Defected ground structures (DGS) and uniplanar compact-photonic band gap (UC-PBG) structures for reducing the size and enhancing the out-of-band rejection of microstrip bandpass ring resonator filters

This paper addresses a modified bandpass ring resonator filter providing compact size, low insertion-loss, wide bandwidth, sharp rejection and suppressed higher order modes. Typically, ring resonator filters suffer from large occupation area and limited out of band rejection. These limitations are tackled in this work by means of a threefold approach. On the one hand, the use of internal folded stubs allows the exploitation of the internal ring area yielding more than 70% of overall size reduction with respect to conventional orthogonal straight stubs solution. On the other hand, the introduction of Defected Ground Structures (DGS) and Uniplanar Compact-Photonic Bandgap (UC-PBG) structures, formed by etching patterns in the ground plane of the filter, enhances by great extends the rejection of higher order modes providing a stopband that reaches 4.7 GHz beside offering a further advantage in terms of size reduction. The two approaches are first separately validated and then simultaneously combined to provide an optimized filter design. Finally, a solution employing via holes to replace the tuning stubs is explored in order to extend the lower stopband. EM simulation, equivalent circuit model, as well as measurement results validate the followed design approach and are in excellent agreement within each other.