Microstrip Band-pass Filter with Adjustable Center Frequency and Fixed Bandwidth Property Using a Modified 3-Section SIR Resonator

This paper presents a novel microstrip bandpass filter (BPF), whose one of the most important feature of the filter is that the bandwidth remains fix as the center frequency changes. Therefore, the structure is suitable for reconfigurable communication systems. The BPF is consist of coupled-lines and a modify three section stepped impedance resonator (SIR) and a LPF that consists of one open-ended stub to suppress spurious harmonics at high frequencies. Even and odd mode analysis has been used for designing process of the BPF. Finally, the proposed filter has a fractional bandwidth of 25% at 3.095 GHz with the insertion loss less than 0.1 dB/ 3.715 dB in the pass-band (simulation/measurement), compact size, low insertion loss, right out of band rejection, high-frequency selectivity. The BPF is fabricated and measured. The excellent agreement between the measured and simulated results demonstrates the validity of the proposed filter and method.