Miniaturized Ultra-Wideband HTS Bandpass Filter Using High-Q CRLH Transmission Line

An ultra-wideband (UWB) high-temperature superconducting (HTS) bandpass filter (BPF) with miniature circuit size and high-selectivity is introduced. The UWB BPF is designed using HTS composite right-/left-handed transmission line (CRLH-TL). Firstly, the CRLH-TLs are designed based on interdigital capacitors and TL inductors. The transmission and dispersion characteristics are analyzed. The high-
${Q}$
property of the CRLH-TLs ensures the low-loss property of the filter. Then an 8-order UWB BPF is constructed by directly connecting four sections of HTS CRLH-TLs and one interdigital capacitor. The synthesis design method is utilized to obtain the elements values in the circuit model of the filter and the physical parameters of the filter can then be determined. Finally, the filter is fabricated and measured for verification. The filter size is only
$0.07\,\,{\lambda }_{g} \times 0.23\,\,{\lambda }_{g}$
at the central frequency. Measured results show that the filter owns 0.18-dB maximum insertion loss, and minimum 17.8-dB return loss at a temperature of 20 K within the 1-dB passband from 3.8 to 8.2 GHz. Three transmission zeroes are generated to improve the filter selectivity.