Miniaturized Dual-band Bandpass Filter Using Co-planar Waveguide Coupling Structure

A dual band planar bandpass filter with co-planar waveguide feeding structure is presented. The size of the filter is reduced by using nested multi mode resonator, and the resonance characteristic is analyzed by odd even mode and weak coupling method, and the relationship between resonance frequencies and physical dimensions is also given. To conveniently feed the four modes, obtain desired coupling and miniaturize circuit size, coplanar waveguide feeding method is employed in the filter design. Analysis of the four resonant modes is given and the mechanism to control the center frequencies and bandwidth of two passbands are also shown, and independently control of frequencies and bandwidth of filter circuit are achieved. A prototype filter operating at 2.39GHz and 4.34 GHz with 3dB fractional bandwidth of 8.8% and 13.4% has been designed and fabricated. The center frequency and bandwidth can be independently controlled, and resonator has a compact size of 0.09λg×0.09λg . Because of simple structure, small size, good isolation between passbands, and easy to be fabricated, the proposed filter is of great value in application.