S-parameter measurements are advancing on many fronts, ranging from operational frequencies to supported interfaces to extreme loading conditions. The operational frequencies for VNA-based systems continuously extend in frequency range and at present support measurement capability beyond 1.1 THz. The development of RF devices and systems is progressing across many domains that require the support of many interfaces, ranging from the conventional connectorized coaxial and waveguide lines to the on-wafer planar domain. At the same time, RF devices are becoming smaller and using novel materials that impose challenging measurement loading conditions, such as extreme impedances. These challenges imposed on many fronts for S-parameter measurement have an increased need for sensitivity and accuracy, forming the basis of the work outlined in this book.
The first three chapters ddress measurement challenges in coaxial VNA test benches and describe advanced accuracy improvement techniques. Starting with chapter 1, a detailed overview is provided for the S-parameter measurement challenges addressed in this thesis. From here, chapter 2 extends with an introduction to uncertainty sources in connectorized Vector Network Analyzer (VNA) test benches, measurement models, and uncertainty propagation techniques. This forms the basis for chapter 3, which presents advanced methods and strategies for the evaluation of the uncertainty contributions corresponding to the VNA, test-port cables, and coaxial connectors. Moving forward from coaxial VNA measurement systems, the emphasis subsequently is on the primary realization of S-parameter traceability. Chapter 4 describes a method employing a coaxial air-dielectric transmission line for uncertainty evaluation purposes. Here, an improved ripple method is proposed for evaluating uncertainties of a calibrated VNA using a transmission line standard in combination with the Time Domain Signal Extraction (TDSE) algorithm [1]. Chapter 5 describes an advanced model for calibrating transmission line standards using mechanical and material parameters and forms the basis of primary TRL calibration of coaxial VNA systems. The accuracy improvement techniques for coaxial VNA systems are finally extended
to extreme impedance devices. Chapter 6 focuses on measurement of such extreme impedances and introduces the novel interferometer design for ultra-low-noise and broadband measurements, including a novel calibration method for interferometer-based broadband VNAs. With accuracy methods for both matched and high-mismatched environments established, the final part of this thesis aims to expand accurate measurements to the on-wafer domain. Chapter 7 describes our advancements in on-wafer measurements, focusing on RF probing to develop a fully autonomous on-wafer measurement capability. Chapter 8 concludes this thesis with a summary of its main findings and recommendations for future work.@en

In this contribution, we describe the modeling approaches and the characterization procedures used to develop accurate standard models for cryogenic, probe-level, calibrations substrates.The key electrical and mechanical parameters of the impedance terminations and the lines used in commercially available impedance standard substrates are first characterized versus temperature. After, these component are simulated using 2.5D EM solvers including their mechanical variation when exposed to cryogenic temperatures, to extract their nominal response at 7 Kelvin. The quality of the resulting calibrations at cryogenic is evaluated first using independent CPW lines on the calibration substrates and then by measuring the response of a transformer-based resonator realized on a Si-based technology.Ambient temperature models are used as a comparison, to highlight the accuracy improvement that can be achieved employing optimized Cryo-EM based models.

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In this contribution, we present a rigorous analysis based on uncertainty propagation techniques to estimate the random variation of the controlled reflection coefficient in mixed-signal load-pull test benches. A digital-to-analog converter is commonly used in these test benches to generate the baseband signal required to synthesize the high-frequency, user-defined injected wave. To study the random noise of the injected wave, which can be mapped to the noise of the controlled reflection coefficient, we employ Jacobian sensitivity functions between the baseband signal and the RF one. First, the baseband integrated rms noise of the up-converter is evaluated, and then the upconverted noise is determined via the derived transfer function. Finally, experimental results to validate the uncertainty control bound of the synthesized reflection coefficients are presented, highlighting a full coverage of the measured reflection coefficients.@en

The addition of RF interferometers to vector network analyzer (VNA) test benches has allowed for realization of low-noise high-frequency measurements of extreme impedance devices. However, when employing correction techniques such as the short-open-load method, the RF response of the interferometer hardware introduces measurement inaccuracies due to unwanted load-dependent inconsistencies. This letter presents a novel VNA calibration method that enables both low-noise and accurate small-signal characterization of highly mismatched devices. The proposed solution is experimentally validated in the 10-18-GHz band, confirming a 23-fold improvement in measurement resolution with absolute accuracy across the entire Γ range of the VNA. The accuracy of the new calibration process is verified via comparison with traceable reference standards supported by state-of-the-art uncertainties.@en

A method is presented for automated probing of on-wafer devices for measurements at millimetre-wave frequencies. The proposed method automatically detects the contact between the measurement probe and on-wafer device, based on the evaluation of variation in the input reflection coefficient. It is shown that, using this automated technique, about five times better measurement repeatability is achieved in millimetre-wave device characterisation.

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In this contribution, we present the performances of an IQ mixer-based RF-interferometer module, called the HΓ-VNA, designed to be used as an add-on to VNAs to improve the measurement sensitivity and accuracy of DUTs presenting extreme impedances (|Γ|>0.8). The calibration procedure used to obtain accuracy improvement is presented, and allows to set both reference and system impedance to any selected one. The procedure is benchmarked by comparing it to a conventional 50 Ω short-open-load calibration performed on a 50-Ohm VNA. For this purpose, a custom on-wafer calibration kit has been realized, using a fused silica substrate, featuring calibration loads with very high impedances (between 3.5 kΩ and 7 kΩ). Experimental results show accuracy improvements when applying the proposed calibration technique to the measurement of high impedance resistors, for frequencies below 4 GHz.

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This paper analyzes and accurately models the complex noise behavior of vector network analyzers (VNAs) when measuring large-mismatch devices and subsequently shows how the VNA measurement noise performance is enhanced through implementation of a high-speed, broadband, active RF interferometer module. The presented VNA noise model provides a solid framework, benchmarked by measurement data, to analyze existing RF interferometer approaches. The performance improvement of the proposed interferometer implementation is then benchmarked in terms of magnitude and phase stability of the renormalized impedance level. A test bench employing the novel add-on RF interferometer module is presented and demonstrated to achieve high-speed cancellation of the scattered wave over a broad frequency band. The first experiment shows ultralow noise in a 1-18 GHz broadband measurement of co-planar waveguide 0.5-Ω and 5-k Ω impedance standards. Employing the proposed hardware setup improves the noise uncertainty for the 5-k Ω impedance standard by a factor of 8 and 20 at 1 and 18 GHz, respectively. In the second experiment, a factor of 2 height-resolution enhancement is achieved in a scanning microwave microscope when the RF interferometer module is added to the instrument.

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In this contribution we analyze the definition of reference planes in probe-level calibrations. The removal of the probe type from the calibration definition is presented first analyzing the transition discontinuities and defining to which extent they have to be incorporated in the calibration error terms. Subsequently a commercial calibration, which is defined specific to a probe topology, is considered and its frequency dependent standard response are computed accurately via a 3D EM simulator. These probe independent standard definitions are then used to compare the accuracy achieved on the same structures, (i.e., CPW lines of different lengths) from two different probe topologies. Finally, the data from both probes are compared, using a worst bound metric, to the data achieved when using the probe specific calibration data showing an accuracy improvement for the probe independent approach, validating the improved identification of the reference plane proposed here.@en