Coupled‐line band‐pass filter with T‐shaped structure for high frequency selectivity and stopband rejection

A coupled‐line band‐pass filter (BPF) with T‐shaped stub structure is presented. Five transmission poles within the passband and eight deep transmission zeros (TZs) from 0 to 2f₀ (f₀ denotes filter's center frequency) are realized through input impedance calculations. With the simple T‐shaped structure, the positions of six TZs can be appropriately adjusted to achieve high frequency selectivity and stopband rejection. For demonstration, a BPF prototype centered at 2.05 GHz is designed and fabricated, whose measured rejection levels are of over 45.5 dB at lower stopband and better than 19.5 dB at upper stopband. The simulation and measurement results are in good agreement, which validates the design idea.     

Compact ultra‐wideband bandpass filter with variable notch characteristics based on transversal signal‐interaction concepts

A novel technique is presented to design highly compact microstrip ultra‐wideband (UWB) bandpass filters that exhibit high selectivity quasi‐elliptical response. The design is based on transversal signal‐interaction concepts that enable the inclusion of single or dual notch‐bands within the filter's passband to eliminate interference from other services that coexist within the UWB spectrum. The filter configuration comprises of two transmission paths which include folded T‐shaped stepped impedance resonators (SIRs) that are capacitively coupled with the input/output lines to enable signal transmission. It is shown that by combining the filters of different passband centre frequencies an UWB filter can be realised with either a single‐ or dual‐notch function. The theoretical performance of the filter is corroborated via measurements to confirm that the proposed filter exhibits UWB passband of 123% for a 3 dB fractional bandwidth, a flat group‐delay with maximum variation of less than 0.3 ns, passband insertion loss less than 0.94 dB, high selectivity, a sharp rejection notch‐band with attenuation of ?23 dB, and a good overall out‐of‐band performance. Furthermore, the filter occupies a significantly small area of 94 mm² compared with its classical counterparts. © 2014 Wiley Periodicals, Inc. Int J RF and Microwave CAE 24:549–559, 2014.     

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.     

Dual‐band filter with high selectivity and wide stopband rejection

This article presents a highly selective dual‐passband filter based on stepped‐impedance‐resonator (SIR) and mixed electromagnetic coupling. First, the surface area of the filter is effectively reduced by the triangular topology. Second, four controllable transmission zeros are introduced by source‐load coupling feed and mixed electromagnetic, which increases the selectivity of the filter. Third, a perturbation structure is added to independently control the resonance points of each passband. Finally, the improved defect ground structure (DGS) is integrated to obtain wide stopband rejection. The measured S‐parameters are well agreement with the simulated results, which show that the center frequencies of the two passbands are 2.4 GHz and 5.2 GHz; and the passband insertion losses are 0.85 dB and 1.6 dB; and the relative bandwidths are 14.6% and 5.7%, respectively. Besides, the structure is with six transmission zeros, and 20 dB suppression for the third harmonic and the fourth harmonic are achieved. Compared with the traditional SIR double‐passband filter, this filter has many advantages, such as simple design, small size, small insertion loss, controllable frequency, high selectivity, and high spurious suppression.     

A novel wideband bandpass filter based on CSRR‐loaded substrate integrated folded waveguide

A novel wideband bandpass filter based on folded substrate integrated waveguide (FSIW) is presented in the article. Five square complementary split‐ring resonators (CSRRs) are etched in the middle layer of the FSIW. By adjusting the physical size of the CSRR structure, the resonant frequency of the CSRRs can be tuned at the same time and the stopband performance can be changed. As transverse electromagnetic (TEM) mode can be transmitted in the stripline, FSIW excited by stripline shows wider passband than that excited by microstrip line directly. To achieve perfect impedance matching, two microstrip lines to stripline transitions are added in two ports of the filter. The proposed bandpass filter exhibits compact size, high selectivity, good stopband rejection, lower radiation loss, and wideband performances. The measured results show that the fractional bandwidth of the filter is about 35.5%. The measured return loss is better than 15 dB from 4.84 GHz to 6.90 GHz, and the insertion loss is less than 1.2 dB. The comparison between the simulated results and the measured ones validate the possibility of the technology that combines the FSIW and CSRR.     

A multi‐stub loaded compact UWB BPF with a broad notch band and extended stopband characteristics

A compact ultrawideband (UWB) bandpass filter (BPF) employing the principle of multiple mode resonance characteristics to create UWB passband with high selectivity and simultaneously having extensive stopband characteristic is presented. Utilizing five stubs attached along with the asymmetric trisection stepped impedance resonator (ATSSIR), the proposed resonator enables seven transmission poles inside the passband. As an additional attributes the projected filter triggers one transmission zero at 5.0 GHz which helps to mitigate WLAN signal interference. No additional circuitry is used to generate a notch band. The proposed prototype of UWB BPF is fabricated and estimated. Simulated and estimated results are in great understanding. The prospective filter displays a deliberate passband from 2.9 to 11.02 GHz. The filter unveils deceptive free wide upper stop band attributes till 25 GHz with least attenuation of 10 dB all through the stop band.     

Analysis and optimization of the unpredictable transmission zero in X‐band filter design

This article focuses on the common problem of uncontrollable transmission zero (TZ) in X‐band filter design. Using uniform impedance rectangular resonator (UIRR) to design an X‐band filter always results in an unpredictable TZ on the low‐frequency side of the passband, which greatly deteriorates the frequency selectivity of the filter performance. Electromagnetic coupling polarity analysis of the UIRR shows that the magnetic crosscoupling between nonadjacent resonators which is opposite to the main coupling plays a major role in the unpredictable TZ. By optimizing the UIRR from the loop structure with a small opening to the folded one, weak couplings between resonators are obtained. A high‐temperature superconducting filter at X‐band using folded UIRR resonator was designed and fabricated with a center frequency of 10.01 GHz and a bandwidth of 58 MHz, and the uncontrollable TZ has been removed successfully from the low‐frequency side of passband. The measured insertion loss is less than 0.4 dB and the return loss is greater than 15 dB.     

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.     

Capacitive Coupled Wide-Notch Stepped Impedance Narrow-Band Bandpass Filter for WiMax Application

The development of wireless communication standards necessitates optimal filter design for the selection of appropriate bands of frequencies. In this work, a compact in size pair of parallel coupled symmetric stepped impedance-based resonator is designed with supporting to the WiMAX communication standards. The coupled resonator is tuned to allow the frequency band between 3.4 GHz and 3.8 GHz, which is centered at 3.6 GHz. A parasitic effect of capacitively coupled feed structure is used for exciting the two symmetrical stepped impedance resonators. The bandwidth and selectivity of the filter are enhanced with the change of characteristic impedances and controlling the coupling gap between resonators. This design offers single narrow sharp passband selectivity as well as multiple stopband harmonic suppression arising as a result of multiple transmission zeros. The designed filter operates with a fractional bandwidth (FBW) of 11.47%. The proposed single narrowband bandpass filter provides better suppression in either side of the tuned frequency (3.6 GHz) without degrading the passband performance. Also, this novel filter offers an insertion loss of about −0.08 dB and a return loss of greater than −30 dB in passband. This approach is useful for eliminating unwanted spurious harmonics responses that enter the desired response. The suggested bandpass filter has been simulated using Advanced Design System (ADS) tool, and the measurement has been made using a network analyzer, and the results are reported.     

A tunable 0.86‐3.83 GHz bandpass filter with high selectivity

This letter presents a tunable bandpass filter (BPF) with wide tuning range of center frequency and high selectivity. The wide frequency tuning range is achieved by two pairs of switchable varactors‐tuned parallel coupled line resonators with direct‐feed structure, which can be switched to lower and higher frequency resonator modes by using p‐i‐n diodes. Since the electromagnetic mixed coupling and frequency‐variant source‐load coupling are incorporated in this configuration, three self‐adaptive transmission zeros (TZs) close to the tunable passband are obtained. Also, three TZs can almost keep the same relative location of passband to achieve continuous high selectivity and good out‐of‐band rejection over the whole frequency tuning range. Meanwhile, by selecting a proper coupling region, a constant fractional bandwidth (CFBW) in the frequency tuning process can be realized. For verification, a tunable 0.86‐3.83 GHz BPF with a 12% CFBW and high selectivity is designed, fabricated and measured. The experimental results show the proposed filter has the advantages of wide tuning range and high selectivity.     

Wideband reconfigurable bandpass filter using coupled lines loaded with varactor loaded stubs

In this article, a novel reconfigurable bandpass filter with tunable passband edge and bandwidth is proposed. The bandpass filter enables the two band edges independently adjustable to meet the needs of different systems. The wide tuning range of the bandwidth is achieved by controlling the tuning mechanism to altering not only the coupling strength of the coupled lines but also the transmission zeroes positions of the resonator. The lower passband edge can shifts from 760 to 840 MHz with the upper passband edge keeping still and the upper edge of the passbands from 981 to 1107 MHz with the lower edge keeping still. The overall tuning range of 3 dB fractional bandwidth is from 15.5% to 36%. The upper stopband attenuation of the fabricated structure can reach to 40 dB within a wide frequency range.     

Substrate integrated folded waveguide controllable mixed electric and magnetic coupling structure and its application to millimetre‐wave pseudo‐elliptic filters

A substrate integrated folded waveguide (SIFW) controllable mixed electric and magnetic coupling structure is proposed and investigated, which is suitable for millimeter‐wave pseudo‐elliptic filter applications. The proposed structure is built by a meandered slot‐line (MSL) combined with an inductive via‐hole in a SIFW cavity, which can support two closely‐spaced resonator modes and achieves a mixed coupling. By adjusting the physical sizes of the mixed coupling structure, the electric and magnetic coupling can be separately controlled and a controllable transmission zero (TZ) can be produced below/above the passband. Furthermore, detailed analysis of the SIFW cavity mode spectrum and the mixed coupling characteristics are presented. Finally, direct‐coupled and cross‐coupled fourth‐order pseudo‐elliptic filters are designed, fabricated and measured to verify the proposed mixed coupling structure. The experimental results show that the proposed filters have good performance, such as high frequency selectivity, compact size, wide upper stopband, and easy integration with other planar circuits.     

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.     

Microwave bandpass filter by feedback interference topology

This paper proposes a signal feedback interference topology to control the characteristics of a coupled line bandpass filter. Control is carried out by varying the ratio of the transmission line parameters of the forward and the feedback couplers. The topology produces a compact filter that does not require a complicated design or fabrication. The results of the insertion and return losses show that the feedback‐controlled design has desirable features in terms of the design frequency, fractional bandwidth, stopband rejection level, roll‐off rate and insertion loss. The test results of the fabricated prototype achieved good agreement with the simulation results of the ideal transmission line design model. Several microwave, modern wireless and mobile/satellite communication systems may benefit from such a filter design due to its compactness and flexibility.     

A folded dual‐mode bandpass filter with circular waveguide

In this article, a folded circular waveguide dual‐mode filter without tuning screws is designed for the fifth‐generation (5G) mobile communication system. The folded filter is composed of two stacked circular cavities operated at 3.5 GHz. Each cavity has two resonant modes, which can generate and control two transmission zeroes at specific frequencies. Through a coupling iris, the two single‐cavity filters are connected together, and can control four poles, which helps to expand the 3dB fractional bandwidth to 11.4%. The measured insertion losses are around 0.5 dB in the passband (from 3.4–3.6 GHz). The experiment results show an excellent agreement with the simulation results. Such folded filters have the advantages of very low insertion loss, compact size, high frequency selectivity, and low cost.     

Design of wideband comb shape substrate integrated waveguide multimode resonator bandpass filter with high selectivity and improved upper stopband performance

In this article, a wideband bandpass filter (BPF) is designed using the comb slotted substrate integrated waveguide (SIW) cavities. The comb‐shaped slots engraved on the SIW cavity are used to constitute a novel multiple‐mode resonator (MMR) that accomplishes a wide passband of operation. Further, a Jerusalem cross defected ground structure (DGS) is introduced to miniaturize it and enhance filter performance in the pass band and stop band. The filter is fabricated on RT/Duroid 5880 having dielectric constant 2.2 and tested to prove the validity of design. The filter achieves 3 dB fractional bandwidth of 48%, return loss above 14 dB and insertion loss of 1.1 dB in the passband. Also, the proposed filter has steep selectivity and wide upper stopband with 25 dB attenuation from 16.7 to 24 GHz.     

The design and test of dual/triple‐mode bandpass filter based on temporal coupled‐mode theory

In this article, the design and test of both a K‐band dual‐mode bandpass filter (BPF) pair and a K‐band triple‐mode BPF are presented based on N‐mode temporal coupled‐mode theory (CMT). The expressions of transmission and reflection responses are analytically derived. All the parameters in the expressions have clear physical meanings and are easily optimized to reach the required filter performance. Aided by eigenmode simulations, concrete structures of the three integrated BPFs are designed and optimized to approach the calculated physical parameters. After the fabrications and measurements of the three BPFs, extended upper/lower stopband with high stopband rejections are achieved, and by increasing the number of resonant modes, improved frequency selectivity and better passband flatness are obtained. The analytical analysis well predicts the simulation and measurement results, which provides an efficient way for BPF designs. © 2016 Wiley Periodicals, Inc. Int J RF and Microwave CAE 26:609–622, 2016.     

Computer‐aided design of microstrip extracted‐pole lossy filter

In this article, computer‐aided design of a new type of microstrip lossy filter is described. The new type of lossy filter is realized by introducing resistive cross couplings into a microstrip extracted‐pole filter to achieve a flat passband. The high selectivity is achieved by introducing 2 transmission zeros using 2 extracted poles, which can be adjusted. An equivalent circuit model is established and its circuit parameters, which are useful for physical implementation of the filter, are determined in light of computer modeling. For demonstration, two 6‐pole filters centered at 2 GHz with fractional bandwidth (FBW) 6% and 20% are implemented. Experimental results, together with a theoretical comparison between different FBWs, and full‐wave electromagnetic simulation results are also presented.     

Wideband and wide stopband superconducting bandpass filter using asymmetric stepped‐impedance resonators connected to open stub

A wideband wide stopband filter is designed using asymmetric stepped‐impedance resonators (ASIRs) connected to a large open stub. The capacitive open stub and the parallel‐coupled microstrip line are used to achieve the strong couplings for large fractional bandwidth (FBW). For a wide‐stopband performance, the proposed filter uses ASIRs to improve the high‐order spurious resonant frequency. The first and last resonators of the proposed filter are further optimized to suppress the spurious resonant frequency caused by open stub. The final filter has a 70% FBW centered at 4.87 GHz with 20‐dB‐rejection stopband up to 15.78 GHz (approximately 3.24 f₀). The measured insertion loss is less than 0.15 dB and the return loss is better than 17 dB.     

Design of high selectivity tri‐band bandpass filter with wide upper stopband

This article presents a dual‐plane structure high selectivity tri‐band bandpass filter (BPF) which consists of a pair of T‐shaped microstrip feed lines with capacitive source‐load coupling as well as spur lines embedded, and three resonators, i.e., a dual‐mode stub‐loaded stepped impedance resonator and two nested dual‐mode defected ground structure resonators. Using the intrinsic characteristics of the resonators and feed lines, nine transmission zeros near the passband edges and in the stopband can be generated to achieve high selectivity. An experimental tri‐band BPF located at 2.4/5.7 GHz [wireless local area networks (WLAN) application] and 3.5 GHz [worldwide interoperability for microwave access (WiMAX) application] has been simulated and fabricated. Good agreement between the simulated and measured results validates the design approach. © 2012 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2013.