Novel method for skirt response improvement of open-stub low-pass filter

A direct circuit replacement method using interdigital low-pass structure directly replaced all open stubs of odd-order open-stub low-pass filter to achieve sharp skirt response. The characteristics of the interdigital structure are analyzed by lumped element equivalent circuits and investigated by full-wave electromagnetic simulator, IE3D. These simulated results were then applied as a guideline for the replacement. Two open-stub low-pass filters were designed by this method to enhance the skirt response near 3-dB cutoff frequency. The measured results show very good improvement on skirt response and the rejection level in stop band.