Compact parallel coupled HTS microstrip bandpass filters for wireless communications

A practical design methodology for a compact parallel-coupled microstrip bandpass filter structure with steep attenuation is introduced using a computer-aided full-wave electromagnetic (EM) simulation based on the method of moments. The structure consists of an array of fully aligned half-wavelength spiral-meander-line resonators. Aimed at the application in the front-end receiver of digital cellular communication service, a 12-pole high-temperature superconductor filter with 2.27% fractional bandwidth (FBW) at 883.0 MHz was designed. In order to utilize the limited wafer area further, a 15-pole filter with 803.0-MHz center frequency, 2.25% FBW is precisely designed with greater EM simulation effort. Both the filters are fabricated using thallium-barium-calcium-copper-oxide thin films on a 2-in lanthanum-aluminate (LaAlO₃) wafer. The S-parameter measurements show a good agreement with the simulated results. At 70 K, the 12-pole filter shows less than 0.4-dB insertion loss, 0.3-dB passband ripple, and better than 12-dB return loss, and the 15-pole filter shows 0.25-dB insertion loss, 0.2-dB passband ripple, and better than 15-dB return loss. Out-of-band rejection for both the filters at 3 MHz below the passband edges is more than 60.0 dB. In order to estimate the power-handling capability of the filters, third-order intermodulation distortion was measured. A sensitivity analysis for the observed frequency shift in the filters is reported, which also shows very similar parametric dependence in both the filters. Also from this analysis, an approach for using the same deign in 0.5% FBW applications is discussed.