In this paper, a new quadruple-mode stepped-impedance square ring loaded resonator (SI-SRLR) is proposed, and a high-performance dual-band differential bandpass filter (BPF) is developed. Resonant characteristics of the SI-SRLR are investigated by the even-/odd-mode method and simulation techniques. Two differential-modes (DMs) of the SI-SRLR are used to form the dual differential passbands. The admittance ratio K of the SI-SRLR is chosen appropriately to prevent the common-mode (CM) interference with DM operations. Moreover, an open-circuited stub is added to enhance the CM suppression, while a source-load coupling is introduced to produce multiple transmission zeros and improve significantly the frequency selectivity of DM passbands. Finally, a dual-band differential BPF is designed, fabricated, and measured. Good agreement between the simulated and measured results verifies well the proposed structure and design method.

In this letter, a novel compact dual-band filtering power divider (DB-FPD) is proposed. It consists of a U-shaped Wilkinson power divider, two pairs of dual-resonance resonators (DRRs), and a pair of spur lines. With the simultaneous use and appropriate design of the coupled feedlines, mixed electric and magnetic couplings between the DRRs and spur-lines with different lengths, multiple transmission zeros are produced which result in two passbands with desired power division, high-frequency selectivity, good isolation, and an ultrawide stopband. A prototype DB-FPD operating at 2.3 and 3.5 GHz with 3-dB fractional bandwidths of 8% and 6% is designed, fabricated, and measured. The measured responses agree well with the design simulations, exhibiting a stopband up to 13.8 GHz (6f₀) with 20-dB rejection level.

In this brief, a novel multi-mode square ring loaded resonator (SRLR) is presented, and a tri-band balanced bandpass filter (BPF) with good performances is developed. The multi-mode SRLR consists of a ring resonator and six open stubs, and its resonant characteristics is obtained by the odd/even-mode method. Compared with conventional SRLR, the symmetrical SRLR has more degree of freedom in controlling the resonance modes. Thus, a wide upper DM stopband can be achieved by choosing proper electrical lengths of open stubs. Moreover, frequency dispersion technique is used to enhance common-mode (CM) suppression in the proposed balanced tri-band BPF. By loading a cross-shaped stub and open-loop stub to the center of the first and second order SRLR respectively, five CM transmission zeros (TZs) are produced. Therefore, a wideband CM suppression can be achieved with the DM response almost intact. To verify this design method, a tri-band balanced filter operating at 2.4, 3.5 and 5.2 GHz has been designed, simulated and fabricated. Measured results exhibit good agreement with the simulated ones.

In this paper, a novel wideband differential bandpass filter (BPF) with slotline surface plasmon polaritons (SPPs) is proposed. It consists of a pair of differential microstrip-to-slotline transition, a slotline-to-SPP transition with subwavelength gradient grooves, and a slotline SPP waveguide, which supports the TEM, TE, and SPP modes, respectively. The dispersion and transmission characteristics of the proposed hybrid SPP structure are analyzed under differential-mode (DM) and common-mode (CM) operations and applied to design wideband differential filter. To validate the design concept, a filter sample has been designed, fabricated, and measured. Both simulated and measured results prove its good performance including a wide 3-dB fractional bandwidth of 147.6% centered at 4.2 GHz (f ₀ ), a wide DM upper stopband to 4.8 f ₀ (S ₂₁ ^dd <; -30 dB), and a wide 40-dB CM suppression from 0.1 to 20 GHz.

A novel design method of microstrip diplexer without extra matching circuit is proposed in this paper. Because of the synthesizable circuit parameters and controllable transmission zeros (TZs) in composite resonator (CR) cell, diplexer A based on CR cell is proposed. By allocating TZs at center frequency of the opposite channel, as well as the using of impedance transformations of J-inverters, high isolation and good passband selectivity are realized in diplexer A without extra matching circuit. In order to improve the stopband performance, diplexer B based on coupled CR cells is proposed. Furthermore, diplexer B using the proposed design method is implemented by directly combining two bandpass filters (BPFs), which simplifies significantly the design process of diplexers. Diplexer A operated at 1/1.2 GHz and diplexer B operated at 1/1.5 GHz are designed and fabricated. The measured isolation characteristics in diplexer A and diplexer B are all higher than 40 dB. Good agreements between simulated and measured results are achieved, which successfully verifies the proposed design method of diplexers.