A balanced-to-balanced (BTB) bandpass filter (BPF) with an ultra-wide upper stopband is proposed in this letter. The proposed BPF is fed by balanced stepped-impedance microstrip-to-slotline transition structures. Good differential-mode (DM) transmission and common-mode (CM) suppression can be achieved intrinsically. To achieve good quality in DM passband and out-of-band suppression, a pair of dual-mode resonators has been designed and adopted. Meanwhile, the proposed balanced BPF exhibits an ultra-wide upper stopband of 162.7%. In order to verify the feasibility of the design method, a balanced BPF with a centre frequency of 1.57 GHz has been fabricated and measured. Measured results indicate that the designed filter achieves an out-of-band rejection better than 15 dB from 1.85 to 18 GHz, and the insertion loss (IL) inside the passband less than 1.4 dB. A good agreement between the simulation and measurement results demonstrates the validity of the design.
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