Modified microstrip filters improving the suppression performance of harmonic signals

Abstract — In this paper, a new parallel-coupled-line microstrip BPF(Bandpass Filter) improving the suppression performance of 2nd harmonic signals is described. Using the consecutive patterns in coupled-line, the desired passband performance is improved and harmonic passband signal is diminished. Recalculation of design parameters(space-gap between lines, line widths and lengths) of conventional filters is not required. Because, after using the conventional design methodology for parallel-coupled-line BPF, new filters can be easily realized by inserting periodic patterns in coupled-line. To evaluate the validity of this novel technique, order-3 Butterworth BPF centered at 2.5 GHz with a 10% FBW(Fractional Bandwidth) and order-5 Chebyshev BPF centered at 10 GHz with a 15 % FBW were used. When five and three square grooves are used, over 30 dB suppression at 2nd harmonic is achieved in simulation and experiment. I