Circuit direct replacement method enhanced skirt response of open stub low-pass filter

An interdigital low-pass structure directly replacing center open stub of odd-order open stub low-pass filter to achieve sharp skirt response is proposed. The characteristics of interdigital structure were first analyzed by lumped elements equivalent circuits and investigated by full-wave microwave circuit simulator, IE3D. These simulated results were then applied as guideline for the replacement. An open stub low-pass filter was developed by this method to enhance the skirt response near 3 dB cutoff frequency and fabricated on RO-4003C substrate. The measured results show very good improvement on skirt response and the rejection level in stop band.