A Compact Superconducting Bandpass Filter at 360 MHz with Very Wide Stopband Using Modified Spiral Resonators

This paper proposes a modified spiral resonator with a tight three-turn spiral and inner/outer tails. The resonator has good spurious response because the mutual inductances between the three turns at the fundamental mode are positive and thus reduce the fundamental resonant frequency f(0), whereas those at the first spurious mode are negative and increase the first spurious resonant frequency f(S). By further optimizing the lengths of the spiral and inner/outer tails of the proposed resonator, a spurious resonant frequency f(S) up to 3.4 f(0) is obtained. Moreover, the couplings between such resonators at spurious modes are much weaker than that at the fundamental mode, which is helpful to suppress the spurious response of bandpass filters consisting of such resonators. Furthermore, three dissimilar types of resonators with the same fundamental frequency but with different spurious resonant frequencies are used to compose a bandpass filter and further suppress the spurious response in the stopband. With these methods, a ten-pole superconducting filter at 360 MHz with a 15-MHz bandwidth is successfully designed and fabricated on a LaAlO3 substrate. The overall measurements show a high performance and agree very well with the simulations. The maximum insertion loss is 0.15 dB, the return loss is greater than 19 dB, and the rectangle coefficient (the ratio between the 40-dB bandwidth and the 3-dB bandwidth) is 1.35. Moreover, the out-of-band rejection is higher than 76 dB up to 2030 MHz, which is 5.64 f(0). The ten-pole filter occupies a compact area of 33 mm x 15 mm, which amounts to only 0.15 lambda(g0) x 0.07 lambda(g0), where lambda(g0) is the guided wavelength of the 50-Omega line on the substrate at f(0).