Systematic connector effects
METAS S-Parameter Metrology
Comprehensive work on the SI traceability of S-parameter measurements [01] in the coaxial line system has led to the realization that the mechanical discontinuities introduced by the coaxial connector have a non-negligible influence on electrical properties. Therefore it is necessary to include the connector in the characterization of VNA calibration standards [02]. Typical changes in S-parameters, which result from this consideration are as an example |ΔS11| = 0.01…0.02 for female DUTs of the 2.4 mm line system. Neglecting the connectors of the standards has different impact for male and female DUTs. Male DUTs are less influenced because the reflection of the male part of the connector is lower than that of the female part. In general it can be stated that the assumption of ideal connectors for calibration standards leads to inconsistent definitions of measurement reference planes and thus to degraded accuracy in measurements. It has further been found out that the pin gap, inherent to sexed coaxial connectors, shows unpredictable behavior due to resonant effect when becoming too small [3]. This is in contradiction to the previously established belief that a connection should be as flush as possible.
As an example: The air-dielectric line, which is considered as a golden standard among coaxial VNA calibration standards, has a center conductor, which is not kept at a fixed position by a supporting bead in order to make the line section calculable from the diameter profiles. The center conductor has thus a variability in the longitudinal position, resulting in arbitrary pin gaps at the connections on both sides. Air-dielectric lines have been used to establish S-parameter traceability and to calculate uncertainties associated with S-parameters (with the so-called Ripple Method) and the assumption was usually that the connectors are ideal. From today’s point of view, air-dielectric lines without a support bead are unstable [4] and are only suitable as standards when additional measures [05] are taken to control the position of the center conductor. And even then it is necessary to include the connector in the characterization of the standard.
Coaxial connectors of primary measurement standards, i.e. calculable standards, which are used to establish traceability to SI units, can be taken into account by mechanically parametrizing them and performing electromagnetic field simulations on them [7]. In this METAS document the mechanical parametrization for a subset of the coaxial connectors covered by the IEEE Std P287 are given. Results of calculations of reflection and transmission coefficients of connector pairs based on typical values for the geometry parameters are shown. Furthermore, sensitivity coefficients of reflection and transmission coefficients with respect to the geometry parameters are displayed and finally minimal pin gap requirements are discussed.
