Compact Superconducting Diplexer Design with Conductor-Backed Coplanar Waveguide Structures

This paper presents a compact superconducting diplexer with conductor-backed coplanar waveguide (CBCPW) structures. The couplings of the CBCPW structure resonators are significantly weaker than those of traditional microstrip resonators. High isolations between two channels are therefore achieved by using the CBCPW structures because of their highly suppressed unwanted cross couplings. The CBCPW diplexer consists of two four-pole channel bandpass filters. The couplings in the CBCPW structures, in which the air layer below the substrate varies in height, are simulated and compared with the coupling simulations of microstrip structures. The comparison shows that the coupling intensities of the CBCPW structures are nearly one order of magnitude lower than those of the traditional microstrip structures. The fabricated diplexer exhibits high performance with a compact core size of only 20 mm x 14 mm, and its measurements agree well with the simulations. The measured insertion losses of both channels are less than 0.19 dB, and the isolations are over 53 dB.