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.     

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 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.     

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.     

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 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.     

Filtering balanced‐to‐single‐ended power dividing network

In this article, the filtering balanced‐to‐single‐ended power dividing networks are proposed. Except the fundamental functions of differential‐mode transmission, common‐mode suppression, and out‐of‐phase single‐ended output ports with isolation, the proposed designs show the advantages of wide controllable range of differential‐mode bandwidth, multiple transmission zeros (TZs), and wide bandwidth for high out‐of‐band suppression. The frequencies of TZs, bandwidth, isolation, and common‐mode suppression can be controlled by the parameters. For demonstration, three prototypes (Deigns I, II, and III) with two, four, or six TZs are implemented. The measured results show that design I (II and III) has an insertion loss of 0.38 dB (0.7 dB and 0.8 dB), an operating bandwidth of 12.5% (7.5% and 6.9%), and a bandwidth for 30‐dB out‐of‐band suppression of 0.06f₀ (0.09f₀ and 0.14f₀). The isolation and common‐mode suppression inside the passbands of the three prototypes are all larger than 17 and 38 dB, respectively.     

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 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 novel differential filtering SIW power divider with high selectivity

A novel differential power divider with bandpass filtering response using the substrate integrated waveguide (SIW) technology is presented. An SIW resonant cavity operated in a balanced resonant mode with odd symmetric electric field distribution is utilized to provide both balanced inputs/outputs and expected common‐mode (CM) suppression in a certain band. Meanwhile, by properly constructing the cross‐coupled topology of SIW resonant cavities, the proposed differential power divider achieves a high‐selectivity bandpass filtering response with two transmission zeros on both sides of the passband. The differential power divider is designed and prototyped on a single‐layer printed circuit board (PCB). The measured center frequency is at 10.6 GHz with 490 MHz 3‐dB bandwidth. A good CM suppression can also be achieved within the operating band. The measured in‐band differential‐mode imbalance for magnitude is ±0.3 dB, while for phase is 0°–4°. © 2015 Wiley Periodicals, Inc. Int J RF and Microwave CAE 26:182–188, 2016.     

A BTB UWB power divider with arbitrary power‐dividing ratio based on three‐slotline structure

In this article, a balanced‐to‐balanced (BTB) ultra‐wide band (UWB) power divider (PD) is proposed, which can realize arbitrary power‐dividing ratio (PDR) with improved transmission bandwidth flatness. The proposed PD is primarily based on microstrip/slotline (MS) transition structures and parallel‐coupled three‐slotline structure. U‐type microstrip feed lines integrated with stepped‐impedance slotline resonators are adopted at the input and output ports, which make the differential‐mode (DM) responses independent of the common‐mode (CM) ones. Meanwhile, superior DM transmission and CM suppression are achieved intrinsically, thereby simplifying the design procedure significantly. By changing the distances between the coupled three slotlines, the PDR between the output ports is controllable. In order to verify the feasibility of the proposed design method, several prototype circuits of the proposed PDs with different PDRs are simulated and a prototype circuit with the 2:1 PDR is fabricated and measured. A good agreement between the simulation and measurement results is observed.     

High selectivity balanced filters with multiple transmission zeros using ring resonators with loaded stubs

In this article, two high selectivity balanced filters with multiple transmission zeros using ring resonators are proposed. Four, and six transmission zeros are easily realized to improve the differential mode passband selectivity and common mode suppression. Over five octaves common mode suppression can be realized by the bandstop performance of the coupled lines and stubs. Two balanced filters with 3‐dB bandwidths of 23.6 and 23.5% are designed and measured. Good agreement can be found between the theoretical and measured results.     

Double‐layer microstrip ultra‐wideband filtering power divider with high selectivity and large isolation

In this article, a compact double‐layer microstrip ultra‐wideband (UWB) filtering power divider with high selectivity and isolation is proposed. The filtering power divider consists of a multimode resonator at the top layer coupled with a pair of branch lines at the bottom through a slotline in the middle ground. The slotline provides strong coupling between the two layers and equally distributes the power to two branch lines. The resistor loaded about a quarter‐wavelength away from the slotline achieves high isolation within UWB range. The UWB filtering properties with controllable transmission poles and zeros as well as power splitting with enhanced isolation have been analyzed. The adjustable transmission zeros of the filter unit enables the bandwidth control of the filtering power divider. Finally, a UWB filtering power divider operating at 3.1 to 10.6 GHz has been designed, fabricated, and measured. It achieves a compact size of only 26 × 28 mm², high isolation about 20 dB, and good out‐of‐band suppression of 40 dB.     

Dual‐wideband filtering power divider based on center‐fed three‐line coupled structure

In this article, a dual‐wideband filtering power divider is proposed by using a center‐fed three‐line coupled structure with three open stubs and two isolation resistors. The center‐fed three‐line coupled structure can generate two wide passbands separated by a transmission zero (TZ). The three open stubs can achieve four TZs around the two passbands, which is conducive to the frequency selectivity. Compared with the reported designs, the bandwidth is extended and the performance of isolation, insertion loss and circuit size can reach balance. The proposed design is implemented with size of 0.22 λ_g × 0.39 λ_g (λ_g is the guided wavelength at the center frequency of the lower passband) which exhibits the 3‐dB fractional bandwidths of 56.5%/24.27% and the insertion loss of 0.51/0.68 dB at the center frequency of two passband (f₁/ f₂) of 1.94/4.2 GHz, while the isolation at f₁/f₂ are higher than 22.5/20.1 dB.     

Dual‐wideband differential bandpass filter with intrinsic common‐mode suppression based on hybrid transmission structure

Based on the microstrip‐line/slot‐line hybrid transmission structure, a novel method of realizing dual‐wideband differential bandpass filter (DDBPF) is proposed and demonstrated in this paper. By virtue of hybrid transmission structure, the proposed DDBPF has intrinsic common mode (CM) rejection. Moreover, the dual‐wideband transmission performance of differential mode (DM) signal can be approached by using two dual‐mode stub‐loaded stepped‐impedance resonators. The two passbands of the proposed DDBPF are respectively centered at 2.76 GHz and 4.12 GHz which fractional bandwidths are 20.7% and 11.4%. The DM harmonic suppression can extend to 43.5 GHz with rejection level of 13 dB. The CM attenuation of DDBPF can reach 60 dB within DM passband. Furthermore, the CM suppression with attenuation level of 20 dB is ranging from 0.5 GHz to 43.5 GHz. The measurement results are in good agreement with the simulation results, which validates the effectiveness of proposed design methodology.     

Compact lowpass filter with sharp roll-off and wide stopband using dual-stepped impedance resonator

A lowpass filter unit with sharp roll-off and wide stopband using dual-stepped impedance resonator （DSIR） is presented. The lowpass filter unit is composed of DSIR, open circuited stubs, and high-impedance transmission lines. DSIR mainly dominates the passband and stopband width while open stubs are used to enhance the suppression level with one transmission zero. Three transmission zeros are introduced to stophand and one closed to passband edge. Thus, a very sharp transition can be achieved. A compact lowpass filter with sharp roll-off rate of 201 dB/octave, wide stopband of 5.9fc, and high suppression level of 30 dB was obtained when two asymmetric units were cascaded.     

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.     

A self‐packaged wideband filtering power divider based on substrate‐integrated suspended line

In this study, a filtering power divider (FPD) is proposed by utilizing one T‐shaped tri‐mode stepped‐impedance resonator with input/output coupling structures based on substrate‐integrated suspended line (SISL). The circuit topology and SISL technology are combined together to reach balance in performances such as compact size, wideband, high frequency selectivity, low loss, good in‐band isolation, wide stopband, and self‐packaging so that there are no obvious flaws. Wide bandwidth and two near‐band transmission zeros are contributed by the proposed circuit topology. Good isolation can be obtained by comparing different coupling schemes with one resistor. An additional transmission zero for extending the upper stopband can be achieved by the two closely placed stubs without increasing the size of the design. Low loss and self‐packaging can be realized by SISL technology. For demonstration, a prototype is implemented with the size of 0.5λ_g × 0.28λ_g, which exhibits the 1‐dB fractional bandwidth of 26.3%, the frequency selectivity of 0.25/0.37 at the lower/upper edges of the passband, and the insertion loss of 1.1 dB (including transition) at the center frequency (f₀) of 3.34 GHz, while the in‐band isolation is higher than 20 dB and the 15‐dB stopband is achieved up to 3.74 f₀.     

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.     

Widely separated dual‐band half‐mode SIW bandpass filter

A novel half‐mode substrate integrated waveguide (HMSIW) based dual‐band bandpass filter (DBBPF) is proposed. Back to back connected two defected ground structure (DGS) resonators on the top layer of HMSIW cavity constitute the passband with two transmission zeros (TZs) at a lower frequency. The higher modes TE₃₀₁ and TE₃₀₂ of HMSIW cavity give the passband response at higher frequency using the mode shifting technique with slot perturbation. The source‐load coupling has been used to create finite frequency TZs to improve the selectivity of the second passband. Therefore, the proposed filter gives two widely separated passbands, center frequencies (CFs) at 5.83 and 18.1 GHz with an attenuation of greater than 10 dB between the passbands. The synthesized filter is fabricated using a low‐cost single layer PCB process, and the measured S‐parameters are almost mimic the EM‐simulation results.