VNAs (vector network analyzers) and TDRs (time-domain reflectometers)—two instruments able to analyze time or frequency domain data to accelerate product-development cycles. The goal of the study was to determine whether the instruments possess similar levels of measurement precision.The company’s interest in the instruments derives from its development and production of products such as cable assemblies for high-frequency RF and microwave applications. In situations such as 100% production testing, repeatability of test results – as well as time taken to complete a test routine – are of great importance.The study notes that the TDR majors on temporal analysis — characterizing impedance or reflection coeffi cient with respect to time. Its quick setup, intuitive controls, and results-oriented operation appeal to a broad range of users. VNAs excel in frequency domain analysis — characterizing amplitude and phase with respect to frequency. Both instruments, Gore’s engineers observe, have the ability to perform time or frequency domain analysis through built-in Fast Fourier Transform (FFT) algorithms or ancillary software

Test Subjects

Using a variety of cable assemblies with a range of insertion loss and VSWR (voltage standing-wave ratio) characteristics, the engineers initially tested six cable assemblies in controlled conditions on each instrument. Next, they evaluated the instruments’ measurement uncerGore tainty in the best-case scenario with the highest-performing assembly. Then, to ensure TDR/VNA test parity, they evaluated the VNA using both one-port s11 reflection and the more traditional two-port s21 transmission method.

The study included a range of measures to ensure that measurement conditions were repeated as closely as possible, so that instrument-related uncertainties would dominate in the final results. This included, in making and re-making connections, “Standard RF- cable assembly care and handling practices; cleaning connectors with alcohol, drying with a moisture-free air source, tightening connectors to proper torque, and careful handling of the cable itself.”

For the TDR tests, Gore used the Tektronix CSA8000 and CSA8200 equipped with an 80E04 TDR sampling head. This equipment uses a launched signal with time domain step, with a rise time of approximately 17.5 psec. For VNA testing Gore selected the Agilent Technologies 50 GHz PNA, model E8364B, configured for two-port calibration, 0.045 GHz to 18.045 GHz, and 801 point sweep. No smoothing or averaging was applied, and IF Bandwidth was set to 100 Hz.

Study Results

Among the study’s findings: “In both the TDR and VNA, instrument-related measurement uncertainty was found to be dependent upon the device under test’s VSWR and insertion loss. The median measurement uncertainty for the VNA was found to be an order of magnitude below that of the TDR....” In addition, “TDR instrument-related uncertainty accounted for 61 percent of the total measurement uncertainty. VNA instrument-related uncertainty made up 22 percent of the total uncertainty….The findings of Gore’s experiments suggested that before making critical production measurements with either a TDR or VNA, it is necessary to understand the interaction of the DUT and measurement system.”

Said Paul Pino, the company’s North American product engineer for test and measurement, “Because precision is crucial in this industry, understanding the accuracy of an instrument is essential. Although our study indicated that one platform operated with significantly lower measurement uncertainty than the other, the important learning from this study is that both instruments are formidable tools, with each having its own strengths and weaknesses.”

You can download the complete 18-page study at gore.com/measurement.