Design of miniature tri-band filter with multiple types of resonators

A microstrip tri-band filter, which is designed by using two kinds of ring resonators and one lumped element resonators, is presented in this paper. This filter is constructed by using inductive coupled lumped resonator for the lowest passband and two folded ring resonators for the other passbands to minimize the size of the filter and flexibly control the characteristics of three pass bands. Design equations that can determine the component values of the new structure of ring filter are derived. A methodology that can determine the components of the filter with arbitrarily assigning transmission zeros and the bandwidths of three passbands is proposed. To verify the proposed design concept, an experimental prototype with three pass bands at 2.4 GHz, 3.6 GHz and 5.0GHz was fabricated on PCB substrate. The size of the filter is about 12×21 mm² and the measured insertion losses in the three passbands are less than 2.0 dB, 2.5 dB and 3.0 dB. The associated return losses are all greater 18 dB over the frequency bands of interest.