Highly Sensitive Defect Detectors and Comparators Exploiting Port Imbalance in Rat-Race Couplers Loaded With Step-Impedance Open-Ended Transmission Lines

Abstract

Defect detectors and comparators, able to detect tiny differences between a reference (REF) sample and a material under test (MUT), are presented in this paper. In one version, the comparator is implemented by means of a rat-race hybrid coupler and a single step-impedance open-ended transmission line connected to one of the ports of the coupler. The comparator exploits the imbalance generated in one of the two pairs of isolated ports of the coupler when the MUT sample differs from the REF sample. The other pair of isolated ports is used for comparator feeding ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta $ </tex-math></inline-formula> -port) and for recording the output signal ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Sigma $ </tex-math></inline-formula> -port), and feeding is achieved by means of a harmonic signal tuned to the operating frequency of the coupler. The resulting device is therefore a two-port structure, and the considered output variable is the magnitude of the transmission coefficient, an easily measurable quantity. By virtue of the step-impedance transmission line, used as the sensitive element of the comparator, the device exhibits very high sensitivity to variations in the dielectric constant between the REF and MUT samples, this being the input variable. A prototype device example, with a maximum sensitivity of 0.72 is reported, and applied to the detection of defects in the REF sample, a dielectric slab, generated by drilling holes of different densities across it. In the second prototype, a true differential-mode sensor, two identical sensing elements (step impedance lines) are connected to one of the pairs of isolated ports of the coupler, and roughly perfect balance is obtained when the MUT is identical to the reference sample. The maximum sensitivity in this case is 1.20, and the device discriminates also small perturbations generated in the REF material. The main relevant aspect of the reported prototypes, phase-variation sensing devices by nature, is the phase-to-magnitude conversion achieved by means of the hybrid coupler. This transforms the reflective-mode phase-variation sensing element to a two-port transmission-mode device where the output variable is the magnitude of the transmission coefficient, an easily measurable quantity.