Synthesis of X-band trisection bandpass filters using hybrid structures of λ/4 and λ/2 resonators

This paper presents the design of compact cross-coupled microstrip trisection bandpass filters based on the hybrid structures of quarter-wavelength (λ/4) and half-wavelength (λ/2) resonators. The desired cross-couplings for transmission zeros (TZs) are realized by parallel-coupled transmission-line sections (J-inverter) or coupled via sections (K-inverter) of λ/4 resonators. The main-line couplings, on the other hand, are formed by mixed-coupled λ/4 and λ/2 resonators. To validate the proposed hybrid configurations, two X-band trisection filters with 6.5% and 6.2% fractional bandwidth were designed. The first filter generates a TZ below the passband, while the second one produces a TZ above the passband. Minimum insertion loss of 1.7 dB and 1.5 dB was observed for the two trisection filters, respectively, while the hybrid configurations caused significant size reduction in the overall filter size. Moreover, the first spurious response for the proposed filter topologies was pushed to 3fo, resulting in a wide stopband.