Design of compact microstrip band-pass filter with ultra-wide stopband

In this article, a microstrip band-pass filter with compact structure and ultra-wide stopband using quarter-wavelength stepped impedance resonators is proposed. Because of the optimal design for stepped impedance resonators, circuit size is reduced and harmonic response is suppressed. To suppress the higher resonant modes further, two open stubs connected to the input/output ports are used to achieve an ultra-wide stopband. The design procedure and additional lateralresonated response are discussed in detail. As the presence of the two attenuation poles are around the passband edge, high selectivity and good performance is realized. The proposed filter with a fundamental frequency f₀ of 0.9 GHz, measured insertion loss of less than 1.5 dB, return loss of better than 16 dB, and wide stopband up to 19 f₀ is demonstrated. The stopband level is lower than 23 dB from 1.1 GHz up to 17.1 GHz.