Dual-Band Superconducting Spiral Filters Including Narrow Bandstop Notches

Dual-band filters designed as bandpass filters containing bandstop notches are described. The design formulation is simplified by considering the resonators in pairs. Two microstrip superconducting spiral filters are presented with 400-MHz center frequency, 15%-16% overall bandwidth, and notches of 1% and 0.1%, respectively. Internal dimensions of the enclosures are only 32 mm times 30 mm times6 mm. Suitable spiral shapes and orientations enable the required wide range of coupling coefficients to be implemented, and also reduce unwanted coupling without using partitions. The remaining low level of unwanted coupling is cancelled using capacitive bridges. A qualitative understanding of spurious interactions between spirals is obtained via electromagnetic simulations.