Design method of closed loop resonator filter using parallel capacitor to reduce size

This paper presents a new design method of closed loop resonator filter by using multiple sub-networks that connects transmission line to capacitor in parallel. This method gives an advantage to reduce the size of closed loop resonator without changing permittivity of printed circuit board (PCB) or folding the microstrip lines. The value of parallel capacitor is used as a design factor to reduce the size of closed loop resonator. This novel design method can only reduce the size of closed loop resonator without degrading or changing the filter characteristics. To evaluate the validity of this novel design method, 2-order Tchebyscheff BPF (f₀=2 GHz, BW=100 MHz) composed by closed loop resonator filter with C_p=1 pF is designed and fabricated. According to design and measurement results, a good agreement between calculated results and measured results is achieved. As a result, 70% reduction in size of the filter is obtained without changing filter characteristics.