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Synthesis and Design of Tunable Bandpass Filters with Constant Absolute Bandwidth Using Varactor-Loaded Microstrip Resonator
International Journal of RF and Microwave Computer-Aided Engineering
Abstract
This article presents two new types of tunable filters with constant absolute bandwidth using varactor-loaded microstrip resonators. First, the second- and third-order Butterworth tunable filters are designed based on the parallel coupled-line J inverters. Second, a fourth-order Chebyshev tunable filter is designed based on the alternative J/K inverters, in this design, two adjacent resonators are coupled with each other through a short-circuited transmission line as the K inverter. The proposed two topologies can be easily extended to high-order tunable filter. Three tunable bandpass filters with J and alternative J/K inverters, respectively, are built with a tuning range from ∼1.8 to ∼2.3 GHz. The measured second-order filter has a 3-dB bandwidth of 160 ± 6 MHz and an insertion loss of 2.4–3.8 dB. The third-order filter shows a 3-dB bandwidth of 197 ± 5 MHz and an insertion loss of 3.8–4.8 dB. The fourth-order filter shows a 3-dB bandwidth of 440 ± 5 MHz and an insertion loss of 2.1–2.6 dB. For all the designed filters, the measured results are found in excellent agreement with the predicted and simulated results. © 2014 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2014.
... The proposed tunable filter is comprised of two cascadedtrisection structures, and two transmission zeros are generated for improving the frequency selectivity; however, the structure of the proposed filter is complicated, and the insertion loss is large. In [14][15][16], fourth-order tunable filters with constant ABW are demonstrated; however, in order to maintain constant ABW, lumped capacitors are used to connect the feeding microstrip lines to the filter, which may increase the insertion loss of the tunable filters. In [17,18], fourth-order tunable BPFs with high selectivity are designed. ...
... However, these aforementioned low-order tunable filters (with two poles) usually suffer from poor selectivity. In order to improve the selectivity of filters, several high-order tunable filters are presented in [11][12][13][14][15][16]. In [11], a miniature tunable four-pole bandpass filter is demonstrated, the selectivity of the proposed filters is improved significantly compared with two-pole filters. ...
In this paper, a compact high-selectivity frequency tunable bandpass filter (BPF) with constant absolute bandwidth (ABW) based on varactor-loaded step-impedance resonators (SIRs) is presented. By introducing cross coupling between resonators, a pair of transmission zeros (TZs) close to the passband are produced and the selectivity of the filter is enhanced significantly. Another pair of TZs are generated to improve the out-of-band rejection by using source-load coupling. The varactor-loaded SIRs are utilized to design the compact fourth-order tunable BPF in order to realize wide tuning range and compact size. In addition, the frequency-dependent coupling feeding structures are employed instead of lumped capacitors used in conventional feeding structures, as a result, the insertion-loss performance is improved. The simulated and measured results are presented and show good agreement. The measured results exhibit a tuning range from 0.8 to 1.14 GHz with a 3 dB constant ABW of about 47 ± 5 MHz, the return loss of the filter is greater than 13.9 dB, and the insertion loss is about 2.7–3.1 dB. Moreover, four TZs are generated, and the proposed tunable filter shows high selectivity with a rectangular coefficient of 2.3–3.1.
... In fact, it is difficult in a resonator to simultaneously achieve a narrow passband for an improved quality-factor and wide stop-band characteristics for harmonic suppression. Thus, not all harmonic suppressing bandpass filters can improve the quality-factor (Q-factor) [9,[12][13][14]. Furthermore, although the external Q-factor is a major parameter of RBPFs, the group-delay peak-based Q-factor, which plays a vital role to reduce phase-noise in microwave LC tank-circuit oscillators, is often ignored by most of the conventionally designed RBPFs [4]. ...
... Although non-linearity in tuning arrangement often provides wide tuning ranges, such an irregular variation in tuning voltages leads to cumbersome operation. Nevertheless, both in non-linearly and linearly tunable RBPFs, large variation in insertion loss is found over their entire tuning ranges [12][13][14][15][16]. Whereas it is expected that insertion loss of RBPFs should be consistent especially for measurement and sensing applications [17][18]. ...
Radio frequency-based measurement components utilize differently designed resonators to adjust various filtering characteristics. We introduce a unique coupling mechanism of a novel even-branched resonator that alone can adjust 6 radiometric filtering characteristics i.e., bandpass operation, harmonic suppression, quality-factor enhancement, switching ability, tunability, and digitized coupling. In our design, even-branched resonators have inter-resonator magnetic coupling while each end of their branch-lines has intra-resonator electrical coupling. Using the prominent magnetic coupling, adjacently coupled- and diagonally coupled- resonators are developed. A short-circuited stepped-impedance parallel technique is applied to achieve enhanced quality-factor and switchable/tunable bandpass filtering features. Characteristics of our radiometric resonator are determined by the odd/even dependency on its branch number. Our resonator couples with its another identical unit only when it has an even number of branches and is coupled by branch-lines that are also even-numbered. With novel digitized coupling characteristics, our resonator provides a 6-in-1 solution for intelligent radiometric systems.
... But filter still possess insertion loss of 1.5 to 2.2 dB due to parasitic effect of RF MEMS. Coupled microstrip resonator with J/K inverter was modified by varactor-loaded semi-lumped metallic vias was proposed by [10]. Reconfigurability in bandwidth is about 440 MHz with insertion loss of 2.1 to 2.6 dB. ...
A compact, ultra-broadband multi-coupled transmission line band-pass filter (BPF) is demonstrated. The proposed multi-coupled transmission line is essentially designed by exploiting parallel-coupled line (PCL) structures. Technically, the proposed filter comprises multi-coupled transmission line and open-stub structures that are connected in a cascade topology. The higher (fH) and lower (fL) edge frequencies of the band are mainly related to the electrical length of the multi-coupled transmission line and the series open stub, respectively. The difference in even- and odd-mode phase velocities is reduced by using multi-coupled line structure acting as coupled CPW mode resonator. In addition, the stop-band rejection is enhanced through an incorporation of multi-coupled transmission line when compared to conventional PCL filter. Moreover, the stub loaded with active capacitor compensates for the loss of passive filter and helps to achieve reconfigurability in bandwidth along with varactor-loaded stub. The proposed filter design is verified through experimental demonstration. Compared with the classical PCL-BPFs, the proposed filter is of a relatively simple and compact configuration. The demonstrated multi-coupled transmission line BPF has about 12.2% 3-dB fractional bandwidth, sharp selectivity and great stop-band rejection. Simulated and measured result exhibits a stop-band rejection lower than − 16 dB till 20 GHz with passband gain of + 0.6 dB, bandwidth reconfigurability of 51 MHz and return loss of 20 dB. The BPF has much reduced circuit area of 43.4 × 31.8 mm2. It is implemented on the FR4 substrate with dielectric constant εr = 4.2, substrate height = 0.825 mm and thickness of the conductor = 0.017 mm.
... 1,2 The varactor diode, with its small size, low cost, and fast tuning, is superior to all other tuning technologies. [3][4][5] A wide stopband suppression, low insertion loss, high selectivity, and a wide tunability range are the key requirements for high-performance tunable RF filters. ...
This paper presents the design of a highly-tunable single-ended and balanced filter with improved upper stopband performance. The proposed filters are designed using a spurline and a U-shaped stepped impedance resonator (SIR) with a varactor diode connected to the open-circuited end. A spurline is introduced in both designs to achieve wide stopband performance by suppressing the higher-order modes. The proposed filters have high tunable ranges (single-ended: 2.35-3.99 GHz; balanced: 2.39-4 GHz), low insertion losses (IL) (single-ended: 0.07-0.2 dB; balanced: 0.21 to 1.1 dB) and wide upper stopband performance (single-ended: 3.79f 1 with a rejection level higher than 20 dB; balanced: 3.79f 1 with a rejection level higher than 20 dB). Performance of both filters are experimentally validated by comparing the experimental results with the simulated ones. Both filters can cover most of the fifth-generation (5G) wireless communication bands with different biasing voltages (capacitance). When both filters are compared with state-of-the-art filters, the proposed filters are found to achieve high tunable ranges with low IL and wide upper stopband performance. K E Y W O R D S 5G filters, balanced-filters, stepped impedance resonator, tunable filters, varactor diode, wide stopband
... Thus far, only a few numbers of high-order tunable filters with wide FTR have been reported to meet the frequency selectivity requirement. For example, directly loading the tuning element on the frequency-fixed filter resulting in the narrow-FTR tunable filter was reported in [13]- [17]; replacing the physical couplings by the high-loss tunable capacitors resulting in a bandwidth (BW)-and frequency-tunable filters with very complex design and control was reported in [18]- [21]; directly cascading two tunable dual-mode filters resulting in a four-pole tunable filter with relatively higher loss was reported in [22]- [24]. The fourth-and sixth-order tunable filters were reported in [25]- [27] by employing the folded or modified-folded topology but without the design generality. ...
This article proposes an advanced synthesis of the frequency-adaptive bandpass filter (FA-BPF) with constant absolute bandwidth (ABW). The proposed synthesis is based on the element-variable coupling matrix (EVCM), which is directly extracted from the classical filter polynomials. The extraction method is presented, investigated in detail, and confirmed by a set of numerical simulations. All frequency-dependent couplings between the resonators in an FA-BPF are uniquely modeled by the new coupling matrix, and the element variation of the EVCM directly accounts for the frequency tuning behavior of an FA-BPF. This one-to-one correspondence tackles the difficulty of the FA-BPF synthesis and design with a different center frequency tuning range (FTR), a different bandwidth (BW), and a different BW variation rate. This article also proposes the predefined coupling technique in the new synthesis. It allows both nonfrequency-dependent matrix and frequency-dependent matrix in an EVCM to be separately implemented in practice. For experimental demonstration, two four-pole/four-transmission zero (TZ) FA-BPFs with identical passband and constant ABW are implemented and then constitute a frequency-adaptive bandpass diplexer (FA-BPD) with two tunable channels that have identical passband and constant ABW. The measurement results exhibited performance superiority of the design and validated the proposed design method.
... Though varactor diodes have the drawbacks of low quality factor, poor linearity, and high driving voltage from 0V to 30V [Genc, 2009], they are widely used for building tunable filters as they have fast tuning speed, high tunability even at lower microwave frequencies, compact size, low cost and good reliability. [Huang, 2014] uses coupled line filters to achieve a tuning range of 500MHz when the working frequency is designed to 1.8GHz. The tuning range of 1.1GHz is realized in [Boutejdar, 2016], which is designed to work at 5GHz. ...
Liquid crystals (LCs) are a promising microwave material due to their advantages of low voltage tuning, low cost, low power consumption. The first objective of this thesis is to develop a general design method for tunable microstrip devices using LCs working at microwave frequencies for wireless communications. As a second objective, the method was then applied to design, simulate and fabricate three types of filter to achieve different bandwidths and to maximise the tuning range. A novel general design method for tunable microstrip devices based LCs is first proposed. The design process consists of two stages, the first uses a lumped element modelling and the second, full wave simulation to optimise the dimensions. The design process was tested and verified by designing tunable bandpass filters using ELC (Electric, inductive, capacitive) resonators. Three types of ELC resonators have been designed, simulated and fabricated. The comparison between the simulation and measured results shows that the proposed design method is effective, providing the electric field of the microwave signal is considered and is compatible with the LC switching. For the second objective, interdigital capacitor (IDC) filters and ring filters were investigated. The IDC filters were designed for 2.4GHz and 5GHz. Compared with LC devices working at 5GHz in the literature, the designed IDC filter using LCs has the largest tuning range. The tunable ring resonators were designed to have compact size and narrow bandwidth so that high frequency selectivity can be realized.
... Examples of tunable BPF in microstrip technology are reported in refs. [11][12][13] while an example of monolithically integrated band-stop filter is reported in ref. [14]. In this work, a highly compact varactor-tuned LC parallel tank was employed, as shown in Figure 2A. ...
This work presents the first example of monolithically integrated phase shifter based on a pass‐band filter architecture. The proposed configuration was realized mapping a classical quarter‐wave coupled filter circuit into its lumped element equivalent. Phase control is achieved by controlling the pass‐band through tunable tanks employing varactor diodes. A demonstrator was prototyped in the 24 GHz ISM band using a 0.25μm SiGe BiCMOS technology. Experimental results show 180° of phase range and maximum transmission losses of 8 dB. The main feature of this configuration is that it allows controlling the transmission losses by design and that its size is extremely compact.
... In many applications, interconnection mismatching may deteriorate performance with the cascaded design. In order to reduce circuit size and improve performance, extensive examples of filtering power dividers (FPDs), 7,8 which are the co-designs of filters and PDs, with various advantages are explored. ...
In this article, a balanced filtering power divider (FPD) that allows for operational agility of the bandwidth (BW) is presented. The differential‐mode power dividing and high common‐mode (CM) suppression can be realized by microstrip‐to‐slotline transition. Two slotline open stubs with different lengths are added in shunt to the main slotline for the transition, which can not only introduce transmission poles for extending and controlling transition BW, but also generate two extra transmission zeros (TZs) near to the passband edges, featuring good filtering response. The two transmission poles can be independently tunable by loading varactors to the open ends of slotline stubs and two TZs will be changed accordingly so that the filtering passband BW is electrically tunable. To verify the theoretical prediction, a prototype of tunable balanced FPD is fabricated and measured. The measured results show that the 3‐dB fractional bandwidth (FBW) of the passband varies from 5.6% to 12.6%, meaning more than a double tuning range for the FBW, and the CM suppression is better than 40 dB across the frequency band of interest.
... Planar filters can be very suitable for reconfigurable/switchable filters due to its characteristic of easy fabrication on the printed circuit board and integration with RF tuning components. [4][5][6][7][8][9][10][11][12] The research of reconfigurable filters can overall be concluded into center frequency tunability and BW tunability. The center frequency of the bandpass filter can be changed in the result of altering the valid electronic length of the resonator under the effect of tuning components. ...
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.
... With the reduction of the system size, complexity and cost, electronically tunable pre-selected microwave filters improve the overall performance of multiband and multifunction wireless communication and smart radar systems. Tunable microwave bandpass filters (BPFs) can be realized by varactor diodes [1], ferroelectric diodes [2], or RF micro-electromechanical systems (RF-MEMS) devices [3]. Moreover, in order to meet the needs of different communication frequency bands. ...
In this article, a quadruple-mode stub-loaded resonator (QM-SLR) is introduced and its four modes are excited using a simple approach, which can provide a dual-band behavior. By changing the length of the loaded stubs, independently tunable transmission characteristics of the proposed quadruple-mode stub-loaded resonator were extensively described for filter design. Moreover, microwave varactors were adopted to represent the length variation of the loaded stubs for the dual-band tunability. The equivalent circuit modeling of the open stub with microwave varactor was given and discussed. Then, adopting the compact quadruple-mode stub-loaded resonator with three varactors, an independently controllable dual-band bandpass filter (BPF) was designed, analyzed, and fabricated. Its separated bandwidths and transmission zeros can be tuned independently by changing the applying voltage of the microwave varactors. A good agreement between simulated and measured results verified the design methodology. The proposed filter possesses compact size, simple structure, and excellent dual-band performances.
This paper presents a dual‐mode tunable bandpass filter (BPF) for global system for mobile communication, universal mobile telecommunications system, wireless fidelity, and worldwide interoperability for microwave access standard applications. The proposed filter consists of a stepped‐impedance resonator, single resonator, and coupled line, which are loaded with varactors. The center frequency and bandwidth of the proposed filter can be tuned with tuning varactors. Furthermore, the measurement results show that the BPF can be tuned over the frequency range of 1.8 to 2.5 GHz. Moreover, the bandwidth can be changed at each certain frequency. Furthermore, using PIN diodes, a bandstop filter is added to the tunable BPF to reduce the out‐of‐band frequencies around the desired frequencies. The values of LC equivalent circuits are calculated, and the results are compared with those obtained from the layout of the proposed structure. Finally, the measurement results justify the simulation results. This paper presents a dual‐mode tunable bandpass filter for global system for mobile communication, universal mobile telecommunications system, wireless fidelity, and worldwide interoperability for microwave access standard applications. The proposed filter consists of a stepped‐impedance resonator, single resonator, and coupled line, which are loaded with varactors. The center frequency and bandwidth of the proposed filter can be tuned with tuning varactors.
This paper presents a systematic design procedure of microstrip higher order tunable bandpass filters (BPFs) that can realize a constant absolute frequency bandwidth (ABW) and transmission zeros (TZs) with the minimum number of varactors and a single controlled dc-bias voltage. To this end, intrinsic frequency dependence of interresonator couplings is employed instead of external controls, but the design has faced a difficulty of realizing ideal frequency curves of coupling coefficients at all the coupling regions, including nonadjacent couplings. To overcome such a difficulty, this paper introduces a frequency-curve shape design approach considering upward- or downward-curved convex that is dependent on the types of interresonator coupling. As illustrative examples, fourth- and sixth-order tunable BPFs are proposed and designed, starting from a coupling-matrix synthesis. Their structural parameters can be efficiently designed with the proposed design method. The frequency agility with keeping a constant ABW, acceptable low insertion loss, a good in-band return loss level, and TZs is numerically and experimentally demonstrated for both fourth- and sixth-order tunable BPFs.
This article presents a wideband tunable balanced bandpass filter (BPF) with stable passband selectivity using optimized varactor‐loaded half‐guided wavelength stepped impedance resonator (SIR). By overall investigation, it can be found that the frequency‐tuning range of the varactor‐loaded SIR is not only controlled by the variation range of the loaded capacitor, but it can be widened by optimizing the SIR. The coupling region between the two coupled SIRs in the proposed BPF is studied for achieving the constant absolute bandwidth (ABW) during the frequency‐tuning process. Meanwhile, the frequency‐dependent source‐to‐load coupling is introduced to realize two self‐adaptive transmission zeros (TZs) on both sides of the tunable differential‐mode (DM) passband so that the proposed filter can keep high selectivity. Benefiting from the constant ABW and two self‐adaptive TZs, the stable DM passband selectivity of the balanced tunable filter can be obtained. For demonstration, a balanced tunable BPF based on the optimized varactor‐loaded SIRs is designed, fabricated and measured. The simulated and measured results, showing good agreement, exhibit a tuning range from 0.542 GHz to 0.926 GHz with 3‐dB constant ABW of around 105 MHz and in‐band insertion loss of 1.5‐2.56 dB.
This paper presents an electrically tunable bandpass filter which provides nearly constant absolute bandwidth (CABW) within the center frequency tuning range. The proposed tunable band pass filter (BPF) is designed on a low cost FR4 substrate with mixed coupled 3rd order open loop ring resonators, high frequency varactor diodes, chip capacitors and some RF chokes. The center frequency of the BPF is tunable from 1.9 GHz to 2.52 GHz with 3 dB absolute bandwidth of 201.5±1.5 MHz. The measured insertion loss of the proposed BPF is found less than 3 dB within the tuning range. There is a good agreement found between the simulated and measured results of the proposed BPF.
A varactor-based tunable diplexer with wide tuning range is proposed in this paper. The proposed diplexer consists of two varactor-loaded open-loop resonator filters. Size reduction and wide tuning range are achieved by using the varactor-loaded open-loop resonator. A simple microstrip T junction at the input and two filters responsible for the respective passbands is applied. Simulated insertion loss of our designed diplexer is obtained between 1.3 to 2.1 and 1.2 to 2.7 dB over the tuning range for Tx and Rx channels filter bands, respectively. Both filters exhibit better than 30 dB rejection in the passband of its counterpart. The varactor diode attached to both filters allow 48% and 50% center frequency tunbility in Tx and Rx filters bands, respectively.
This paper presents a two-pole dual-band tunable bandpass filter (BPF) with independently controllable dual passbands based on a novel tunable dual-mode resonator. This resonator principally comprises a λ/2 resonator and two varactor diodes. One varactor is placed at the center of the resonator to determine the dominant even-mode resonant frequency; the other is installed between two ends of the resonator to control the dominant odd-mode resonant frequency. These two distinct odd- and even-mode resonances can be independently generated, and they are used to realize the two separated passbands as desired. Detailed discussion is carried on to provide a set of closed-form design equations for determination of all of the elements involved in this tunable filter, inclusive of capacitively loaded quarter-wavelength or λ/2 resonators, external quality factor, and coupling coefficient. Finally, a prototype tunable dual-band filter is fabricated and measured. Measured and simulated results are found in good agreement with each other. The results show that the first passband can be tuned in a frequency range from 0.77 to 1.00 GHz with the 3-dB fractional-bandwidth of 20.3%-24.7%, whereas the second passband varies from 1.57 to 2.00 GHz with the 3-dB absolute-bandwidth of 120 ± 8 MHz.
A compact topology for bandpass filters based on coupled slow-wave resonators is demonstrated. A study of fixed bandpass filters leads to design rules and equations. Measurements on a 0.7-GHz fixed bandpass filter, consisting of three coupled slow-wave resonators, demonstrate the validity of the proposed topology and validate the theory, since the agreement between simulations and measurements is very good. Designed for a Q-factor of 5, this filter shows a Q of approximately 5.2. At the center frequency, insertion loss is 0.6 dB and return loss is greater than 20 dB. A 0.7-GHz tune-all bandpass filter is also designed and tested. The performance of this electronically tuned filter, which incorporates semiconductor varactors, is promising in terms of wide continuous center-frequency and bandwidth tunings. For a center-frequency tuning of plusmn18% around 0.7 GHz, the -3-dB bandwidth can be simultaneously tuned between ~50 and ~78 MHz, with an insertion loss smaller than 5 dB and a return loss greater than 13 dB at the center frequency. The surface areas of the fixed and tunable 0.7-GHz filters are, respectively, ~16 and ~20 cm<sup>2</sup>
This paper describes a discretely tunable filter topology based on lumped-distributed coupled transmission lines, particularly suitable for microelectromechanical systems switching devices. This topology is capable of simultaneous wide-band center frequency and bandwidth tuning, limited only by the electrical size of the transmission lines and the placement density of the switching devices. Low fractional bandwidths can be achieved without the need for large coupled-line spacings due to the antiphase relationship of the lumped capacitive and distributed electromagnetic coupling coefficients. The positions of the additional poles of attenuation due to the lumped capacitive coupling can be selected either above or below band leading to the choice of a narrow bandwidth design having good high-side performance or a design with compromised upper stopband performance, but with no bandwidth tuning limitations. The interaction between a pair of lumped-distributed coupled transmission lines is analyzed and the resulting model is used to develop a filter synthesis procedure. The synthesis procedure and filter performance are validated through theoretical and experimental comparisons using a filter with low-side attenuation poles
This paper proposes a unified synthesis method for the bandpass filters based on the alternative J/K inverters and λ/4 microstrip line resonators. Firstly, an even-order Chebyshev bandpass filter prototype with equal terminations is transformed to two dual networks based on hybrid J/K inverters and λ/4 resonators. In this work, two adjacent λ/4 resonators are coupled with each other through a semi-lumped via-hole as the K inverter; the tailored method is employed and effectively demonstrated to extract the J/K inverters' values and their respective associated effective lengths. As its application examples, the proposed synthesis method is applied to design two classes of even-order Chebyshev bandpass filters with fourth and sixth order on microstrip-line topology, which operate at 2.4 GHz with the fractional bandwidths of 10% and 15%, respectively. Finally, both classes of filters are fabricated and measured. Simulated and measured results provide a good verification for the theoretical counterparts.
In this paper, a tunable bandpass filter using cross-shaped multiple mode resonators (MMRs) and N:1 transformer based external quality factor tuning structures is proposed. The use of a cross-shaped MMR simplifies inter-resonators control while two Qe tuning structures are investigated and incorporated with the MMR to implement simultaneous center frequency agility and narrow and wide bandwidth tuning. Compared with traditional tunable filters, the proposed architecture requires less tuning elements and is easier to realize wideband and high-order tunable filters. Two examples (Filter I and II) are presented to validate the design. Both filters use a single MMR and six tuning elements to achieve a third-order wideband tunable filter. Filter I reports 58% center frequency tuning with constant bandwidth and 14%-64.4% fractional bandwidth (FBW) tuning when center frequency locates at 1 GHz. Filter II achieves larger frequency agility and wider FBW tuning of 82.9% and 95%, respectively, for the same bandwidth and center frequency of Filter I.
Intrinsically switched tunable filters are switched on and off using the tuning elements that tune their center frequencies and/or bandwidths, without requiring an increase in the tuning range of the tuning elements. Because external RF switches are not needed, substantial improvements in insertion loss, linearity, dc power consumption, control complexity, size, and weight are possible compared to conventional approaches. An intrinsically switched varactor-tuned bandstop filter and bandpass filter bank are demonstrated here for the first time. The intrinsically switched bandstop filter prototype has a second-order notch response with more than 50 dB of rejection continuously tunable from 665 to 1000 MHz (50%) with negligible passband ripple in the intrinsic off state. The intrinsically switched tunable bandpass filter bank prototype, comprised of three third-order bandpass filters, has a constant 50-MHz bandwidth response continuously tunable from 740 to 1644 MHz (122%) with less than 5 dB of passband insertion loss and more than 40 dB of isolation between bands.
This paper presents a novel approach to the design of tunable dual-band bandpass filter (BPF) with independently tunable passband center frequencies and bandwidths. The newly proposed dual-band filter principally comprises two dual-mode single band filters using common input/output lines. Each single BPF is realized using a varactor-loaded transmission-line dual-mode resonator. The proposed filter also offers switchable characteristics to select either of the passbands (either the first or the second passband only). To suppress the harmonics over a broad bandwidth, defected ground structures are used at input/output feeding lines without degrading the passbands characteristics. From the experimental results, it was found that the proposed filter exhibited the first passband center frequency tunable range from 1.48 to 1.8 GHz with a 3-dB fractional bandwidth (FBW) variation from 5.76% to 8.55% and the second passband center frequency tunable range from 2.40 to 2.88 GHz with the 3-dB FBW variation from 8.28% to 12.42%. The measured harmonic results of the proposed filters showed a rejection level of 19 dB, which is up to more than ten times of the highest center frequency of the first passband without degradation of the passbands.
In this paper, two- and three-pole microstrip LC constant fractional bandwidth (CFBW) and constant absolute bandwidth (CABW) tunable bandpass filters are proposed. The equivalent-circuit models are presented to study the tunable mechanism. The filter can be reconfigured by changing the capacitance of the LC resonators. Based on electric coupling coefficient compensation, second- and third-order tunable equivalent-circuit models with CFBW/CABW are presented. For demonstration, a semiconductor varactor diode loaded microstrip LC resonator is adopted in our work to design the second- and third-order tunable filters. The S-parameters, group delays, and third-order intercept points for different center frequencies of the filters are presented. Each resonator requires only one varactor diode for both central frequency and resonator coupling coefficient control.
This paper presents a four-pole elliptic tunable combline bandpass filter with center frequency and bandwidth control. The filter is built on a Duroid substrate with epsilon(r) = 10.2 and h = 25 mils, and the tuning is done using packaged Schottky diodes. A frequency range of 1.55-2.1 GHz with a 1-dB bandwidth tuning from 40-120 MHz (2.2-8% fractional bandwidth) is demonstrated. A pair of tunable transmission zeroes are synthesized at both passband edges and significantly improve the filter selectivity. The rejection level at both the lower and upper stopbands is > 50 dB and no spurious response exists close to the passband. The measured third-order intermodulation intercept point (TOI) and 1-dB power compression point at midband (1.85 GHz) and a bandwidth of 110 MHz are > 14 dBm and 6 dBm, respectively, and are limited by the Schottky diodes. It is believed that this is the first four-pole combline tunable bandpass filter with an elliptic function response and center frequency and bandwidth control. The application areas are in tunable filters for wireless systems and cognitive radios.
A three-pole microstrip combline filter with a center frequency of 1.75–2.25 GHz and a tunable bandwidth of 70–100 MHz is presented. In this design, an extra transmission zero is created using a non-adjacent resonator coupling, and is placed on the lower side of the passband to improve the filter selectivity. The filter bandwidth is also tuned by controlling the non-adjacent resonator coupling. The measured $S$ -parameters, third-order intercept point and power handling, for different center frequencies and bandwidths, are presented and with good agreement with simulations. This topology is simple to implement and requires only one additional tunable component between the first and third resonator for bandwidth control.
This paper proposes a varactor-tuned microstrip combline bandpass filter (BPF) that is loaded with lumped series resonators instead of the short-circuited end of combline. This configuration has the advantage of enhancing the tunability of the suggested BPF by varactor diodes in series resonators, which leads to a wide tuning range even with a small capacitance ratio $(C_{\rm ratio})$ of the varactor. Therefore, a low controlled bias voltage varactor-tuned BPF can be achieved. In addition, due to the controllable slope parameter of the proposed resonator, the coupling between resonators can be controlled. As a result, the proposed tunable BPF can keep nearly constant bandwidth over the wide tuning range. The insertion loss resulting from the varactors is compensated using an active capacitance circuit with negative resistance. Theoretical analysis and design approach are described. Experimental results of a demonstrative BPF show a 500-MHz tuning range (1.7–2.2 GHz) with $C_{\rm ratio}$ of only 2.6 (1–5-V bias voltage) while maintaining nearly constant absolute bandwidth and low insertion loss. The measured performance of the fabricated filter shows good agreement with the simulated one.
This paper presents a compact highly linear tunable second-order quasi-elliptic filter with constant 3-dB bandwidth. The proposed filter is thoroughly analyzed to clearly describe the filter equivalent circuit and the tuning mechanism involved. In addition, the tunable resonator configuration employed is shown to improve filter linearity, especially for low bias voltages where distortion is normally stronger. A quasi-elliptic tunable filter was designed, built, and tested for illustration and verification. With a 3-dB bandwidth variation of only 4.6%, the filter had a frequency coverage from 1.45 to 1.96 GHz, an insertion loss better than 2.5 dB, and measured dBm throughout. The experimental results are in excellent agreement to theory and simulations.
In this paper, we propose and develop a partially shielded tunable filter structure using a varactor-loaded split-ring resonator. The novel physical topology of the filter is suitable for vertical stacking of filters and enables a compact multilayer tunable filter for applications requiring a very wide tuning range. The capacitance range and ratio needed to achieve the designated tuning range are presented, and analytical solutions are given for the second and third higher order modes for synthesis in the spu- rious-free filter operation range. Two prototypes are developed. A partially shielded tunable filter using varactor-loaded split-ring resonators is developed to verify the design method and to demon- strate the wide tuning range of a tunable filter. To further extend the tuning range, a filter bank with three stacked tunable filters at different frequency bands are designed and developed. With slight overlap between neighboring bands, a continuous tuning range of more than 6:1 is achieved.
This paper presents a novel approach to the design of tunable dual-band bandpass filter with broadband harmonic suppression characteristics. The proposed filter structure offers the possibility of two tunable passbands, as well as a fixed first passband and controllable second passband. The tunable passband frequency usually causes a shift of the harmonics, which need to be suppressed to improve out-of the passband characteristics. In order to suppress the harmonics over a broad bandwidth, defected ground structures are used at input and output feeding lines without degrading the passbands characteristics. Both theory and experiment are provided to validate the proposed filter. From the experimental results, it is found that the proposed filter exhibits a first passband center frequency tunable range of 34.14% from 0.85 to 1.2 GHz with the almost constant 3-dB fractional bandwidth (FBW) of 13% and second passband center frequency tunable range of 41.81% from 1.40 to 2.14 GHz with the 3-dB FBW of 11%. The measured results of the proposed filters show a rejection level of 20 dB up to more than ten times of second passband frequency can be obtained, thereby ensuring broad harmonics rejection characteristics without degradation of passbands. The measurement data have good agreement with the simulation.
This paper presents a three-pole tunable combline bandpass filter with center frequency, bandwidth, and zero control. The filter is designed on a Duroid substrate with ε r =10.2 and h =25 mil . A frequency range of 1.5-2.2 GHz with a 1-dB bandwidth tuning from 50 to 170 MHz (2.2%-11.2% fractional bandwidth) is achieved. The transmission zero can also be controlled, and a zero location of 1.37-1.64 GHz is demonstrated at center frequency fo of 2.05 GHz. The measured third-order intermodulation intercept point and 1-dB power compression point at midband (1.85 GHz) and a bandwidth of 110 MHz are >;15 and 8 dBm, respectively. To our knowledge, this is the first three-pole combline tunable bandpass filter with center frequency, bandwidth, and transmission zero control.
This paper presents a high- Q two-pole tunable filter with frequency and bandwidth control. The filter is implemented using suspended-stripline ring resonators and planar RF microelectromechanical systems capacitive networks. A tuning range of 3.7-5.95 GHz with an absolute 1-dB bandwidth of 110±20 MHz (3.6%-1.7% fractional bandwidth) is first demonstrated with an insertion loss <; 2.5 dB . The bandwidth can be controlled at any frequency, and a 1-dB bandwidth of 90-515 MHz is also demonstrated at 5.5 GHz. The measured Qu is 115-250 at 3.7-5.95 GHz. The measured third-order intermodulation intercept point and 1-dB power compression point are >; 35 dBm(Δ f = 300 kHz) and 20 dBm, respectively. To our knowledge, this filter represents the state-of-the-art in tuning performance and Q for planar designs at 2-6 GHz.
This paper presents a new type of varactor-tuned dual-mode bandpass filter. Since the two operating modes (i.e., the odd and even modes) in a dual-mode microstrip open-loop resonator do not couple to each other, the tuning of the passband frequency becomes simple with a single dc-bias circuit while keeping nearly constant absolute bandwidth. Design equations and procedures are derived, and two two-pole tunable bandpass filters of this type are demonstrated experimentally.
This paper presents high-performance RF microelectromechanical systems (RF MEMS) tunable filters with constant absolute bandwidth for the 1.5-2.5-GHz wireless band. The filter design is based on corrugated coupled lines and ceramic substrates (ε<sub>r</sub>=9.9) for miniaturization, and the 3-bit tuning network is fabricated using a digital/analog RF-MEMS device so as to provide a large capacitance ratio and continuous frequency coverage. Narrowband (72 ± 3 MHz) and wideband (115 ± 10 MHz) 1-dB bandwidth two-pole filters result in a measured insertion loss of 1.9-2.2 dB at 1.5-2.5 GHz with a power handling of 25 dBm and an IIP<sub>3</sub> >> 33 dBm. The filters also showed no distortion when tested under wideband CDMA waveforms up to 24.8 dBm. The designs can be scaled to higher dielectric-constant substrates to result in smaller filters. To our knowledge, these filters represent the state-of-the-art at this frequency range using any planar tuning technology.
This paper presents a novel approach to design frequency-agile bandpass filters with constant absolute bandwidth and passband shape, as well as a suppressed second harmonic. A novel mixed electric and magnetic coupling scheme is proposed to control the coupling coefficient variation. Theoretical analysis indicates that it is able to achieve desired coupling coefficients between the proposed resonators at various frequencies so as to obtain constant absolute bandwidth. Moreover, this half-wavelength resonator has a Q higher than the quarter- and half-wavelength counterparts, thus resulting in low insertion loss. A filter of this type is designed to validate the proposed idea. To remove the spurious responses of the filter, a method is then introduced to suppress the second harmonic without degrading the passband performance. For demonstration, two frequency-agile filters with 60- and 80-MHz constant absolute bandwidth are implemented with the frequency tuning range from 680 to 1000 MHz. Comparisons of experimental and simulated results are presented to verify the theoretical predications.
This paper presents corrugated coupled lines for miniaturized fixed and tunable microstrip bandpass filters. The novel approach uses microstrip corrugated coupled-line concept to synthesize a coupling coefficient, which maintains a nearly constant absolute bandwidth across the tuning range. A miniaturized two-pole varactor tuned filter is demonstrated with a frequency coverage of 1.32-1.89 GHz and an insertion loss < 3 dB with a constant 1-dB bandwidth of 70 ?? 4 MHz across the tuning range. In addition, a three-pole comb-line 4.7% fixed filter at 1.94 GHz shows a 3:1 resonator spacing reduction over the conventional approach with an insertion loss of only 1.1 dB. This technique will allow the design of miniaturized small bandwidth fixed and tunable microstrip filters.
This paper presents harmonic-suppressed tunable bandpass filters with two movable transmission zeros. For tunable bandpass filters, tuning the passband frequency will cause the harmonic to shift, complicating the harmonic suppression. To overcome this problem, lumped elements are utilized to realize harmonic suppression without degrading passband performance. It is found from theoretical analysis that at even-order harmonic frequencies, the lumped elements could not only decrease the resonator Q and dissipate RF power, but also control even-order impedance and cause mismatching at filter input/output ports. Both of the factors help reject even-order harmonics. Meanwhile, the features at fundamental resonant frequencies are nearly not affected by these elements, indicating harmonic suppression could be achieved without affecting passband performance. This property is experimentally verified by comparing the responses of tunable bandpass filters with and without harmonic suppression. Finally, a harmonic-suppressed tunable bandpass filter with constant bandwidth and passband shape is designed. A novel input and output coupling structure with a bandpass response is employed to maintain constant bandwidth and help reject both even- and odd-order harmonics. For each tuning state, two transmission zeros are created near the passband, ensuring high selectivity.
This paper presents a miniature high-Q tunable evanescent-mode cavity filter using planar capacitive RF microelectromechanical system (MEMS) switch networks and with a frequency coverage of 4.07-5.58 GHz. The two-pole filter, with an internal volume of 1.5 cm<sup>3</sup>, results in an insertion loss of 4.91-3.18- and a 1-dB bandwidth of 17.8-41.1 MHz, respectively, and an ultimate rejection of > 80 dB. RF-MEMS switches with digital/analog tuning capabilities were used in the tunable networks so as to align the two poles together and result in a near-ideal frequency response. The measured Q<sub>u</sub> of the filter is 300-500 over the tuning range, which is the best reported Q using RF-MEMS technology. The filter can withstand an acceleration of 55-110 g without affecting its frequency response. The topology can be extended to a multiple-pole design with the use of several RF-MEMS tuning networks inside the evanescent-mode cavity. To our knowledge, these results represent the state-of-the-art in RF-MEMS tunable filters.
This chapter presents a comprehensive treatment of subjects regarding coupled resonator circuits. The subjects cover the formulation of the general coupling matrix, which is of importance for representing a wide range of coupled-resonator filter topologies, the general theory of couplings for establishing the relationship between the coupling coefficient, and the physical structure of coupled resonators. This leads to a very useful formulation for extracting coupling coefficients from EM simulations or measurements. Formulations for extracting the external quality factors from frequency responses of the externally loaded input/output resonators are derived next. Numerical examples are provided to demonstrate how to use these formulations to extract coupling coefficients and external quality factors of microwave coupling structures for filter designs.
Low-loss tunable filters with three different fractional-bandwidth variations were designed and fabricated on epsiv<sub>r</sub> = 2.2, 0.787 mm Duroid substrates for 850-1400-MHz applications. A detailed analysis for realizing predefined bandwidth characteristics is presented, and a design technique to take into account the source and load impedance loading is discussed. It is found that independent electric and magnetic coupling makes it possible to realize three different coupling coefficient variations with the same filter structure. The proposed topology is different from the comb-line design in that all three filters have identical electrical lengths, the same varactors, and the same filter Q values. Three different filters are built using Schottky varactor diodes with a tuning range from ~ 850 to ~ 1400 MHz. The constant fraction-bandwidth filter has a 1-dB bandwidth of 5.4%plusmn0.3% and an insertion loss of 2.88-1.04 dB. The decreasing fractional-bandwidth filter shows a 1-dB bandwidth decrease from 5.2% to 2.9% with an insertion loss of 2.89-1.93 dB (this is effectively a 40-45-MHz constant absolute-bandwidth filter). The increasing fractional-bandwidth filter shows a 1-dB bandwidth increase from 4.3% to 6.5% with an insertion loss of 3.47-1.18 dB. The measured Q of the filters are between 53-152 from ~ 850 to ~ 1400 MHz. The measured third-order intermodulation intercept point ranges from 11.3 to 20.1 dBm depending on the bias voltage. To our knowledge, these planar tunable filters represent state-of-the art insertion-loss performance at this frequency range.
A general-purpose circuit model of a microstrip interdigital
capacitor (IDC) is presented in this paper for use in the design of new
quasi-lumped miniaturized filters. This computer-aided-design-oriented
model is developed as a versatile admittance π-network with the
short-open calibration technique that we have recently proposed for
accurate parameter extraction of a circuit from its physical layout.
This technique is self-contained in our method of moments, which
accounts for frequency dispersion and fringing effects. A J-inverter
topology is further conceived to explicitly formulate the coupling
behavior of three types of IDC's. This model provides a unique way for
the IDC-related circuit synthesis and optimization based on the accurate
equivalent-circuit network extracted from the field theory algorithm. It
is validated theoretically and experimentally through an example of a
line resonator connected with two IDC's. The proposed scheme is used in
the design and optimization of new low-loss miniaturized quasilumped
integrated circuits, namely, two types of three-pole direct-coupled
bandpass filters. Our measured and predicted results show interesting
features of the proposed filter structure such as size reduction and
suppression of harmonic resonance if the line resonator is attached by
series-connected equivalent inductance
tunable filters for wireless applications
- Rf-Mems Ghz
GHz RF-MEMS tunable filters for wireless applications, IEEE Trans Microwave Theory Tech 58 (2010), 1629–1637.
- G Chaudhary
- Y Jeong
- J Lim
G. Chaudhary, Y. Jeong, and J. Lim, Dual-band bandpass filter with independently tunable center frequencies and bandwidths, IEEE Trans Microwave Theory Tech 61 (2013), 107-116.
- X G Huang
- L Zhu
- Q Y Feng
- Q Y Xiang
- D H Jia
X.G. Huang, L. Zhu, Q.Y. Feng, Q.Y. Xiang, and D.H. Jia,
Tunable bandpass filter with independently controllable dual
passbands, IEEE Trans Microwave Theory Tech 61 (2013),
3200-3208.
- M Sanchez-Renedo
- R Gomez-Garcia
- J I Alonso
- C Briso-Rodriguez
M. Sanchez-Renedo, R. Gomez-Garcia, J.I. Alonso, and C.
Briso-Rodriguez, Tunable combline filter with continuous
control of center frequency and bandwidth, IEEE Trans
Microwave Theory Tech 53 (2005), 191-199.
- Hong