Design of a Compact Balanced Tri-band Bandpass Filter Using Simple Planar Resonator with Wide Stop-band and High Selectivity

While most balanced multiband band-pass-filters have some remarkable properties, they provide a trade-off between design goals. Also, few of them have provided a structure capable of satisfactory harmonic suppression. Stepped Impedance resonators (SIR) are famous and widely utilized to relocate or cancel high order harmonics, and they provide advantages when used in designing bandpass filters. They have shown benefits in achieving common mode (CM) suppression along with compact size and wide stopband. To design a tri-band balanced bandpass filter, a Ring-SIR structure is used as the main building block of the filter, the SIR is first analyzed and the design formulas are presented and based on that, design graphs are extracted. Using the graphs, it is possible to design a tri-band filter and have the ability to control its pass-bands’ center frequencies. To achieve high pass-band selectivity, coupled feeding configuration is chosen and external quality factor is analyzed and respective graphs are presented. Results show more than 20dB isolation with pass-bands centering at 1.7, 2.55, and 4.48 GHz, and insertion losses of 1.44, 1.77 and 2.11 dB respectively. The corresponding FBWs are 3.95%, 4.10% and 1.56% respectively. Great out-of-band performance is achieved with a wide stop-band stretching from 4.8 to 8.76 GHz. The CM rejection for three pass-bands are better than 12.9, 18.9 and 43.5dB, respectively.