Compact Bandpass Filter with Multiple Harmonics Suppression Using Folded Parallel-Coupled Microstrip Lines

A compact bandpass filter with wide stopband and multiple transmission zero is presented. The device uses a low impedance feeding network, folded parallel coupled microstrip lines (FPCML) with stepped impedance and two capacitive coupled open stubs. The low impedance feeding network is utilized to enhance the coupling of the PCML structure and to achieve sharp lower and upper cut-off frequencies, while the folded open stub embedded with high/low impedance at end of PCML structure able to extend the upper stopband. Meanwhile, the two capacitive coupled open-ended stubs are able to produce multiple transmission zeros at the stopband to enhance the rejection response and achieve harmonic suppression. The proposed filter with a centre frequency of 2.45 GHz is designed and optimized using ADS Momentum. The filter shows excellent performance with 0.1 dB insertion loss and more than 15 dB attenuation in the wide stopband, with 20% fractional passband.