Design and Fabrication of a Novel Compact Bandpass Filter to Improve Spurious-Free Band

In this paper, a new micro strip slow wave open loop resonator is proposed. This resonator overcomes the substantial limitation of the conventional slow wave open loop resonators, which limits the spurious-free band of those resonators. The new proposed resonator improves the spurious-free band up to 8 times of the fundamental frequency, which is 3.16 times of the conventional slow wave open loop resonators. For the proposed resonator the coupling factor and the external quality factor curves, which are the design curves of a narrow band filter, are extracted using a full wave simulator, i.e. HFSS. These curves are utilized for the design of two three-pole Chebyshev band pass filters. One of these filters is fabricated. Simulation and measurement results are reported which are in good agreement. Designed filters provide 75% and 83% of footprint reduction in comparison with the conventional open loop resonator filters. Different parameters which can improve the spurious-free band are explained in the paper.