Superconducting coplanar delay lines
Two 25 ns wideband HTS delay lines with a novel double-spiral meander line (DSML) structure are designed, fabricated and measured. One is based on the conventional coplanar waveguide (CPW), and the other based on a conductor-backed coplanar waveguide (CBCPW). Systematic design work is presented in this thesis on the calculations of transmission-line parameters, the selection and optimisation of delay line patterns, and the modelling of the transitions and connections at the input/output. Simulations show that the DSML structure has better transmission efficiency over a wide frequency range than the conventional double-spiral line (DSL). The bandwidth and dispersion of such a meander structure are investigated. The fabricated delay lines are first characterised as resonators with a fundamental mode at ~20 MHz. The surface resistances of the superconductors and the temperature- and power-dependent properties are investigated by measuring the Q-values of more than 1000 harmonics from 20 MHz to 20 GHz. Then, the delay lines are fully connected as they would be used in the application and measured thoroughly in both the frequency and time domain. The performance of the CBCPW delay line is the best ever demonstrated in terms of the widest resonance-free band (2 to 18 GHz), low insertion loss (0.06 dB/ns at 60 K and 10 GHz), small ripple (<1 dB up until 16 GHz), and small dispersion (< 2 ns in the variation of group delay between 2 and 18 GHz). This is the first coplanar delay line successfully demonstrated without using bonding-wires to join the in-plane grounds. The spurious reflecting elements in the DSML structure are identified by the time domain analyses. The results from this measurement are compared comprehensively with those from both resonance measurement and simulations.