A planar bandpass filter design with wide stopband using double split-end stepped-impedance resonators

In this paper, we propose a compact planar bandpass filter design with wide stopband. The double split-end quarter-wave-length (λ/4) resonator is introduced to reduce the resonator size while providing additional transmission zeros. Optimal split-end length is determined to provide transmission zeros that attenuate the two lowest spurious resonance frequencies. The singly loaded quality factor (Q_si) of the optimal-length λ/4 stepped-impedance resonator (SIR) is also analytically derived. Using the proposed technique to design an Nth-order filter, N+1 controllable transmission zeros are generated and used to suppress spurious frequency responses. The experimental design shows that the sixth-order filter can provide a very broad stopband of 8.5 times the fundamental frequency with at least 37.8 dB of attenuation using the SIRs with a stepped-impedance ratio (R) of 0.528.