Compact microstrip bandpass filter with two transmission zeros based on λ/8 coupled-line sections

Compact microstrip bandpass filter with two transmission zeros is proposed based on one-eighth wavelength (λ/8) coupled-line sections. The proposed filter occupies only half of the circuit area of conventional parallel coupled-line bandpass filter, and exhibits good stopband rejection with no repeated passband at twice center frequency. By introducing a cross-coupled shunt inductor, two transmission zeros at upper and lower stopbands may be created without increasing the circuit area. The desired locations of transmission zeros can be adjusted by varying the value of cross-coupled inductance. Specifically, a microstrip coupled-line bandpass filter centered at 2.45 GHz with a 3dB bandwidth of 9.6% and two transmission zeros is implemented.