Chebyshev Wideband Balanced Bandpass Filter with High Suppression of Common Mode and Harmonic Using Series Short-Ended Slotline Stubs

A Chebyshev wideband balanced bandpass filter (BPF) with excellent common-mode (CM) suppression and an extended upper stopband is proposed, which is made up of a hybrid microstrip–slotline vertical transition structure and series short-ended slotline stubs. By means of the intrinsic CM rejection feature of slotline stubs, the CM noises of the circuit are effectively suppressed with a rejection level better than 31.7 dB over the frequency range of 1.7 to 14.5 GHz, while the differential-mode (DM) signal can pass through smoothly. In terms of odd-/even-mode equivalent circuits, the corresponding odd-/even-mode resonant frequencies can be solved expediently. Moreover, the synthesis algorithm is employed to achieve Chebyshev equal-ripple responses in the balanced filtering circuit, which significantly enhances the design efficiency of the proposed balanced filter. In addition, lowpass filters (LPFs)—which consist of two sets of dumbbell-shaped series slotline segments—are adopted to further improve the harmonic suppression level of the DM filtering circuit and significantly expand the upper stopband. Finally, the proposed Chebyshev wideband balanced BPF is fabricated and tested to verify the validity of the synthesis theory; experimental results agree well with simulation responses.