Microstrip Bandpass Filter with Wide Stopband and High Out-of-Band Rejection

This paper presents a compact microstrip bandpass filter exhibiting a wide stopband, high selectivity and easily tunable passband. The filter comprises of asymmetric resonator structures, which are interconnected by an inter-digital capacitor to enable the realization of a wide bandwidth with high rejection level. High selectivity and easily tunable passband are obtained by optimizing the parameters of the interdigital capacitor. The filter has high out-of-band rejection 30 dB, less than 0.6 dB of insertion-loss, up to 5.5 GHz spurii free and more than 15 dB of return-loss. Full-wave electromagnetic simulator ADS^TM based on Method of Moment technology is used to analyze and optimize the prototype bandpass filter. The proposed technique was verified practically to validate the design methodology. The experimental results of the prototype circuit are presented and a good agreement was obtained comparing with the simulation results. The dimensions of the proposed filter are 32×24 mm². The filter’s characteristics and compact size make it suitable for wireless communication systems.