Design of a high-order narrowband superconducting balanced filter based on asymmetrically coupled stepped-impedance resonators

Abstract

This study investigates a sixth-order narrowband superconducting balanced filter with high frequency selectivity and wide stopband based on asymmetrically coupled stepped-impedance resonators (SIRs). Three types of coupled SIR pairs with different characteristics are investigated. The differential-mode (DM) phase shifts, coupling coefficients, and common-mode (CM) levels of the three SIR pairs are then analyzed. The spurious frequencies of DM responses can be pushed far away from the fundamental frequency to widen the stopband by properly choosing the impedance ratio of the SIR and using dissimilar SIRs with dispersed harmonics. Moreover, an inherent CM suppression can also be obtained because of the different resonant frequencies in DM and CM equivalent circuits. Results also show that the electromagnetic coupling pair can contribute a weak coupling coefficient for narrowband design without increasing the circuit size. The structure of the electric coupling pair is beneficial for high-order circuit interconnection. The magnetic coupling pair possesses the highest CM rejection. Moreover, the weak coupling properties are well-utilized for narrowband design. For validation, a sixth-order balanced bandpass filter is designed using the three coupled SIR pairs, and a cross-coupling structure is further introduced to produce transmission zeros (TZs) and enhance the passband selectivity. The magnitude and phase delay of the DM transmission paths are analytically illustrated to explain the TZ production. Eventually, the proposed balanced filter is implemented by a high-temperature superconducting technique to reduce the insertion losses and is measured at the temperature of 77 K. The measured results show that the fractional bandwidth of the passband is 1.6$%$ with less than 0.37 dB in-band insertion loss. The $-60$ dB bandwidth/$-3$ dB bandwidth rectangle ratio is less than 1.45 while the in-band CM suppression is better than 40 dB. The $-$20 dBłinebreak stopband is up to about 5$f_{0}^{d}$ ($f_{0}^{d}$ is the DM fundamental frequency).