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This work, for the first time, presents an absorptive frequency-tunable substrate-integrated waveguide (SIW) filter. For the filter design, an absorptive bandpass filter prototype tailored to an SIW structure has been formulated. The proposed absorptive bandpass filter prototype is capable of having a return loss larger than 20 dB at all frequencies although it has a simple matching circuit. The filter prototype has been attained by comprehensive formula-based synthesis of a new matching circuit equivalent to an existing one. To validate our filter prototype, a second-order absorptive bandpass filter that can alter its center frequency has been designed, fabricated, and measured. This article demonstrates approaches to find physical dimensions by comparing each section of the filter structure with the corresponding part of the prototype. The fabricated absorptive bandpass filter has a compact structure by virtue of its simple matching section. The frequency-tunable SIW resonators allow our absorptive bandpass filter to adjust its center frequency from 1.76 to 2.69 GHz while having absorptive performance. The measured reflection of the filter centered at 2.50 GHz is smaller than -10 dB between 1.56 and 3.27 GHz (2.10:1). It is shown that theory, simulation results, and measured responses are in good agreement.

This article reports on the RF design and practical development of RF co-designed bandpass filters/isolators (BPFIs). They are based on series-cascaded nonreciprocal resonant stages, microwave resonators, and multiresonant cells. The nonreciprocal stages are shaped by an in-parallel cascaded transistor-based path and a transmission line (TL) that result in a zero-phase resonance in the forward direction and high isolation (IS) in the reversed one. The operating principles of the nonreciprocal resonant stage are presented through circuit analysis and coupled-resonator diagrams (CRDs) that facilitate their design with coupled-resonator-based filter design methods. Various CRD-synthesized examples of low- and high-order transfer functions with and without transmission zeros are shown. They demonstrate the applicability of the proposed method for alternative-type of filtering responses with and without gain in the forward direction and high levels of IS in the reversed one. For proof-of-concept demonstration purposes, five prototypes were designed, manufactured, and measured at $S$ -band.

A novel band-pass filter topology in waveguide technology is presented in this work. The proposed filter design is based on a periodic structure that uses modified sections of a single-ridge waveguide (SRW) as the unit cell to produce the desired frequency response. Two-step height profiles are included in the central part of the SRW, which provide useful parameters to yield a simple design method to achieve the required filtering characteristics. The suggested topology and design process are used to achieve band-pass filter responses with different fractional bandwidth and rejection characteristics. A 54% fractional bandwidth band-pass filter centered at 5.4 GHz is implemented using low cost 3-D additive manufacturing techniques, which allow fast prototyping and the fabrication of complex geometries. Experimental measurements are in agreement with the expected simulated response of the designed band-pass filter.

This article introduces a new technique for filter design and tuning using the phase of the input impedance (PII) as the design parameter. This novel method is applicable to both narrow and wideband filters. Compared with conventional methods, this approach requires less computation time and provides a clear step-by-step procedure for identifying the proper interresonator coupling and the resonant frequencies of the resonators. In practice, the physical realization of the filter always has a nonideal I/O port, which can introduce an unexpected unknown transmission line between the physical reference plane and the port of the corresponding inverter in the circuit model. In this article, the PII response is used to determine the equivalent electrical length of this unknown transmission line. The technique is demonstrated through the design of a wideband planar filter with a fractional bandwidth of 72%, and the tuning of a 4–2 combline filter with two transmission zeros. Finally, a wideband diplexer is developed using the proposed technique.

A dual-band filter of a large frequency ratio is proposed, studied, and developed in this article, which is based on mode composite coplanar waveguide (MCCPW). Both low and high bands of the dual-band filter can be designed independently with a high degree of design freedom. The filter can achieve more than ten times the frequency difference between the two bands, which is usually hard to accomplish by conventional dual-band filter design techniques. All harmonics between the two bands can be suppressed by a cascaded low-pass filter and impedance matching structure to form a qualified dual-band frequency response. The characteristic of the proposed MCCPW dual-band filter makes it a promising candidate for multiband systems, especially in 5G communication systems. The demonstrated MCCPW dual-band filter is fabricated with a single-layer substrate. Measured results are in good agreement with simulated results, which validate the feasibility and superiority of the proposed filtering technique.

This article presents a type of bandpass filter (BPF)-integrated single-pole double-throw (SPDT) switch using the frequency selective star-junction matching circuit and the switched magnetic coupling (SMC) technique, i.e., the lowpass phase shifter (LP-PS) and the quasi-elliptic lowpass filter (QE-LPF) with specified phase shift. Firstly, a high selectivity third-order BPF-integrated single-pole single-throw (SPST) switch with cross coupling and mixed electric and magnetic coupling (MEMC) is designed by the classical coupled-resonator filter design theory. A 34-dB OFF-state suppression (OSS) is achieved due to the detuned resonators, and the OSS can be further increased to 60 dB by the introduced SMC. Then, the BPF-integrated SPDT switches are designed by using proposed frequency selective star-junction matching circuit to cascade the above designed BPF-integrated SPST switch with or without SMC. The proposed frequency selective star-junction matching circuit is used to not only provide the required phase shift for meeting the impedance matching condition, but also have lowpass frequency response for expanding the upper stopband of ON-state channel, the rejection bandwidth of OFF-state channel, and the port-to-port isolation bandwidth. As examples, four BPF-integrated SPDT switches are designed and fabricated to validate the proposed design approaches, which show the improved electrical performance.

High-performance microwave and millimeter-wave filters’ design is a challenging task because the filter characteristic is rather sensitive to the variation of geometric dimensions and electrical sizes. A common practice in filter design is to optimize the design variables starting from a set of initial values. However, if the initial values are not sufficiently close to the optimal solution, the optimization often fails to provide any satisfactory result. To deal with this problem, for the first time, the homotopy method is introduced to microwave and millimeter-wave filters’ optimization problems in this article. The homotopy method formulates a series of intermediate optimization problems, which can guide the optimizer to approach the optimal solution for the target filter design. In this article, the artificial neural network (ANN) is adopted as the surrogate model to the time-consuming electromagnetic model to speed up the homotopy filter optimization process. Two design examples are given to demonstrate the homotopy optimization technique based on the ANN model, including an all-pole filter and a generalized Chebyshev filter with a frequency-dependent coupling. Both filters with optimized geometric dimensions are simulated, and the all-pole filter is fabricated and measured. The simulation and measurement results verify the accuracy of the ANN model and validate the homotopy optimization method.

Any filter of order $N$ can be readily tuned for a constant absolute bandwidth (BW) using $2N+1$ independent tuning elements. This significantly increases the size and the cost, and also adds to the complexity of the tuning control mechanism. This article demonstrates the feasibility of realizing a high- $Q$ tunable coaxial filter, which is tuned by a single rotational tuning element irrespective of the filter order. The proposed filter has low variations in the absolute BW and insertion loss (IL) over a relatively wide tuning range. A prototype four-pole filter is developed at 2.5 GHz with a fractional BW of 4% to verify the concept. The measured tuning range of the filter is 20%, within which the BW variation is better than ±10% and IL variation is better than 0.05 dB. The proposed concept is easily expandable to filters with higher order. Furthermore, the concept is adopted to design a tunable diplexer which can be tuned using a single tuning mechanism. The proposed high- $Q$ tunable filter is promising for use in the frequency-agile communication architecture at the cellular base-station and aerospace applications.

This paper characterizes the performance of a miniaturized folded ridged half-mode substrate integrated waveguide (SIW) and explores its use in quarter-mode bandpass filter (BPF) design. By folding the fundamental mode of a ridged half-mode SIW (RHMSIW) around the capacitive ridge structure, an additional 50% of miniaturization can be achieved over RHMSIW and up to 87.5% compared with the standard SIW. The monomode bandwidths of both folded ridged half-mode and ridged half-mode waveguide technologies are studied, yielding 1:4.28 or greater fundamental to the second-order mode ratios for all cases and approximately twice the monomode bandwidth compared with half-mode technology. Furthermore, the study of folded ridged half-mode and quarter-mode substrate integrated cavities is presented, yielding miniaturizations compared with standard substrate integrated cavities with respect to free-space lambda equal to 96.6% and 98.0%, respectively. The design of BPFs using folded ridged quarter-mode cavity is demonstrated, with two different prototypes measured to study the choice of substrate thickness with respect to filter performance. By increasing the substrate thickness while maintaining similar cavity miniaturization, an improvement of 26.1% in unloaded quality factor between the filters is realized experimentally.

In this paper, a newly shunted-line stepped-impedance resonator (SLSIR) is studied and applied to realize high-order balanced filter design for radio astronomy application. The proposed SLSIR not only has more flexible control of the internal coupling between adjacent units but also provides more design freedom for the cascade connection of high-order filter. Then, the operating theory of three kinds of electrically, magnetically, and electromagnetically coupled SLSIR pairs has been discussed and utilized to high-performance filter design. Hence, fourth-order balanced filter using electromagnetically coupled SLSIR pairs is investigated and designed, which exhibits an excellent filtering performance under differential-mode (DM) operation as well as a high common-mode (CM) suppression level. In addition, the production mechanism of DM transmission zero is modeled and analyzed. Based on the analysis above, two dissimilar types of coupled SLSIR pairs with the same fundamental frequency but different harmonics are adopted to design an eighth-order balanced filter. Finally, the high-order balanced filter is fabricated on 0.5-mm-thick MgO wafer with double-sided YBCO thin films. Measured results show that the eighth-order balanced filter operates at 2.53 GHz with the corresponding in-band insertion loss of 0.1 dB. Results well verify the proposed structure with high selectivity, low insertion loss, and wide stopband performances.

In this paper, a systematic research campaign on eighth-mode substrate integrated waveguide (EMSIW) filters is reported. The sizes of these EMSIW resonant cavities are only one-eighth or one-sixteenth of a conventional SIW resonant cavity. Two different coupling topologies, i.e., electric coupling and magnetic coupling, are employed between two EMSIW resonant cavities, which are analyzed theoretically via the response of the structure and the relationship between the cavities. Both coupling topologies enable structural variations that possess advantages and flexibility of the second-order EMSIW bandpass filters (BPFs) designing. By utilizing these basic coupling mechanisms, multilayer board technology, and other practical techniques, a class of triple-order SIW BPFs with the merits of compact size and high selectivity are demonstrated. Specifically, several triple-order EMSIW BPFs composed of pure EMSIW cavities or comprised by combining EMSIW and quarter-mode SIW cavities are simulated, fabricated, and measured for verification.

In this paper, an analytic design method for a frequency- and bandwidth-tunable absorptive bandpass filter is introduced. The presented design method relies on the transformation from the normalized absorptive low-pass filter prototype to a bandpass filter structure with parallel resonators and admittance inverters. The inverter values for producing a predefined frequency response are given in closed-form expressions so that a filter design needs no heuristic approach and can be carried out in an analytic manner. This work also presents a new tunable stub resonator structure capable of changing not only its resonant frequency but also the slope parameter. As the slope parameter can be adjusted, it is allowed to use static inverters in designing a filter whose bandwidth can be tuned. For demonstrating our design method for a reconfigurable absorptive bandpass filter, a third-order filter having the target to have the frequency tuning range from 1.35 to 1.65 GHz and the bandwidth tuning range from 5% to 10% has been designed and measured.

This paper reports on the RF design of fully reconfigurable quasi-elliptic-type single- and multi-band microwave bandpass filters (BPFs) and duplexers. They are based on series-cascaded resonators and multi-resonant/multi-band cells that produce frequency-reconfigurable poles and transmission zeros (TZs). By appropriately tuning the resonant frequencies of their constituent resonators, multiple levels of transfer-function adaptivity can be obtained. These capabilities include center-frequency and bandwidth tuning, as well as intrinsic RF switching-off (i.e., without the use of RF switches). As opposed to conventional BPF tuning mechanisms, the aforementioned reconfiguration characteristics avoid the use of tunable couplings. The basic operating principles of the engineered resonator-cascade BPF concept are presented through a three-pole/two-TZ example. Afterward, methods for expanding the proposed filter design approach to higher order, multi-band, and multi-port (e.g., duplexers) realizations are explained through coupling-matrix-based analysis. For practical-demonstration purposes, four L-band tunable microstrip prototypes were developed and measured. They include: 1) a five-pole/four-TZ BPF; 2) a five-pole/two-TZ BPF; 3) a dual-band BPF with two three-pole/two-TZ passbands; and 4) an RF duplexer.

Frequency-dependent coupling (FDC) emerges as a new method to introduce the finite-position transmission zeros (TZs) in the synthesis and design of filtering networks. In this paper, several fundamental building blocks with FDCs are introduced and studied. The higher order filters with many TZs can be constructed by cascading these fundamental building blocks. As compared with the conventional coupled-resonator filters with only constant couplings, the inclusion of FDCs can potentially create more TZs with the same coupling topology. It is also shown that to realize the same TZs, a filter can be realized in a simplified structure with less cross-couplings by using FDCs. In this paper, one complete filter design procedure is illustrated, including the steps of the circuit model synthesis by the method introduced in this paper, physical dimensioning of the filter by the electromagnetic simulation and measurement of the fabricated filter. The measurement results are in very good agreement with the theoretical predictions, which validates the modular design strategy for filters with FDCs.

This paper presents a type of bandpass filter (BPF)-integrated single-pole double-throw (SPDT) switch using common shorted stepped-impedance resonator technique. The BPF-integrated ON-state with reverse-biased p-i-n diodes can be synthesized by classical coupled-resonator filter design theory, and the OFF-state with forward-biased p-i-n diodes is detuned which exhibits high OFF-state suppression (OSS). The transmission zeros due to the direct coupled feed, the cross-couplings and the cancelling effect of mixed electric and magnetic coupling are produced to improve the ON-state passband selectivity significantly. As examples, the third-order/fourth-order BPF-integrated SPDT switches at center frequency $f_{0} = 1$ GHz with ripple bandwidth of 110/100 MHz are designed, which show low insertion losses of 1.08/1.58 dB, sharp skirts, high roll-off rate of sidebands, high OSSs of 40.9/40.1 dB as well as high port-to-port isolation of 57.4/47.8 dB. Then, the switched 50- $\Omega $ matching circuits are in series with two output ports of above BPF-integrated SPDT switches, resulting in the absorptive OFF-state. The fabricated third-order/fourth-order BPF-integrated SPDT switches with absorptive OFF-state have the matched return losses of 30/28.2 dB, and their OSSs and port-to-port isolations are also improved.

In this paper, an adaptive synthesis using the particle swarm optimization (PSO) for implementations of resonator-coupled filters is proposed. The coupling matrix of in-band filtering response is achieved and optimized by the PSO-based synthesis. Meanwhile, the stopband coupling matrix of the filter is predicted based on the combination of extra resonant nodes representing stopband spurious and parasitic effect of input/output ports. In addition, an enhanced accurate prediction of filter performance is calculated by the proposed approach, considering the practical fabrication tolerance on filter design. To verify principles mentioned earlier, various resonator-coupled filters are implemented. The calculation, EM-simulation, and measurement of filters show good agreements in both passband and stopband.

This paper presents a novel configuration for a high-Q tunable waveguide (WG) filter with a constant absolute bandwidth (BW). The key feature of this filter is that it is tuned by a single tuning element. This paper investigates the theory of coupling behavior of single septum and double septa to achieve constant absolute BW. The theory is presented for both the coupling matrix-based and inverter-based filter topologies. The filter design methodology is lucidly presented covering the design strategy adopted for interresonator coupling, input–output coupling and tuning element. The tuning mechanism of the proposed filter is explained with measurement results presented for a $Ku$ -band tunable WG filter designed at 15 GHz with a 2% BW to achieve 15% tuning range. The filter promises to be useful in emerging 5G millimeter-wave applications, where the filter size is very small to accommodate many mechanical tuning elements. Furthermore, the proposed design methodology is scalable, i.e., the tuning mechanism is independent of the filter order.

The additive manufacturing process of multimaterial extrusion offers performance advantages using functional materials including conductors while making accessible the third dimension in the design of electronics. In this work, we show that the additional geometrical freedom offered by this technique can be exploited for the design and realization of filters made of 3-D resonators that exhibit enhanced characteristics. The coupling properties of 3-D grounded square split ring resonators (SRRs) are initially explored. We demonstrate by simulations and experiments that SRRs with finite height display significantly stronger coupling compared to equivalent thin printed circuit structures. The observed trend can be exploited for designing filters with wider operational bandwidths for a given footprint, or miniaturized layouts and enhanced compatibility with fabrication limits for minimum feature size and spacing without performance degradation. This concept is demonstrated by presenting results of full-wave simulations for sample bandpass filters with identical footprint but formed by coupled 3-D square SRRs of different heights, showing that filters with taller resonators exhibit increasingly wider bandwidths. Two filter prototypes with center frequencies at 1.6 and 2.45 GHz are manufactured by multimaterial 3-D printing. The measured characteristics of these prototypes are found to be in good agreement with numerical simulations taking into account the effect of the lossier metallic and dielectric materials used in 3-D printing and confirm the predicted larger bandwidth of the filters made of 3-D SRRs with marginally higher insertion losses.

The realization of planar wideband bandpass filters with an extremely wide upper stopband is described. The filter design differs from conventional approaches in which it is based on the choice of a wideband bandstop (BS) filter, to which a number of shunt-shorted stubs are connected at specific nodes; the stubs provide transmission zeros at the origin. Two examples of wideband bandpass filters with extremely wide upper stopbands are presented: filters based on a fifth-order Chebyshev wideband BS filter and a pseudoelliptic function ultra-wideband-BS filter. Relative passbands in excess of 100% are achieved, while the equivalent of at least four harmonic passbands is suppressed.

© 1963-2012 IEEE. In this paper, an adaptive N-path filter design technique is presented. The filter designed by this method can be reconfigured to different shapes, e.g., Butterworth or Chebyshev with variable bandwidth (BW), based on the user requirements. In addition, an analytical synthesis procedure is introduced to realize high-order bandpass filters (BPFs) based on N-path architecture with a prescribed set of specifications. A proof-of-concept BPF is fabricated and measured in a 65-nm CMOS process. The filter can be reconfigured to realize different filter shapes by changing coupling capacitors. The BW of the proposed filter is also tunable. The passband of the filter is tunable from 0.2 to 1.2 GHz with a gain of 14.5-18 dB, a noise figure of 3.7-5.5 dB, and a total power consumption of 37.5-52.7 mW. The channel BW can be varied from 20 to 40 MHz and the filter out-of-band third-order intercept point (Δ f=50 MHz) is 25 dBm.

This paper proposes a unified synthesis methodology and the related architecture for fully passband-tunable dual-mode filters with wide tuning range. The methodology and architecture relate the well-developed frequency-fixed synthesis technologies to tunable filters, and can be applied to any tunable filter design and analysis by an element-variable coupling matrix. This general methodology accounts for bandwidth (BW), center frequency (CF), and return loss (RL) (passband ripple or stopband rejection) tuning, and helps to implement the passband-tunable filter. For validation, a novel tunable dual-mode bandpass filter (BPF) based on the architecture is presented. The proposed filter has the ability to arbitrarily construct the two-pole passband within the frequency tuning range as predicted. In addition, three transmission zeros (TZs) are generated by the proposed structure, which results in a highly selective passband and a reconfigurable stopband. A tunable filter with a tuning range from 0.8 to 1.2 GHz is designed and fabricated. The measured filter presents an elliptic response during the frequency tuning. The 50% CF tuning range (0.8–1.42 GHz) with a constant absolute BW1 dB and a BW1 dB tuning range from 50 to 500 MHz at the fixed CF are measured. Good agreement among matrix calculation, circuit model simulation and measurement demonstrates the validity of the proposed method and structure. In addition, different states of the stopband with the same passband are obtained to further broaden its application.

A new approach for the design of a balanced bandpass filter (BPF) is presented in this paper. Stemming from the mode degeneration properties of a square patch resonator, a complete set of resonant modes in a right-angled isosceles triangular patch resonator (RAITPR) is thoroughly investigated so as to construct a modified RAITPR with one electric wall and two magnetic walls along its three sides for filter design. This proposed resonator can support the half-TM10-mode of the RAITPR, and thus, it is named as the half-mode RAITPR here. By properly adopting the relationship between one RAITPR and two coupled half-mode RAITPRs, a second-order coupling topology is presented. Its working principle is then explained by using the field distribution of the desired resonant modes involved. A few high-order balanced filters are in final implemented and fabricated to evidently demonstrate their good differential-mode filtering selectivity and high common-mode suppression. Good agreement is achieved between the simulated and measured results for all the designed three balanced BPFs. To further improve the concerned out-of-band rejection performance, the feeding scheme of the modified structure has been discussed.

Quasi-optical (QO) filter design is commonly based on the direct coupled bandpass filter topology with distributed resonator elements. Conventionally, prototypes with symmetrical impedance inverters are employed during synthesis, and as a result, this leads to resonators with custom lengths across the filter. This can be impractical from a QO filter implementation perspective, since spacers of arbitrary thickness may not be readily available. In this paper, we propose a novel impedance inverter equivalent network, which allows the development of prototypes where transmission lines across the filter structure to have predefined fixed lengths. Based on this technique, we developed a fourth-order QO filter prototype that composed of 2-D periodic arrays with subwavelength features. Its performance is verified with free-space measurement, and the measured results show good agreement with both full-wave and circuit simulation. Thus, it is shown that the proposed impedance inverter circuit can simplify the implementation of QO filters of higher order.

This paper presents a comparison and performance evaluation of three different technologies for the implementation of tunable filters utilizing semiconductor varactors, commercially available barium strontium titanate (BST) thin film and screen-printed BST thick-film varactors. To identify the most suitable technology for applications in low power receiving up to high-power transmitting stages, the performance evaluation is carried out with the same tunable hairpin filter design. While the center frequency of the proposed tunable filter structure is tuned by varactors loading the filter resonators, the bandwidth is controlled by coupling varactors between adjacent resonators. The filters are designed for a center frequency range from 700 MHz to 1 GHz and for bandwidth tuning from 60 to 150 MHz. The tunable filters are evaluated and compared with regard to their tunability, quality factor, linearity (intermodulation products of third order), and power handling capability. In conclusion, the tunable hairpin filter based on semiconductor varactor diodes offers the largest center frequency tuning range of 390 MHz with a transmission between −1.5 and −7 dB, whereas the BST thin- and thick-film-varactors-based filters exhibit high linearity with an IIP3 between 39 and 60 dBm. Moreover, large signal characterization reveals that the BST thick-film-varactors-based filter structure has the best power handling capability of up to 41dBm.

In this paper, a compact dual-mode hairpin ring resonator (HRR) with two controllable resonances is proposed to design high-order dual-band high-temperature superconducting (HTS) bandpass filters. Its noncoupled dual-mode resonant characteristics and the mechanism for inherent transmission zero (TZ) production are studied. Moreover, a general dual-path coupling scheme is introduced for high-order coupling realization and applied to the third-order and eighth-order dual-band filter design. Also, two coupling structures for the adjacent HRRs with different orientations are proposed to excite TZs between the passbands for high selectivity. Besides, dual-parallel input–output feeding structure is adopted to tune the external quality factors of the dual-mode HRR individually so that the two passband are realized easily using high-order structure. For demonstration, an eighth-order dual-band HTS filter both operating at 1.9 and 2.6 GHz for mobile communications application are designed and analyzed. Compared with the third-order one, the selectivity of the eighth-order filter has at least 4.2 times improvement and five TZs are produced for high band-edge selectivity. Finally, the eighth-order dual-band filter is fabricated on MgO substrate with YBCO thin films. Measured results agree well with the simulations and show the excellent performance with 0.12-dB minimum insertion loss and better than −75-dB rejection level up to 5 GHz. In addition, the nonlinear characteristics of the fabricated HTS filter is observed by experiment, which shows the temperature dependence of filter center frequency performance and microwave insertion loss.

Microfluidically repositionable selectively metallized plates (SMPs) are utilized for the first time to design and realize microstrip line combline filters with wide frequency tuning and high power-handling capability. A continuous frequency tuning range of 2.7:1 is achieved by utilizing tight coupling between the SMP and open-ended microstrip line resonators in order to change their length for tuning from 2.5 to 4 GHz and capacitively load one of their open ends for tuning from 1.5 to 2.5 GHz. The unloaded quality factor (Q u ) of the resonators is maintained relatively constant (210–190) across the entire tuning range allowing for the low insertion loss (IL) and high power-handling capability. Specifically, a ~5% constant fractional bandwidth fourth-order filter design is discussed and experimentally verified. The filter exhibits less than 3-dB IL across the 2.7:1 tuning range. It is shown that the filter’s power handling capability can reach up to 15 W under constant average power excitation.

This paper presents a new class of filter responses called pseudo-Butterworth filter response. A detailed mathematical analysis on the pseudo-Butterworth response has been carried out. In addition, the comparison between the typical complete Butterworth response and the new pseudo-Butterworth response is provided. The theory for the pseudo-Butterworth filter response indicates that bandwidth tuning maintaining excellent return loss performance can be executed adjusting not all coupling structures of a filter. As an example, a bandwidth tuning method for a second-order filter is presented. For verifying the presented bandwidth tuning theory, a second-order substrate-integrated waveguide (SIW) resonator filter has been designed, fabricated, and measured. As an example of applications to higher order filters, a theory for the fourth-order pseudo-Butterworth responses and a bandwidth tuning method for the fourth-order filters have been developed. A fourth-order filter has also been fabricated and measured for verifying the presented bandwidth tuning technique.

This paper investigates L-shaped iris (LSI) embedded in substrate integrated waveguide (SIW) structures. A lumped element equivalent circuit is utilized to thoroughly discuss the iris behavior in a wide frequency band. This structure has one more degree of freedom and design parameter compared with the conventional iris structures; therefore, it enables design flexibility with enhanced performance. The LSI is utilized to realize a two-pole evanescent-mode filter with an enhanced stopband and a dual-band filter combining evanescent and ordinary modes excitation. Moreover, a prescribed filtering function is demonstrated using the lumped element analysis not only including evanescent-mode pole, but also close-in transmission zero. The proposed LSI promises to substitute the conventional posts in (SIW) filter design.

[EN] This paper is focused on the synthesis of capacitively loaded coupled lines for the design of bandpass filters with compact size and spurious suppression. The filters consist of cascaded pairs of coupled lines periodically loaded with patch capacitors. The loading capacitances provide a slow-wave effect to the coupled lines useful for filter miniaturization, whereas periodicity introduces bandgaps that can be controlled in order to achieve spurious suppression. Filter design is achieved following an automated process based on aggressive space mapping (ASM) optimization. The two reported examples, a third-order filter with central frequency f0=1.0 GHz, 8% fractional bandwidth, and 0.1-dB ripple level and a fifth-order filter with central frequency f0=1.8 GHz, 8% fractional bandwidth, and 0.1-dB ripple level, demonstrate the potential of the proposed ASM-based design approach, as well as the effectiveness of the loading capacitances to reduce filter size and reject the spurious bands. Significant size reduction in both filters is achieved, whereas the out-of-band rejection is better than 30 and 45 dB in the third-order and fifth-order filters, respectively, up to at least 4f0 ., This work was supported in part by MINECO-Spain under Project TEC2013-47037-C5-1-R, Project TEC2013-40600-R, and Project TEC2013-49221-EXP, in part by the Generalitat de Catalunya under Project 2014SGR-157, and in part by FEDER funds. The work of F. Martin was supported by the Institucio Catalana de Recerca i Estudis Avancats. The work of M. Orellana was supported in part by the Universidad de Costa Rica, in part by MICITT, and in part by CONICIT

A new kind of compact dual-mode dual-composite right-/left-handed resonators is proposed. Dual-band bandpass filters (BPFs) are consequently designed with over 88% size reduction. To assist the filter design, an analytical design method is further presented, with which one filter can be designed flexibly according to the desired frequencies and bandwidths (BWs). To alleviate the design difficulties, a synthesis design technique and a feedback-based optimization process are both employed. In addition, the independent control of frequencies, BWs, and coupling coefficients are also realized, which provide large freedom for designing BPFs. To ensure the feasibility of the design method, the valid ranges of center frequency and BW ratios are first determined. Furthermore, four compact BPFs are designed with a large/small center frequency ratio (3.11/1.6) and a large/small BW ratio (2.52/0.52). Good agreement between the measurements and simulations are observed, indicating the validity of the proposed design method.

A design methodology for optimized minimum inductor bandpass filters (BPFs) is presented. Design curves are provided to determine the filter order for a required out-of-band rejection as a function of bandwidth (BW). Coefficients for BWs from 10% to 100% in steps of 10% for different filter orders and two different transmission zero placements are provided. For both cases, the required in-band return loss is larger than 10 dB. Using the derived coefficients, filters with required rejections can be designed for a given center frequency. To validate the design methodology, five discrete element filters with different BWs are designed and fabricated at a center frequency of 1.5 GHz. Furthermore, to show the application of the minimum inductor filters in complementary metal–oxide–semiconductor (CMOS), an S-band BPF in 0.13- $\mu \text{m}$ CMOS, with 60% BW, is designed and fabricated. The out-of-band rejection of the fabricated filter is larger than 50 dB.

This paper presents a tunable active bandpass filter (BPF) with adjustable transmission zeros (TZs) close to the passband for software-defined radio (SDR) applications. The RF front-end frequency selectivity is enhanced by the creation of TZs that also improve the out-of-band (OOB) input-referred third-order intercept point (IIP3). The filter is based on two-path signal cancellation and consists of a tunable BPF in parallel with two tunable bandstop filters. This combination ensures the correct amplitude and phase relationships across a wide tuning range to create adjustable TZs without sacrificing the gain of the passband. This paper presents in detail the design considerations and guidelines, as well as analysis of the filter performance in the presence of nonidealities such as parasitics and imperfect clock signal shape. The proposed filter is implemented with high- $Q~N$ -path filter blocks in a 65-nm CMOS process. The passband of the filter is tunable from 0.1 to 1.4 GHz with a 3-dB bandwidth of 9.8–10.2 MHz, a gain of 21.5–24 dB, a noise figure of 3–4.2 dB, and a total power consumption of 50–73 mW. TZs are created on both sides of the passband with a minimal offset of 25 MHz and are tunable across a 20-MHz range with up to a 60-dB rejection. The measured blocker 1-dB compression point is 8 dBm and the OOB IIP3 is 23 dBm. The reported filter provides a promising on-chip filtering solution for multistandard multifrequency SDR applications with improved interference mitigation capabilities.

This paper proposes a novel and unified synthesis approach and design procedure for microwave filters with extracted pole and nonresonating node (NRN) technique. By introducing a general circuit topology, the traditional extracted pole and NRN filters can be unified to a general prototype. With the new design process, the exact mapping relationship between the electromagnetic (EM) model of the pole producing element and the circuit model can be achieved. All the physical dimensions of the filter structure can be obtained by a fully analytical approach. No full EM optimization or adjustment is needed in the filter design. A three pole TE $_{\mathrm { {101}}}$ mode waveguide filter with one finite transmission zero is designed to illustrate the new procedure. In addition, a seven-pole TE $_{\mathrm { {011}}}$ mode filter with four finite transmission zeros covering all three possible types of arrangements is designed and fabricated to demonstrate the versatility of the proposed method. The simulated and measured results verified the efficiency and the accuracy of the proposed design method.

A channel mismatch calibration method is proposed for use in time-interleaved digital real-time oscilloscope (DRTO) applications. Linear equations are derived using Fourier transforms of the separated signals from each of the time-interleaved analog-to-digital converters (TIADCs). Thus the errors in the TIADCs can be easily calibrated by inversion of a matrix, as opposed to most previous work where additional filters are employed. The calibration accuracy of the proposed method is limited only by the noise produced after the TIADC circuitry, while other methods depend on the filter design. A transfer function measurement method is then proposed for application to commercially available DRTOs. Two-tone signals are measured using DRTOs from various suppliers to validate the proposed method. The occurrence of signals at spurious frequencies is considerably reduced, as demonstrated by the calibrated results.

This paper presents an integral-equation formulation specialized for the analysis of capacitive waveguide circuits, which include arbitrarily shaped conducting and homogeneous magnetic/dielectric objects. The technique benefits from the symmetry of the structure by formulating a 2-D scattering problem with oblique angle of incidence, combined with the use of the parallel-plate Green’s functions. As practical applications, the paper proposes novel low-pass filter designs loaded or coated with dielectric and magnetic homogeneous materials. If the filter is properly designed, the use of these materials could improve the filter response, selectivity, or out-of-band performance, and power-handling capabilities. Some novel filter design implementations, showing these kinds of benefits, are presented for the first time in this paper. For validation, a commercial full-wave simulator is employed, showing the validity, accuracy, and computational efficiency of the novel software tool.

In this paper, a novel approach for field coupled inline transitions between different waveguides is proposed. The immittance inverting characteristics of these transitions are simultaneously utilized for filter design. Based on scattering parameters, an exact relation is derived between the desired inverter constants and the even and odd mode characteristic impedances. The well-known approximation introduced by Cohn is improved. Further it will be shown that the introduced inline transitions can be effectively modeled with a contra-directional symmetrical coupler. The description of the field coupled waveguide transitions as inverters allows a “straight on design method” for general field coupled structures and accelerates the optimization process. The integration of the inline transitions into the filter design is applied for two different types of transitions, namely, the transition between a parallel-plate waveguide and a rectangular waveguide (RWG) and the transition between a microstrip line and a RWG, using a ridged waveguide section for the coupling. For both configurations a third-order bandpass filter was realized by using three coupled waveguide resonators. Two of the coupling structures were made up of inline transitions. To verify the approach and quantify losses, a filter was fabricated and its scattering parameters measured. With this new approach low-loss waveguide filters can be integrated into a planar RF circuit while minimizing space. No delicate manufacturing processes are required.

This paper presents a technique that enables the design of high-order microwave bandstop filters with a total through-line length of significantly less than one quarter-wavelength at the filter center frequency. The total through-line length using this technique can be zero regardless of the filter order in theory, but it is limited in practice by the finite line length required to obtain a desired coupling magnitude between the through line and a resonator. A fifth-order quasi-elliptic reflection-mode bandstop filter design is shown with total through-line length of less than 1/15th of a wavelength in suspended-stripline technology. The measured response has a 42-MHz 30-dB bandwidth at a center frequency of 3 GHz and four reflection zeros. The bandstop filter design technique described in this paper shows great promise for reducing the size and weight of microwave bandstop filters.

A synthetic transmission-line N-path bandpass filter technique that lowers the in-band insertion loss and increases the out-of-band rejection of the traditional N-path filter is analyzed. The performance limitations of the traditional N-path filter in terms of in-band insertion loss and limited rejection are reviewed. Measurement results from 0.1 to 1.6 GHz of a single-ended, four-stage synthetic transmission-line N-path filter fabricated in a 65-nm CMOS technology that achieves less than 5-dB insertion loss across the tuning range, 30–50-dB out-of-band rejection, in-band third-order distortion input-referred intercept point (IIP3) of $+$ 29 dBm, and a $+$ 11-dBm input-referred 1-dB compression point $({\rm P}1{\rm dB}_{\rm Jam})$ out-of-band jammer tolerance are presented.

A new concept of tuning transmission zeros (TZs) in 4-pole tunable bandpass filters is demonstrated. In this work, the tuning elements that were conventionally used for bandwidth control are also utilized for adjusting the TZs at the same time. As a result, the selectivity of the tunable filter can be controlled without increasing the filter order and complexity. A 4-pole stripline filter is used to demonstrate this concept. The filter can be continuously tuned from 1.25 to 2.1 GHz and two TZs are employed to improve the stopband suppression to $> {\hbox{60 dB}}$ at 200 MHz from the passband. These TZs can also be moved from the upper stopband to the lower stopband without any auxiliary tuning elements to significantly change the stopband suppression. The high rejection filter is useful in wireless systems and cognitive radios with large interferers.

We propose a temporal coupled-mode theory for the design of microstrip bandpass filters with multi-resonant modes and reveals the mechanism of the filters based on multi-coupling and energy conservation theorem. As an example, the combined effect of the two orthogonal modes in dual-mode filters is analyzed in detail. By simply tuning the coupling efficiencies of the two resonant modes, two K-band integrated microstrip bandpass filters have been designed and demonstrate opposite asymmetric transmission responses. Both simulation and experiment results agree well with the theoretical analysis, which provides a new approach for filter design and optimization.

This paper addresses, for the first time, the digital representation of spectrally periodic signal-interference microwave passive filters. Starting from the theoretical rational transfer function of a synthesized transmission-line signal-interference analog filter, the real-valued coefficients of its digital-domain filter counterpart are derived. This discrete-time formulation for this type of microwave filters allows to clarify some confusing concepts into them, such as filter order. Moreover, some particular phenomena observed in associated experimental proof-of-concept planar prototypes, such as the appearance of out-of-band/spurious resonance peaks, are also explained by this study as zero-pole quasi-cancellations. In addition, it is demonstrated here that the use of the adjective “transversal” for this class of microwave passive filters could be no rigorous as it is understood in a digital sense since its discrete-time modeling commonly obeys an infinite-impulse-response structure. For validation, the expounded theoretical framework is applied to three examples of signal-interference passive filters exhibiting quasi-elliptic-type single-passband, six-passband, and low-pass responses.

A new low-pass waveguide filter topology is proposed based on capacitive posts using rounded shapes. The new structure is designed using a technique that benefits from a full-wave integral equation simulation tool for arbitrarily shaped capacitive microwave circuits. This integral equation method enables one to apply an efficient filter design technique employing iris geometries other than rectangular discontinuities, such as the one based on rounded posts proposed in this work. A prototype using circular posts has been manufactured and measured obtaining excellent results as compared with predictions, thus demonstrating the practical feasibility of the new proposed filtering structure. A new filter using elliptic capacitive posts was compared in terms of multipactor risk against similar filters based on traditional rectangular windows and on rectangular posts. Simulation results indicate that the new proposed concept using rounded shapes leads to geometries that can double multipactor thresholds as compared with traditional filters.

In this paper, a miniaturization technique and a tuning concept that enable the realization of highly miniaturized and frequency reconfigurable helical filters are reported for the first time. The proposed filter design is based on tunable helical resonators with an internal volume of only 8.6 cm 3 and variable capacitive loading. A second-order bandpass filter was built and measured as a proof-of-concept demonstrator. The helical filter is manufactured with a CNC process and tuning is achieved through piezoelectrically actuated tuners, which are bonded over the helix apex. A measured center frequency tuning between 257-365 MHz was realized with an insertion loss between 0.3-3.2 dB including connector losses. The measured filter bandwidth varied between 6%-7.7%. Tunable resonators were also manufactured so as to extract the unloaded quality factor. It was measured between 392-639 for a center frequency tuning of 2.5:1 (156.5-395 MHz).

This study presents the first reported design and implementation of triple-band Marchand balun filter. The novel balun filter is constructed from the short-circuited coupled line with a triple-band resonator to demonstrate the triple-band admittance inverter characteristic. To improve the amplitude and phase responses within three passbands, the compensation techniques for phase-angle, impedance-matching, and susceptance-slope discrepancies are employed in the balun filter design. Four types of triple-band balun filters with significantly different bandwidths and passband separations are designed and the results are consistent with the specifications. The design procedure for the triple-band Marchand balun filter is established to obtain the amplitude and phase balances in three operating bands with arbitrarily controlled center frequencies and fractional bandwidths. The proposed Marchand balun filter realized by the microstrip coupled-line sections and the defected ground structure stubs is measured and a good agreement among equivalent-circuit calculation, full-wave simulation, and measurement is observed, demonstrating the validity and versatility of the proposed design methodology and balun filter configuration.

This paper presents a direct approach to the synthesis of a general Chebyshev bandpass filter that matches to a frequency variant complex load. The approach is based on the power wave renormalization theory and two practical assumptions, which are: 1) the prescribed transmission zeros are stationary and 2) the reflection zeros are located along the imaginary axis. Three necessary conditions that stipulate the characteristic polynomials associated to the filter are derived through renormalization of the load reference impedances. It has been shown that these three conditions can only be satisfied by an ideal filter circuit model separated by a piece of interconnecting stub from the complex load. The length of the stub will be optimally designed in the sense that the designed filter will best match to the complex load over a given frequency range. The proposed method offers a deterministic, yet flexible way for optimally designing a diplexer or a multiplexer with a realistic loading effect. The effectiveness of the method is demonstrated by two design examples.

In this paper, an exact synthesis and design procedure has been proposed for a class of wideband dual-band bandpass filters (BPFs) with both controllable in-band ripple factor and isolation between the dual passband. Two multi-mode resonators (MMRs) with short-circuited stubs are parallel connected to form the basic structure of the proposed dual-band BPFs. Up to two pairs of transmission zeros (TZs) could be created with reference to the dual wide passbands. By choosing the impedances of the two paralleled MMRs, one pair of TZs could be moved from imaginary to real frequencies, whereas the second pair of TZs could be adjusted by folding one of the MMRs to form coupled lines. For these two prototypes, short-circuited stubs are utilized to excite the two paralleled MMRs, but not adequate to obtain a determined coupling degree. To provide an additional freedom for controlling the in-band ripple factor, a combination of short- and open-circuited stubs is used. A generic synthesis procedure is developed for the filter design. First, a generalized Chebyshev transfer functions with one TZ at the origin and symmetrical with respect to the origin has been discussed and frequency transferred to form a dual-band filtering function. Next, by equating of the dual-band circuit response and the filtering function, the circuit parameters could be obtained. Three filter prototypes have been designed and measured with good matching between these two sets of results.

For an efficient exploitation of the RF spectrum, very sophisticated analog-to-digital front-ends for modern cognitive-/software-defined radios featuring high sampling rate and dynamic range are required to properly handle broadband frequency-sparse signals. For this purpose, a receiver solution based on a mixed-analog/digital-domain channelizer can be advantageous, with added benefits regarding system reconfigurability and parallelization of multiband signal processing. Here, a novel receiver architecture using this approach for the commented application is described, with a special focus on the IF hybrid filter bank as its core element. The analog part of the hybrid filter bank is a 50–100-MHz eight-channel analog multiplexer inspired in the human hearing system. The associated digital block can be co-synthesized as a discrete-time filter bank to invert the action of the IF analog channelizer. As a benefit, hardware imperfections inherent to the analog circuitry can be easily counteracted at the digital level. Furthermore, such a channelization enables low-cost analog-to-digital converters to be utilized for the sampling of the signal sub-bands to be later digitally processed for the overall signal reconstruction. For validation, measured and simulated results of a proof-of-concept prototype for the analog channelizer are shown. Besides, the inversion of the measured transfer functions of this multiplexing device by means of the application of the co-designed digital filter bank is demonstrated. An experiment is also carried out showing the reconstruction of a band-limited noise signal with the developed hybrid filter bank.

The λ/2 strip-resonator directional filter has good balance between compactness and robustness against manufacturing error. However, there have been difficulties in generalizing the strip-resonator directional filter to realize multipole filtering responses. This paper presents the theory, experiments, and improvement of a new structure of strip-resonator directional filter based on a band-reject filter. This new structure allows the realization of multipole responses and theoretically does not limit the types of filter responses. A three-pole elliptic directional filter prototype is studied thoroughly to show that its performance can be predicted accurately by a conventional filter structure. We also propose a modification of the delay-line part using composite right-/left-handed transmission line to improve the isolation. An isolation level of 30 dB is shown to be achieved consistently.

A TE011Ka-band tunable filter with a stable and continuous tuning performance is presented in this paper. Both bandwidth and center-frequency tunability are demonstrated by cascading six-pole pseudo-low-pass and pseudo-high-pass tunable filters. A novel mode-splitter resonator and coupling configuration enabling cross-coupled planar TE011 filter realization is introduced in this paper. The concept can be applied to back-to-back coupled TE011 resonators as well. The filter design is verified through fabrication of multiple tunable filters that demonstrates 500 MHz of tuning range with a stable RF tuning performance.