Compact microstrip lowpass filter with ultra-wide stopband based on dual-plane structure

摘要

A microstrip lowpass filter (LPF) with ultra-wide stopband suppression and compact size based on dual-plane structure is presented. The LPF is composed of high-impedance resonators loaded by folded patches on the topside plane, combined with hybrid slot and step-impedance resonators on the ground plane. Multiple transmission zeros are created in the stopband region by different resonators. Thus, an ultra-wide stopband with 11th-harmonic suppression is achieved. A demonstration filter with 3 dB cutoff frequency (fc) at 2.45 GHz has been designed, fabricated and measured. Results show that the relative stopband bandwidth is 163.8% with attenuation level of 20 dB. The insertion loss is lower than 0.3 dB and the return loss is better than 17 dB in the passband. The size of the LPF is only 8.90 mm × 8.93 mm, which corresponds to 0.115λg × 0.116λg (λg is the guided wavelength at fc).