Design of Compact HTS Dual-Band Bandpass Filters Using Dual-Function Feeding Structure With Wide Stop-Band Response

We have developed a compact high-temperature superconducting four-pole dual-band bandpass filter (DBPF) using a dual-function feeding structure with a wide stop-band response. Use of shorted stub-loaded hairpin resonators enabled its achievement. The coupling coefficients are flexibly controlled by using a pair of H-shaped coupling elements between the resonators. A dual-function feeding structure is newly developed for flexibly adjusting the external coupling for the two bands. A tapped-line coupling is used to adjust the external coupling for the first band, and a coupled-line coupling is used to adjust the external coupling for the second one. The DBPF was designed to operate at 0.8 GHz with a 16-MHz (2%) bandwidth for the first band and at 2.0 GHz with a 40-MHz (2%) bandwidth for the second band. A compact (30 mm × 28.25 mm) filter was fabricated using YBa
₂
Cu
₃
O
_y
thin film on a CeO
₂
-buffered Al
₂
O
₃
substrate. The measured results agree well with the simulated ones.