Sliding backshorts for planar circuits

An adjustable planar imbedding circuit based on coplanar transmission lines is presented to deal with the impedance mismatch associated with the use of the Superconductor-Insulator-Superconductor tunnel junction. The planar embedding circuit is developed with movable noncontacting shorting elements that consist of thin metallic plates with optimized arrangements of rectangular holes. The rectangular holes are placed along the insulated metallic transmission line to provide a periodic variation of the line impedance. A large reflection coefficient is demonstrated with a scale model of the sliding backshort, and values of more than -0.5 dB are reported. The shorting elements are incorporated into a low-frequency tuning circuit to test the practical tuning ranges. The backshort can be used to fashion tuning stubs with variable post-fabrication electrical lengths which relaxes the design constraints of planar integrated circuits.