High-Selectivity Balanced Bandpass Filter With Mixed Electric and Magnetic Coupling

In this letter, a novel balanced bandpass filter (BPF) with mixed electric and magnetic coupling is proposed, which is implemented by using
$\lambda/4$
stepped-impedance resonators (SIRs). Specially, balanced coupled-line is utilized to realize the passband response in differential-mode (DM) operation and meanwhile give a wide and deep stopband in common-mode (CM) operation. Second- and third-order equivalent half-circuits are investigated and simulated, which exhibits an excellent filtering performance for DM as well as a high level suppression in CM. A prototype balanced filter operating at 5.2 GHz is realized, which has controllable transmission zeros (TZs), sharp skirts, low inserted loss and compact size for DM. Under CM excitation, the measured level of suppression of signal is over 41 dB within the passband, over 38 dB from 0 to 9 GHz and over 26 dB from 0 to 16.5 GHz.