Technique for tuning center frequency of HTS microstrip filters without significantly changing bandwidth

We have developed a technique for tuning high-temperature superconducting (HTS) microstrip filters in which additional electric pads are placed at the open ends of the resonators for tuning the center frequency. Pads are placed along one side of the open ends of the resonators to enable the effective lengths of the resonators to be adjusted, thereby shifting the center frequency. Pads are also placed around the I/O coupled-line elements to enable the coupling strength between the coupled-line and resonator to be adjusted, thereby reducing the insertion loss caused by tuning. The pads consist of line-and-space patterns. The former are electrically connected to the resonators, and the latter are electrically connected to the coupled-line elements. Simulated application of this technique to a 3-pole superconducting microstrip combline bandpass filter with half-wavelength resonators, a 5.0-GHz center frequency, and a 120-MHz 3-dB bandwidth shifted the bandwidth 500 MHz. However, the 3-dB bandwidth was reduced about 14 MHz. Use of a modified 3-pole microstrip interdigital bandpass filter mitigated the reduction to 2 MHz. Using this modified filter with additional electric pads is thus an effective way to tune the center frequency of HTS bandpass filters without significantly changing the bandwidth.