Balun Bandpass Filter Research Articles

Dual‐band and triple‐band balun bandpass filters using open‐circuited stubs loaded on ring resonators

Dual‐band and triple‐band balun bandpass filters using open‐circuited stubs loaded on ring resonators

A dual‐band filtering structure for highly selective reconfigurable bandpass filter and filtering balun

This article proposes a filtering structure consisting of two half-wavelength resonators and two open-stub loaded resonators, which generates two third-order passbands. Multiple transmission zeros are introduced by the newly developed coupling scheme, resulting in extremely sharp roll-off desirable for highly selective filters. The proposed structure is applied to design a PIN-diodes switch-controlled reconfigurable dual-band bandpass filter (BPF) with four-state filtering responses: both passbands ON, both passbands OFF, high-frequency passband ON, and low-frequency passband ON. Stepped-impedance open stubs and one-end-grounded coupled lines are studied and employed in the design to suppress unwanted responses. In addition, two filtering structures are placed symmetrically to design a dual-band balun BPF. Double-sided parallel-strip lines are added in the input port of the filter, and their inherent out-of-phase feature enables the balun filter to convert unbalanced signal to balanced signal at desired frequencies. The measured results of both fabricated devices agree well with the simulated results. The switchable BPF and balun BPF have two passbands (2.8 and 4.8 GHz) with high selectivity with the minimum roll-offs from −3 to −15 dB are 1.70 × 103 and 2.71 × 103 dB/decade, respectively. The insertion loss of the switchable BPF is 2.3/2.0 dB and the minimum signal suppressions level OFF state is measured as −18.2 dB. The insertion loss of the balun BPF is 1.8/1.4 dB, and the phase and magnitude imbalances are less than 0.3 dB and 5°, respectively. The excellent performances of the implemented filter and filtering balun successfully demonstrate the advantages and design efficacy of the proposed filtering structure for different applications.

Multilayered Input-Reflectionless Quasi-Elliptic-Type Wideband Bandpass Filtering Devices on Diplexer-Based Structures

Classes of input-reflectionless wideband bandpass filters (BPFs) and balun BPF with quasi-elliptic-type (QET) responses are reported. They consist of two signal-transmission parts in multilayered diplexer-based topologies, as follows: 1) a BPF channel shaped by a two-/three-port reflective-type BPF/balun on a microstrip-to-microstrip vertical transition and 2) an input-absorptive bandstop filter (BSF) channel built with a shunt resistively terminated lowpass filter (LPF) that is composed of hybrid microstrip sections and open-circuit-ended microstrip stubs. Two real-frequency out-of-band transmission zeros (TZs) are generated in these RF filtering devices using a cascaded short-circuit-ended two-section microstrip line and two shunt open-circuit-ended half-wavelength microstrip lines, respectively. Using a higher order LPF network, enhanced passband amplitude flatness and improved stopband power absorption ratio levels for the devised BPFs are attained. As design examples, two third-order BPFs with a shunt resistively terminated microstrip T-junction and a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula> -shape structure, respectively, are first provided. Afterward, a higher order BPF based on two cascaded replicas of a third-order BPF unit is designed to show highly increased stopband power attenuation levels and enhanced power absorption ratio profile within the stopband-to-passband transitions. Subsequently, their application to an input-absorptive QET fourth-order wideband balun BPF is presented. The operational principles of these BPFs and balun are detailed by the developed design procedures, in which their associated impedance-type design parameters are synthetically determined. As practical validation, four microstrip prototypes corresponding to three 2-GHz wideband BPFs and one 1-GHz broadband balun BPF are manufactured and tested. These input-reflectionless wideband filtering components experimentally feature the desired merits in terms of QET responses, enhanced passband amplitude flatness, and improved stopband power absorption ratio levels.

Compact wideband balun bandpass filter based on half‐mode corrugated slot

A design method of compact wideband balun bandpass filter (BPF) with high selectivity based on corrugated slot is proposed here. To realize wideband balun response in compact size, metal strips are deployed parallel to the half-mode corrugated slot (HMCS). Besides, the coupling of corrugated slot to the metal stripline provides the conversion of slot to microstrip structure at the balanced ports. By placing a pair of resonators at the balanced ports, a wideband balun BPF response with high selectivity can be realized. In addition, a U-shaped resonator is also loaded at the end of the corrugated slot to suppress harmonics. Afterwards, a compact wideband balun BPF centred at 2.42 GHz with 3-dB FBW of 45% is designed, fabricated, and measured. The effective size is 0.13λg × 0.39λg (λg is the guided wavelength of the centre frequency) and the measured results indicate that the magnitude imbalance is less than 0.45 dB and the phase imbalance is less than 4.8° within the operation frequency band.

Compact Dual-Mode Balun Bandpass Filter with Improved Out-of-Band Performance

A compact dual-mode balun bandpass filter with improved out-of-band performance is proposed based on a ring resonator. In order to excite the two degenerate modes and obtain filtering balun performance, three parallel-coupled lines are placed at three excitation ports along the ring resonator. By properly arranging the angles between the excitation arms, four transmission zeros can be realized to improve stopband performance. Finally, a prototype of the designed balun bandpass filter is implemented and fabricated. The measured results show a good agreement with those obtained from the simulation, thus verifying our proposal with good in-band filtering and out-of-band performance.

A Balun Bandpass Filter to Facilitate the Design of Dual-Polarized Dipole Antenna

A dipole antenna based on a balun bandpass filter (BPF) is developed in this paper. The balun BPF employs two U-shaped resonators settled on the left side of the open-circuited transmission line and two L-shaped stubs to produce signals with equal amplitude and inverse phase. In this way, the volume of the balun BPF is reduced by half, and the distance between two output ports is dramatically decreased. Then, the balun BPF is integrated with a dipole. Instead of the traditional Γ-shaped line with a wide balun ground, two thin microstrip lines with width of 1 mm are adopted to connect the dipole and the balun BPF. The antenna bandwidth is further extended due to the fusion of the resonance of the dipole and balun BPF. As a result, the proposed antenna can operate from 4350 to 5025 MHz (covering the n79 band of 5G NR, 4400 MHz–5000 MHz), yielding a good filtering performance in the stopband. The measured half-power beamwidth is ranging from 61° to 63° and the measured gain is ranging from 7.95 to 8.5 dBi in the passband. This new balun BPF and the dual-polarized dipole based on it have great potential to be applied in 5G MIMO systems.

A Dual-Band Balun BPF Using Codirectional Split Ring Resonators

In this letter, a novel dual-band microstrip balun bandpass filter (BPF) is designed by using interdigital capacitor-loaded codirectional split ring resonators (CDSRRs). The utilized resonator is analyzed by coupled line theory and exhibits independently controllable dual resonance behavior. The proposed balun BPF is constructed by coupling two identical CDSRRs to rotationally symmetrical balanced output ports. Two transmission zeros (TZs) between the passbands can be observed by means of the CDSRRs. The designed balun BPF centered at 1.59 and 1.92 GHz was also fabricated and measured. The measured results show a good agreement with the predicted results. Phase and magnitude imbalances between the balanced output ports have been measured as better than 180 ± 5° and 0.4 dB, respectively. The proposed dual-band balun BPF exhibits high filtering performances in terms of closely spaced passbands, selectivity, and clean upper stopband.

Design of a new balun bandpass filter with single‐band balance and dual‐band filtering characteristics

AbstractIn this article, a new type of balun bandpass filter with single‐band balance and dual‐band filtering function is presented. For this purpose, a novel compact dual‐mode open loop resonator with short‐circuited stubs and interdigital capacitors is proposed. Two identical resonators are coupled to two output ports in order to obtain 180° phase difference between the output ports. By means of the changes in the short‐circuited stubs and interdigital capacitors, control of the passbands at two output ports can be achieved. In addition, as the short‐circuited stubs are located at the same wave traveling paths, phase difference cannot be obtained in the first passband and only the second passband has balun function. The designed balun bandpass filter was fabricated and tested for the experimental verifications. Phase difference and magnitude imbalance have been measured within 180° ± 2° and 0.5 dB, respectively. The measured results exhibit a very good agreement with the predicted results.

Analysis and Design of a New Self-Packaged Wideband Balun Bandpass Filter With the Functionality of Impedance Transformation

In this paper, a new self-packaged wideband balun with the high selective bandpass filtering response and impedance transformation characteristic is presented. By virtue of circuit transformations, corresponding two-port equivalent circuit with asymmetrical port impedances is put forward. The analysis results of this two-port network exhibit that the proposed new design is able to achieve a quasi-elliptic equal-ripple response with three transmission poles and four transmission zeros. In addition, the frequency response can be directly determined according to the desired design specifications of impedance transformation ratio k, ripple fractional bandwidth (FBW), and in-band return loss RL. For validation purposes, a demonstrated wideband balun bandpass filter (BPF) working at 2.0 GHz with 50–100- $\Omega $ impedance transformation has been implemented by utilizing the multilayer liquid-crystal polymer (LCP) bonded printed circuit board (PCB) lamination technology. Theoretical, simulated, and measured results are recorded in good agreement, well verifying the design method.

Vertically‐integrated differential filtering patch antenna excited by a balun bandpass filter

This study develops a differential-fed filtering patch antenna, which features superior frequency selectivity, compact size, low cross-polarisation, and symmetrical radiation patterns. The proposed antenna consists of a rectangular patch on the top layer, which is differentially fed by a balun bandpass filter on the bottom layer. A pair of rectangular coupling slots were etched on the middle layer for energy coupling. The patch antenna performs not only as the radiator but also as the last resonator of the whole filter circuit for compactness. A prototype of the proposed filtering antenna operating at 2.45 GHz was fabricated and tested. The measured in-band peak gain is 5.9 dBi with steep roll-off at band edges, and the out-of-band gain is less than - 25 dBi. Moreover, symmetrical radiation patterns and low cross-polarisation levels of - 22 dB are also realised for the proposed filtering antenna.

Wideband Balanced-to-Unbalanced Bandpass Filters Synthetically Designed With Chebyshev Filtering Response

In this paper, two wideband balanced-to-unbalanced (balun) bandpass filters (BPFs) are synthetically designed with three- and four-pole filtering responses, respectively. Based on the simplified even- and odd-mode equivalent circuits of the proposed three-pole balun BPF, the exact impedance value of the cascaded microstrip sections connected with two balanced ports is first determined in order to attain a perfectly matched S-matrix. By the virtue of the modified two-port transmission-line network with asymmetrical port impedances, a quasi-Chebyshev filtering response for this three-pole balun filter is theoretically presented. Next, to improve the in-band performance, especially at the high frequency, a four-pole balun BPF designed with a folded slotline resonator is proposed. Via the virtually short-ended microstrip stubs of the modified two-port equivalent network, the four-pole frequency response can be directly analyzed with the Chebyshev polynomial function. For these two proposed wideband balun filters, their theoretical and simulated results are well confirmed by the circuit model responses with the same impedance values of the simulated microstrip and slotline resonators and the extracted turns ratios of the employed transformers. Finally, two prototype wideband balun BPFs are fabricated and measured to verify our design predictions.

Balun bandpass filter on slot-line resonators with intrinsic balanced performance in amplitude and phase

AbstractIn this paper, a novel class of balun bandpass filters is presented on slot-line resonator (SLR) toward intrinsic balanced performance in amplitude and phase. The proposed balun filter consists of three inductive-coupled SLRs in the middle, one single-ended unbalanced port, and one balanced port with two terminals. According to the distinct field conversion from the slotline to the microstrip feeding line, good balanced performance in amplitude, and phase performance can be intrinsically realized over a wide frequency range. To validate the proposed technique, a prototype third-order balun filter is designed, fabricated, and measured. Both the simulated and measured results have demonstrated that the proposed balun filter can not only achieve good frequency selectivity but also exhibit intrinsic balanced performance in amplitude and phase.

A Wide Stopband Balun Bandpass Filter with Its Application to Balanced Quasi‐Yagi Antenna

A new microstrip balun bandpass filter (BPF) with wide stopband is presented. It is realized by utilizing the out‐of‐phase feature of standing‐wave distribution on a half‐wavelength (λ/2) open‐ended microstrip line and the resonant characteristics of the triple‐mode resonators. For demonstration, a prototype balun BPF operating at central frequency (f0) of 2.34 GHz with fractional bandwidth (FBW) of about 16.7% is designed. The presented balun BPF not only exhibits nice balanced response with 0.47 dB magnitude imbalance and 4.3° phase difference but also achieves 30‐dB harmonic suppression up to 6.40 GHz (2.7f0). Afterward, this presented design is embedded into a balanced quasi‐Yagi antenna to obtain high‐selective radiation and wide stopband rejection. Experimental results of the implemented antenna indicate less than −10 dB reflection coefficient, 4.49–5.38 dBi gain, and 12.71–19.62 dB front‐to‐back ratio within the operating bandwidth.

Design of a Dual-Mode Balun Bandpass Filter With High Selectivity

In this letter, a new dual-mode balun bandpass filter (BPF) with high selectivity is proposed. The balun BPF is mainly composed of two different types of microstrip-to-slotline transition structures, two identical dual-mode stub-loaded resonators, and a pair of balanced outputs. First, to convert unbalanced signals from port 1 into balanced signals between port 2 and port 3, two different types of microstrip-to-slotline transition structures are utilized. Afterward, two identical stub-loaded resonators are embedded in the circuit for realizing the dual-mode bandpass response. Furthermore, four transmission zeros generated outside the passband are wisely introduced and well elucidated, thus ensuring high selectivity performance. For demonstration, a sample of balun BPF centered at 2.78 GHz ( $f_{0})$ with 0.5-dB magnitude imbalance and 5° phase imbalance is designed, fabricated, and measured. Both simulation and measurement are in good agreements.

Wideband balun bandpass filter with broadside‐coupled microstrip/slotline resonator structure

In this Letter, a new design scheme for wideband balun bandpass filter (BPF) is presented originating from the conventional Marchand-type balun prototype. The designed balun BPF is basically composed of three microstrip lines on the top side of the substrate and two half-wavelength slotline resonators etched in the ground plane. A distinctive wideband performance is attained by utilising the broadside coupling between these microstrip lines and slotline resonators. For demonstration, one fabricated prototype balun BPF operating at 3.75 GHz shows a broad bandwidth of 93% with nice in-band balanced performance. Final results from simulation and experiment are both recorded in good agreement.

A Wideband Balun Filter on a Triple-Mode Slotline Resonator With Controllable Bandwidth

In this letter, a wideband balun bandpass filter (BPF) composed of a triple-mode slotline resonator and a couple of microstrip–slotline cross-junction is presented. The proposed triple-mode resonator consists of a half-wavelength ( $\lambda $ /2) uniform impedance resonator (UIR) and two different short-circuited slotline stubs installed in middle of slotline UIR. The first three modes of the cross-shaped slotline resonator are then excited and utilized to achieve the balanced-to-unbalanced passband. By tuning the short-circuited stub length, its controllable bandwidth can be realized. Owing to the intrinsic 180° phase difference at two output ports of the microstrip–slotline cross-junction, a wideband passband is achieved with improved magnitude balance and out-of-phase properties. To validate our design concept, a wideband balun BPF operating at 4 GHz with 0.6-dB magnitude imbalance and 8° phase imbalance is designed and fabricated. Both simulated and measured results are found in good agreement.

A 0.4–3-GHz nested bandpass filter and a 1.1–1.7-GHz balun bandpass filter using tunable band-switching technique

This paper presents a second-order tunable single-ended (unbalanced) bandpass filter (BPF) with continuous 0.4–3-GHz coverage and a tunable balun BPF with continuous 1.1–1.7-GHz coverage for software-defined radio transceivers with the use of band-switchable and radio frequency (RF)-micro-electromechanical systems (MEMS)-tuned resonators. The BPFs are realized with two pairs of RF switches for coarse-tuning and RF-MEMS-tunable capacitors for fine-tuning. On the one hand, for the tunable single-ended BPF, a transition between three bands is enabled using two pairs of RF switches. On the other hand, for the tunable balun BPF, a transition between two bands is enabled using one pair of RF switches. Furthermore, the three-band switchable single-ended BPF is constructed in a nested two-filter bank structure for expanding the tuning range without increasing the footprint. In addition, to complement the discrete band gaps, RF-MEMS capacitor-tuned resonators are used, and a continuous tuning range of nearly the entire ultra-high-frequency band is achieved. The filter bank is fabricated on a Duroid substrate with εr = 3.5 and h = 0.787 mm. The filter bank has an insertion loss of 3.2–6.8 dB and a 1-dB bandwidth of 65–450 MHz with a continuous tuning range of 0.4–3 GHz.

Design of Balanced and Balun Filters Using Dual-Mode Cross-Shaped Dielectric Resonators

In this paper, a dual-mode cross-shaped dielectric resonator (DR) that consists of two identical rectangular DRs is investigated to design balanced and balun bandpass filters (BPFs) for the first time. According to the dominant TE $_{11\delta }$ -mode electromagnetic field of a single rectangular DR, the driven signals for the DR through two probes on the two sides of the DR should be antiphase. Equivalently, the extracted signals from the DR operating at TE $_{11\delta }$ mode are positively out-of-phase. Based on this, a four-port balanced TE $_{11\delta }$ -mode BPF using a single dual-mode DR cavity can be constructed. Furthermore, by converting the balanced BPF into a three-port device, a novel balun BPF using the same DR cavity is designed, showcasing excellent performance of amplitude balance and 180° phase difference. To validate the proposed idea, the balanced and balun BPFs are designed and implemented, respectively. Both of them have a miniaturized circuit size due to the adoption of the dual-mode DR. Good agreement can be obtained between their respective simulated and measured results.

Design of a Balun Bandpass Filter with Asymmetrical Coupled Microstrip Lines

AbstractA new microstrip coupled-line balun topology and its application to the balun bandpass filter (BPF) with a triple mode response are proposed in this paper. The involved balun structure is composed of two back-to-back quarter-wavelength (

Study of a New Planar-Type Balun Topology for Application in the Design of Balun Bandpass Filters

A comprehensive study of a new planar-type balun prototype is presented in this paper for application in the design of balun bandpass filters (BPFs) with widely used open-type resonators. The proposed balun is in general composed of two identical back-to-back quarter-wavelength ( $\lambda $ /4) coupled line sections. The design concept originates from the analysis of the standing wave pattern on a half-wavelength ( $\lambda $ /2) open-circuited transmission line. After an intuitive illustration is given, its working principle is presented to validate the feasibility of the proposed balun. Afterward, two types of the coupled line prototypes are analyzed to implement the proposed balun with filtering response, whereas a basic rule is given to realize the transmission zeros. To further demonstrate the validity of the proposed balun prototype, two practical balun BPFs with common dual-mode resonators are designed and fabricated. As expected, transmission zeros are generated near the desired passband of our presented balun filters, thereby improving their frequency selectivity. Both simulated and measured results exhibit good filtering and balun performance.