Probes in HF metrology

[摘要] ENGLISH ABSTRACT: Flanged coaxial probes are widely used to conduct accurate, broadband permittivity measurementsof various dielectric materials. A metrology study, discussed in [1], revealed thatsmall perturbations in measured permittivity data, are due to escaping common-mode (CM)current that propagates onto exposed VNA feed cabling. This is not considered in publishedpermittivity extraction algorithms, like the National Institute of Standards and Technology(NIST) full-wave code that assumes an infinite flange radius. To characterise this effect wevalidate a finite volume time domain (FVTD) CST simulation model of an SMA coaxialprobe, by probing sensitive E-fields in a metallic shielding cylinder, placed around it.For this process, electro-optic (EO) E-field sensors are considered and a Mach-Zehndertype sensor is designed. Manufacturing difficulties discontinues this approach, but the revisitedextended centre conductor E-field probing technique proves successful. The techniqueentails a high dynamic range, two-port VNA measurement. Through CST we gainknowledge of the physics behind the CM-problem and the behaviour of fields around thecoaxial probe. Different shielding environments are simulated to establish their ability toimpede CM-current coupling onto measurement cabling.To study the CM-effect on extracted permittivity results, the investigation is extendedto Short-Open-Load (SOL) calibrated face-plane measurements of dielectric solids. A CSTmodel, which considers escaping CM-energy, is used to generate open circuit (OC) calibrationcoefficients and to serve as an independent extraction method. We inspect the effectof different shielding environments and through CST, extract accurate permittivity resultsfor e00, to a degree not previously achieved for such systems. This allows comment on theinfinite-flange-radius assumption of the NIST method and proves the significance of theCM-effect.

[发布机构] Stellenbosch University