Topology bandpass filter with spiral capacitors

A new topology bandpass filter in coplanar strip (CPS) line is reported. It incorporates a new spiral capacitor to reduce the filter size. Single layer inductors are implemented using narrow electrode bent to save space. To emulate high capacitance value on a specific planar electrode topology, the conventional interdigitated structure was first analyzed. It was found that its effective capacitance is significantly larger at frequencies near self-resonance. To use this phenomenon, the filter center frequency needs to be close to the self-resonant frequency of the capacitor. We thus introduced a new spiral capacitor topology where narrow electrodes are coupled with spiral turns to increase the filter inductances so that the self-resonance frequency can be made closer to filter center frequency. Simulation results show that filter size can be significantly reduced using the spiral capacitor instead of the conventional interdigitated capacitor. For demonstration, an UNII 5.15-5.85 GHz bandpass filter was designed and fabricated on an alumina substrate. It takes up only 1.7mm × 1.7mm. The measured responses agree well with the simulation results.