Miniaturization of microstrip ring filter using lumped capacitors

Microstrip ring filter with reduced size is designed by adding four lumped capacitors to its initial topology. Depending on the value of the capacitors, the center frequency of the initial filter can be shifted to about 50% of its initial value and result in a smaller filter as compared to the one realized directly at the desired frequency. However, the shift in frequency has to face the matching level problem in the filter passband response. As a solution, the impedance of the quarter wavelength transmission line is proposed to be modified. The value of the lumped capacitor and the required line impedance of the ring are calculated using the synthesis of tunable ring filter. Implemented on FR4 substrate, the layout and simulation results of miniaturized ring filter are presented in this paper, showing a size reduction of about 66% with 2.5 GHz center frequency.