Refined Geometrical Characterisation of High Frequency Standards

PTB is continuously working on improving the dimensional measurements required for the traceability of high frequency (HF) standards. For this purpose, the existing measurement setups are optimized, and the measurement ranges are expanded.

Various geometrical characterisations of the calibration standards are necessary to achieve traceability of the measurement quantities “HF impedance” and “HF scattering parameters”. This becomes clear using the example of the illustrated sketch of a beadless coaxial air line. Among other parameters, the outer diameter of the inner conductor and the inner diameter of the outer conductor over the length of the line, as well as the total length of the line itself, are required. The measurement result for the outer diameter of an inner conductor of an air line of type 2.4 mm is shown in the figure as an example. In the range up to approximately 1.5 mm there are slots as part of the coaxial jack. These connectors pose a particular challenge in the geometrical characterisation.

Figure: Schematic representation of the cross section of an air line (top), and measurement result for an inner conductor diameter versus length for four angles (bottom). Shaded red areas in the graph represent the expanded measurement uncertainties, the grey area illustrates the standardised tolerance zone.

In conjunction with the extension of the measuring range for HF scattering parameters, improvements are also continuously being made in the field of geometrical characterisation of standards. Both new coaxial standards with large geometric dimensions (e.g., for connectors in the field of mobile communications) as well as very small coaxial structures for extending the measuring range to higher frequencies are being characterised. The improvements achieved within the last three years are summarised in the table.