A Narrowband Bandpass Filter With Wide Stopband Using Mixed Lumped and Distributed Circuits

ABSTRACT A narrowband bandpass filter (BPF) using mixed lumped and distributed circuits is proposed with wide stopband and small circuit size. By loading four lumped capacitors on the two pairs of coupled lines and two open stubs, the periodic harmonics generated by the coupled lines and open stubs can be effectively suppressed, resulting in the achievement of wideband stopband. To validate the design, a narrowband BPF prototype with center frequency f 0 at 1 GHz is fabricated, whose measured 3‐dB fractional bandwidth is 5% (0.97–1.02 GHz). The in‐band insertion loss is less than 1.5 dB and return loss is greater than 32 dB. The rejection level of stopband is better than −20 dB at the frequency range from 1.05 to 6.22 GHz.