An ultra-narrowband HTS bandpass filter

This paper presents the measured results of an HTS ultra-narrowband bandpass filter with a 100 kHz bandwidth. This filter, with a fractional bandwidth of 0.014% and a center frequency of 700 MHz, represents a previously unrealized portion of filter parameter space. This filter was realized using a five-pole Chebyshev filter design and was fabricated using an HTS microstrip configuration. This filter required very high Q, very weak coupling, and a strong reduction in parasitics. Unloaded Qs of 135,000 were achieved with a resulting insertion loss of 1.37 dB. Even resonator Qs of 20,000 would lead to a filter loss of 9 dB. A noise figure of 1 dB was achieved for a complete system with this filter and a cryogenic low-noise amplifier.