Differential-Mode to Common-Mode Conversion Detector Based on Rat-Race Hybrid Couplers: Analysis and Application to Differential Sensors and Comparators

A microwave device able to detect differential-mode to common-mode conversion (or vice versa) in a four-port balanced circuit is proposed. For mode conversion detection, the balanced circuit should be fed by a differential-mode signal, generated by means of a rat-race balun from a single-ended signal. By connecting the pair of output ports of the balanced circuit under test to one of the two pairs of isolated ports of a second rat-race coupler, conversion to the common-mode (if it exists) can be detected and recorded in the Σ-port. Since the signal level at the output (single-ended) port depends on mode conversion efficiency, the complete two-port structure, including the pair of rat-race couplers plus the balanced circuit in between, can be used as a comparator or differential microwave sensor based on mode conversion. In such a sensor, the working principle is symmetry disruption (caused, e.g., by an asymmetric dielectric load), and the output variable is the transmission coefficient, related to the level of asymmetry. A detailed analysis, considering an arbitrary unbalanced four-port test structure (to account for symmetry disruption), is carried out. Then, such analysis is particularized to the case of a pair of unbalanced and uncoupled matched lines, and the conditions for sensitivity optimization are obtained. Finally, a differential sensor and comparator based on a pair of balanced (and meandered) matched lines, designed according to the guidelines for sensitivity optimization, is fabricated and validated.