A balanced dual‐band bandpass filter with independently tunable differential‐mode frequencies

A balanced dual‐band bandpass filter (BPF) with independently tunable differential‐mode (DM) frequencies is proposed in this letter. The proposed BPF is composed of complementary split‐ring resonators (CSRRs) etched on the ground and varactors loaded on the resonators. A balanced stepped‐impedance microstrip‐slotline transition structure is introduced to transfer the DM signals successfully and block the common‐mode (CM) signals transmission. Good DM transmission and CM suppression can be achieved. Moreover, by changing the reverse bias voltages of the varactors loaded on coupling CSRRs, two DM resonant frequencies of the proposed balanced BPF can be tuned independently. To verify the feasibility of the design method, a balanced BPF with DM frequency ranging from 0.80 GHz to 1.12 GHz and 1.55 GHz to 2.05 GHz is fabricated and measured. Good agreement between the simulation and measurement results demonstrate the validity of the design.     

A novel balanced wideband power divider based on microstrip‐slot transitions

A novel design of a balanced wideband power divider (PD) with enhanced common‐mode (CM) suppression is proposed. The top and bottom layers of the structure contain tapered microstrip line. Those microstrip lines are coupled via slotline in the ground plane, which is located at the middle layer. With appropriate placement of the slotline, the coupling between the slotline mode and the differential‐mode (DM) signals can be maximized, while that between the slotline mode and the CM signals can be minimized. Simulated and measured results show that the proposed PD has equal power division, low insertion loss, and good return loss. In the measurement, the fractional bandwidth of the measured ?10 dB (DM) return loss is about 101% (1.82–5.35 GHz), the insertion loss for the DM signals is less than 5 dB, the suppression of the CM signals is higher than 45 dB, and the DM isolation is better than 10 dB over the fractional bandwidth. © 2013 Wiley Periodicals, Inc. Int J RF and Microwave CAE 24:437–442, 2014.     

Wideband and spurious‐suppressed differential bandpass filter based on strip‐loaded slot‐line structure

In this article, a wideband and spurious‐suppressed differential bandpass filter based on strip‐loaded slot‐line structure is presented. By means of the differential microstrip‐slot‐line‐microstrip transition, the proposed filter has a wideband bandpass filtering response. Simultaneously, the utilization of the strip‐loaded slot‐line extends its upper stop‐band. The proposed bandpass filter has wider upper‐stopband, wideband bandpass response, and intrinsic high common‐mode (CM) suppression. To verify the design concept, one filter example has been designed, fabricated, and measured. It has a differential‐mode (DM) 3‐dB fractional bandwidth of 157% with a low 0.82 dB minimum insertion loss. What's more, it shows a very wide 20 dB DM stop‐band bandwidth of 6.5 f_0d. The experienced results are in good agreement with the theoretical and simulated results.     

Novel planar balanced bandpass filter with wideband common‐mode suppression and in‐band common‐mode noise absorption

This paper presents a novel planar balanced bandpass filter (BPF) with wideband common mode (CM) noise suppression and in‐band CM noise absorption using coupled lines (CLs) with short‐circuited stubs to realize high selectivity and wideband differential mode (DM) filtering performance. Two one‐quarter wavelength stubs loaded with grounded resistors are introduced to realize wideband CM noise suppression. Thus, CM noise can be suppressed under a certain level at all frequencies. Four resistors are used to achieve CM noise absorption by dissipating the CM noise into heat, which can avoid the noise being reflected to the communication system and realize a wide absorption bandwidth with 90% absorption efficiency. For demonstration, an absorptive balanced BPF operating at 3.5 GHz with wide 3‐dB fractional bandwidth (FBW) of 79.43% is fabricated and experimentally validated. It is worth noted that the absorptive balanced BPF can realize broadband CM noise suppression from 0 to 8 GHz, and the CM noise is well absorbed more than 10 dB from 2.41 to 4.63 GHz. Besides, wideband CM noise absorption with 90% efficiency from 2.51 to 4.60 GHz is realized, which indicates potential applications in improving the performance of the balanced radio frequency (RF) circuits. Good agreements between the simulated and measured results are observed.     

Direct synthesis of compact wideband differential bandpass filter on composite triple‐mode resonator

In this paper, a direct synthesis approach is proposed for design of a compact wideband differential‐mode (DM) bandpass filter (BPF) on a composite triple‐mode resonator. By virtue of intrinsic common‐mode (CM) suppression in slotline portion, only the DM transmission performances need to be focused on in our design, so as to facilitate the design process. Consequently, a synthesis approach based on an equivalent simplified network is established to design this DM BPF, resulting to directly determine all the circuit element values. A wideband DM BPF is then designed to validate effectiveness of proposed synthesis method. Finally, two circuit prototypes have been designed, fabricated, and measured. The synthesized, simulated and measured results agree well with each other over a wide operating band, which has demonstrated the proposed DM BPFs' attractive performances, such as wide bandwidth of operation, sharpened frequency selectivity, and good CM suppression.     

A tunable balanced dual‐band BPF with quasi‐independently controlled center frequency and bandwidth

In this paper, a balanced dual‐band bandpass filter (BPF) with high selectivity and low insertion loss performance is presented by employing stub loaded resonators (SLRs) and stepped impedance resonators (SIRs) into balanced microstrip‐slotline (MS) transition structures. The balanced MS transition structures can achieve a wideband common‐mode (CM) suppression which is independent of the differential‐mode (DM) response, significantly simplifying the design procedure. Six varactors are loaded into the resonators to achieve the electrical reconfiguration. The proposed balanced dual‐band BPF can realize quasi‐independently tunable center frequencies and bandwidths. A tuning center frequency from 2.48 to 2.85 GHz and a fractional bandwidth (20.16%‐7.02%) with more than 15 dB return loss and less than 2.36 dB insertion loss are achieved in the first passband. The second passband can realize a tuning center frequency from 3.6 to 3.95 GHz with more than 12 dB return loss and less than 2.38 dB insertion loss. A good agreement between the simulated and measured results is observed.     

A new dual‐band single‐ended‐to‐balanced filtering power divider

This article proposes a new dual‐band single‐ended‐to‐balanced (SETB) filtering power divider (FPD), which shows the excellent characteristics of wideband common‐mode (CM) suppression and good selectivity. By employing the structure of double‐sided parallel‐strip line with a mid‐inserted conductor and a T‐shaped defected ground structure etched in the mid‐inserted conductor, out‐of‐phase behavior and high CM suppression can be achieved successfully. Besides, to realize dual‐wideband filtering performance and high selectivity, two pairs of step impedance stubs (SIS) loaded quarter‐wavelength central line‐terminal‐shorted three parallel‐coupled microstrip lines structure are adopted. Meanwhile, two pairs of resistors are introduced so as to realize excellent isolation. To verify effectiveness of the design method, a prototype of dual‐band SETB FPD which operates at 3.2 and 4.9 GHz is designed, fabricated, and tested. Final results exhibit that the new dual‐band SETB FPD possess high selective dual‐band differential mode response, wideband CM suppression, and excellent isolation between the balanced output ports.     

A highly selective balanced wideband bandpass filter based on nested split‐ring resonators

A balanced wideband bandpass filter (BPF) with a high frequency selectivity, controllable bandwidth, and good common‐mode (CM) suppression based on nested split‐ring resonators (SRRs) is proposed in this article. The proposed nested SRRs are applied to form three transmission poles (TPs) that can achieve a wide differential‐mode (DM) passband centered at 3.0 GHz. Meanwhile, two transmission zeros (TZs) are generated to realize a high frequency selectivity of the DM passband. Moreover, TPs and TZs can be quasi‐independently controlled by changing the physical lengths of SRRs and the gaps between them, which can greatly improve the flexibility and practicality of the design. The proposed balanced BPF is fed by balanced microstrip‐slotline (BMS) transition structures. For the CM signals, the BMS transition structures can achieve a good wideband CM suppression without affecting the DM ones, thereby simplifying the design procedure. In order to validate its practicability, a balanced wideband BPF is fabricated and a good agreement between the simulated and measured results is obtained.     

Wideband balanced microstrip‐to‐microstrip vertical transition

A wideband balanced microstrip‐to‐microstrip vertical transition is proposed. This vertical transition is based on the back‐to‐back hexagonal microstrip lines with a slot on the common ground. The etched slot is used to achieve the vertical transmission of differential‐mode signal and the suppression of common‐mode (CM) signal. The back‐to‐back hexagonal microstrip lines enable the transition to obtain wide bandwidth. One prototype shows the fractional bandwidth (|S_dd11| ≤ ?15 dB) of 63%, the minimum (maximum) insertion loss of 0.7 dB (1.9 dB), and the minimum CM suppression inside passband of 27 dB.     

A balanced dual‐band bandpass filter with a wide stopband and controllable differential‐mode frequencies and fractional bandwidths

In this article, a balanced microstrip dual‐band bandpass filter (BPF) is designed. The proposed filter is achieved by employing a microstrip U‐shape half‐wavelength resonator, a folded stub‐loaded resonator and balanced microstrip/slotline transition structures. The center frequencies and the fractional bandwidths of the two differential‐mode (DM) passbands can be controlled independently by changing the physical lengths of the two resonators and the gaps between each resonator, respectively. The balanced microstrip/slotline transition structures can achieve a wideband common‐mode (CM) suppression. Meanwhile, the DM passbands are independent from the CM responses, which significantly simplify the design procedure. In addition, a wide DM stopband is also realized. In order to validate the design strategies, a balanced dual‐band BPF centered at 2.57 and 3.41 GHz was fabricated and a good agreement between the simulated and measured results is observed.     

Dual‐band balanced coupler with wideband common‐mode suppression

A novel compact dual‐band balanced coupler with differential‐mode power division, broadband common‐mode, and common‐to‐differential‐mode conversion suppression is proposed. In these double‐functionality balanced‐coupler architectures, double‐sided parallel‐strip line 180° phase inverters are used to realize the broadband common‐mode rejection. Moreover, the frequency is tunable by changing the characteristic impedance of the transmission line. For practical verification, a balanced couplers (ε_r = 2.65, h = 0.5 mm, tan(δ_D) = 0.003) operation at 0.9/1.8 GHz is constructed in microstrip technology and tested.     

A 1‐to‐n way single‐ended‐to‐balanced substrate integrated waveguide filtering power splitter

An approach to 1‐to‐n (n = 3, 4…) way single‐ended‐to‐balanced filtering power splitter (SETBFPS) is proposed. The properly placed balanced ports with 0.5λ_g (λ_g is the substrate integrated waveguide [SIW] guided wavelength at f₀) space make the TE32nd 103 and TE32nd 105 modes of n 32nd‐mode SIW multimode resonators form differential‐mode (DM) passband of the SETBFPS. Compared with the state‐of‐art single‐ended‐to‐balanced power splitters, the proposed approach has all the functions of 1‐to‐n way, filtering, and common‐mode (CM) suppression. A 1‐to‐3 way prototype is exemplified at 3.5 GHz with the minimum insertion loss (IL) of 0.09 dB, a fractional bandwidth (FBW) for a 15‐dB return loss of 35%, and a FBW for 15‐dB CM suppression of 52%. Low IL and wide bandwidth can be observed.     

A balanced dual‐band BPF with independently controllable frequencies and bandwidths

A balanced second‐order dual‐band bandpass filter (BPF) with independently controllable center frequencies and bandwidths based on coupled stepped‐impedance resonators (SIRs) is designed in this article. To obtain a dual‐band differential‐mode (DM) response, two pairs of SIRs with different resonant frequencies are employed in the design. The bandwidths of the two DM passbands can be independently tuned by adjusting the coupling gaps and coupling lengths of the corresponding resonators. In addition, three transmission zeros are realized to enhance the selectivity of the DM passbands. The microstrip‐slotline transition structure is utilized to achieve a wideband common‐mode (CM) suppression. Moreover, the DM responses are independent of the CM ones, which significantly simplify the design procedure. Finally, a balanced dual‐band BPF is designed to validate the design method and a good agreement between the simulated and measured results is observed.     

A balanced tri‐band bandpass filter with high selectivity and controllable bandwidths

In this article, a balanced tri‐band bandpass filter (BPF) with high selectivity and controllable bandwidths is proposed. Two differential‐mode (DM) passbands are formed by applying stepped impedance resonators into the design. Uniform impedance resonators are introduced to realize the third DM passband. Moreover, frequencies and bandwidths of DM passbands can be independently controlled by the lengths of resonators and the gaps between them. In addition, good DM transmission can be realized while high common‐mode suppression is achieved intrinsically without affecting the DM parts, thereby simplifying the design procedure significantly. In order to validate the practicability, a balanced tri‐band BPF operating at 2.45 GHz, 3.5 GHz, and 4.45 GHz is fabricated and designed. A good agreement between the simulated and measured results is observed.     

A bandwidth‐agile balanced filtering power divider

In this article, a balanced filtering power divider (FPD) that allows for operational agility of the bandwidth (BW) is presented. The differential‐mode power dividing and high common‐mode (CM) suppression can be realized by microstrip‐to‐slotline transition. Two slotline open stubs with different lengths are added in shunt to the main slotline for the transition, which can not only introduce transmission poles for extending and controlling transition BW, but also generate two extra transmission zeros (TZs) near to the passband edges, featuring good filtering response. The two transmission poles can be independently tunable by loading varactors to the open ends of slotline stubs and two TZs will be changed accordingly so that the filtering passband BW is electrically tunable. To verify the theoretical prediction, a prototype of tunable balanced FPD is fabricated and measured. The measured results show that the 3‐dB fractional bandwidth (FBW) of the passband varies from 5.6% to 12.6%, meaning more than a double tuning range for the FBW, and the CM suppression is better than 40 dB across the frequency band of interest.     

Compact ultra‐wideband bandpass filter with configurable stopband based on diamond‐shape resonator

An ultra‐wideband compact bandpass filter (BPF) with configurable stopband by tuning transmission zeroes is proposed in this paper. The ultra‐wideband bandpass response is based on a diamond‐shape resonator consisting of a pair of broadside coupled diamond‐shape microstrip lines, within which a diamond shape defected ground structure (DGS) is etched in the middle. Flexible transmission zeros realized by open and short stubs can be easily adjusted to improve band selectivity and harmonic suppression. Measurement result shows that the dedicated device has a 3 dB fractional bandwidth of 148% (0.94‐6.36 GHz) with 20 dB rejection stopband from 6.87 to 9.7 GHz (77.5%) which agrees good with the simulate performance. The overall size of the proposed BPF is 0.27 λ_g × 0.23 λ_g.     

A novel compact wideband filter using three‐mode dual‐ring resonator

A novel compact wideband filter using three‐mode dual‐ring resonator is presented in this article. The resonator is constructed by two quarter wavelength transmission lines and a cascaded half wavelength dual‐ring. Formulae based on even‐ and odd‐mode analysis are derived to analyze the locations of the transmission poles and zeros of the resonator. Due to the transmission zeros in the lower and upper stopbands, the proposed filter exhibits sharp attenuations near the passband as well as very wide stopband. The filter is successfully realized by full wave EM simulation and fabricated. The measured responses of the filter agree well with the design simulation, and show that the fabricated filter has an insertion loss of better than 1.5 dB in the passband and two rejections of greater than 20 dB in the stopbands from 0 to 10GHz.     

Multi‐mode resonators with quad‐/penta‐/sext‐mode resonant characteristics and their applications to bandpass filters

In this article, a new class of dual‐/tri‐band and ultra‐wideband (UWB) bandpass filters (BPFs) using novel multi‐mode resonators are proposed. The classical even‐/odd‐mode method is applied to analyze the resonant characteristics of the proposed resonators, which exhibit controllable resonant modes with different dimension parameters under the same configuration. According to the analysis, three resonators with quad‐/penta‐/sext‐mode resonant characteristics are obtained by choosing the specific dimension parameters. Then, the quad‐mode resonator is used to design a dual‐wideband BPF centred at 2.39/5.14 GHz with 3‐dB fractional bandwidths (FBWs) of 36.9%/18.9%, and the penta‐mode resonator is utilized to design an UWB BPF with 3‐dB FBW of 102.2%, whereas the sext‐mode resonator is applied to design a tri‐band BPF with centre frequencies of 2.09/3.52/5.46 GHz and 3‐dB FBWs of 11.3%/20%/12.1%. All these three filters are fabricated and measured, and the measured results are in good agreement with the simulated ones.     

Dual‐band circularly polarized antenna with harmonic suppression performance

A dual‐band circularly polarized (CP) antenna with harmonic rejection property is proposed in this paper. Four T‐shaped slits and two corner cuts are etched on the proposed microstrip patch antenna. Those structures can be used to tune the resonant frequencies of TM₀₁ mode and TM₀₃ mode of the antenna into the desired bands of 2.45 and 5.8 GHz with CP radiation. A shunt transmission line is employed not only to improve the impedance matching at 5.8 GHz but also to suppress the radiation at 4.9 GHz (second harmonic of 2.45 GHz). Meanwhile, two L‐shaped slits are etched on the feeding line to realize the harmonic rejection at 11.6 GHz (second harmonic of 5.8 GHz). The simulated and measured results show that this antenna has good dual‐band CP radiation property and harmonic suppression performance, which makes it a good candidate for the wireless energy harvesting system.     

Design of a wideband filtering power divider with good in‐band and out‐of‐band isolations

In this article, a wideband filtering power divider with good in‐band and out‐of‐band isolations is designed based on a hybrid Wilkinson and Gysel structure. To achieve a good in‐band response, two additional in‐band transmission poles can be introduced by installing the coupled‐line structures at each port. By mounting a stepped impedance open stub at the input port, two transmission zeros are generated near the passband to improve the passband skirt. Furthermore, the out‐of‐band rejection and isolation are achieved by the other two transmission zeros, which are produced by the open stub and the three coupled‐line sections mentioned above. Additionally, a good in‐band isolation is realized by the isolation resistor between output ports. For the demonstration, a wideband filtering power divider centered at 1.5 GHz with a 56% fractional bandwidth and 20‐dB isolation is designed and fabricated. The simulated and measured results are in good agreement with each other.