Modal calibration of GSSG probes

This work describes the use of modal Short-Open-Load calibrations to characterize symmetric and nearly symmetric GSSG probes for use in de-embedding and creating probe tip calibrations. Three methods have been developed that leverage the use of a modal representation and symmetric interface. The first method assumes perfect probe symmetry and uses a second tier technique to characterize the differential and common-mode 2-port behavior of the probe using a Short-Open-Load one-port modal calibration approach. This is extended in the second method where the probe mode conversion is also determined from the symmetric Short, Open, and Load standards under the condition of imperfect probe symmetry (low mode conversion). In both cases the single-ended error terms are determined allowing general de-embedding or calibration augmentation. Finally we demonstrate a useful first tier calibration approach using an imperfect single-ended calibration to the probe-tip, followed by immediate modal second-tier using the original standards data corrected by the imperfect first tier. Combined with four-port SOLT-SOLR this modal cleanup approach provides for the first time a practical way to calibrate to a symmetric and coupled reference plane while using only commercially available symmetric calibration standards.