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Calibration


The Rohde & Schwarz ZNA network analyser


are shown directly on the screen along with the measured S-Parameters. At the core of this development is a calculation engine based on solid measurement uncertainty science. METAS is the National Metrology Institute


(NMI) located in Bern, Switzerland. For the past decade, the institute has been providing their measurement uncertainty software “VNA Tools” to other NMIs and Metrology labs around the world. Their research and work on S-parameter traceability have put them at the forefront of VNA Measurement Uncertainty Science, and the worldwide adoption of VNA Tools is the direct result of this effort. This work is reflected in the simplicity of the modern network analyser interface on Rohde & Schwarz instruments. The result is that test engineers can easily


calibrate the VNA, using the same familiar process that has been used for the past few decades. No extra steps are required. After connecting the device under test, the measured S-parameters are presented with an immediate overlay of the measurement uncertainty that is updated in real time with each measurement sweep. No extra effort is required from the operator. In addition, the METAS-derived R&S ZNA-K50


specification. How can suppliers prove that their measurement results are correct? There may be friction if the customer measures the device and gets different results. How can this be resolved? Then there is the scenario where calibration


standards become damaged. There may be a long time interval where the faulty standard continues being used before it is noticed that measurement data is no longer correct. How many devices might need to be recalled and retested? If there are pass/fail criteria, then knowing the


measurement uncertainty allows devices that are closer to the limit line to be passed with confidence. This also saves time and money on expensive re-work. A key factor here is knowing when to perform a new user calibration. Production facilities typically carry out a new calibration once per day, or once per shift. This may be overkill, as scheduled calibrations do not


provide information about whether re- calibration was really necessary. They are a brute force measure. Proof of the actual measurement uncertainty and reliability requires an additional measurement step to be used as a reference. This process is referred to as a verification measurement (usually using verification kits) and is only recently supported in network analysers such as the R&S ZNA and R&S ZNB.


Solution: Real time S- PaRameteR meaSuRement unceRtainty oPtion


Up to now, calculating measurement uncertainty for a device’s S-Parameter results was only possible in a metrology lab. With new options such as the R&S ZNA-K50 and R&S ZNB-K50 Measurement Uncertainty Option, this calculation is done automatically, and the uncertainty bands


and R&S ZNB-K50 options can be applied to perform verification testing. In combination with the VNA Tools calculation engine, this makes verification testing as easy to perform as a calibration itself. Users simply select the desired verification kit, the VNA then guides the user through a verification test; including the creation of a test archive that contains an uncertainty database for the measurement setup, raw measurement results, calibrated measurement results, and a pass or fail indication. Calibration and verification data, along with the measurements of the DUT are all available to METAS VNA Tools software via the VNA Tools project directory. The result? Everything can be inspected,


reports can be generated, results can be scrutinised. Furthermore, advanced users can apply their own vector correction algorithms to the raw data and compare these data to the METAS results. Metrology based calibration and verification exposes the influence of test port cables and measurement drift over time enabling better-informed decisions about how frequently a new user calibration should be performed. Over the long run this saves time and cost by eliminating unnecessary recalibrations. It also maintains the life of standards within calibration kits as they need not be reconnected so often. Finally, users can export a data project from


the whole measurement process to be analysed offline in VNA Tools if the user wants to understand further what the contributing factors are to the measurement uncertainty. At last, accredited calibration and verification standards are available. It is a new level of assurance for designers, manufacturers and their customers and it is in agreement with the new EURAMET VNA calibration guidelines.


ZNA-K50 sweeps with uncertainty bands Instrumentation Monthly March 2021


Rohde & Schwarz UK www.rohde-schwarz.com


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