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In this paper, we propose a dual-mode frequency selective rasorber (FSR) with tunable transmission and absorption windows at the X-band, which shows a broad passband in each transmission window. The proposed tunable FSR consists of a lossy absorption layer, a lossless transmission layer, and an air gap between them. The top frequency selective surface (FSS) layer is a cross-shaped meandering line with resistors and varactors for tunable absorption, and the bottom layer is a cross-shaped gap with varactors to achieve tunable bandpass. The equivalent circuit model (ECM) is investigated, and the 3D full wave simulation is performed. The results are based on simulations, and the simulation results show that the passband can be tuned from 12 to 8  G H z  with an insertion loss between 0.5 and 1.4  d B  by sweeping the capacitance of the varactors. The proposed design decreases the chances of detection by adversary devices and assures spectrum-safe communication, thereby creating new avenues for radar stealth and target concealment. [ABSTRACT FROM AUTHOR]

This study proposes a high-temperature superconducting (HTS) bandpass filter with a continuously tunable bandwidth and center frequency. The proposed filter combines several gallium arsenide varactors and a dual-mode resonator (DMR). The even and odd modes of the DMR can be tuned simultaneously using a single bias voltage. The capacitive value of varactors in the circuit is tuned continuously under continuous voltage and frequency tunability. External couplings and the interstage can be realized using an interdigital coupling structure; a fixed capacitor is added to the feeder to improve its coupling strength. A low-insertion loss within the band is obtained using HTS technology. Additionally, the proposed filter is etched on a 0.5 mm-thick MgO substrate and combined with YBCO thin films for demonstration. For the as-fabricated device, the tuning frequency range of 1.22~1.34 GHz was 9.4%; the 3-dB fractional bandwidth was 12.95~17.39%, and the insertion loss was 2.28~3.59 dB. The simulation and experimental measurement results were highly consistent. [ABSTRACT FROM AUTHOR]

To meet the demand for simultaneously supporting the microwave and millimeter-wave technologies in emerging industrial wireless systems, research on dual-band components with a large frequency ratio has been conducted recently. However, it is challenging to implement independent frequency tunability on bandpass filters with both compactness and high performance. This article presents a dual-band bandpass filter with a large frequency ratio, independently tunable frequencies, and an ultrawide stopband. This is achieved by artfully incorporating a harmonic-controllable varactor-loaded microstrip BPF and a wide-stopband dielectric-loaded cavity BPF with the bandgap-embedded aperture coupling mechanism. Extremely compact size can be achieved without additional filtering elements. Two pairs of transmission zeros can be allocated beside two passbands for good selectivity due to the separate path and cross-coupling in the resonators. For validation, a prototype is designed, fabricated and measured. It can provide two independent tunable frequency ranges (3.22–3.52 GHz and 25–27.06 GHz), exhibiting a large frequency ratio up to 8.3. Mid-band suppression better than 20 dB and ultra-wide stopband up to 32.5 GHz with 20 dB suppression level are achieved. Low insertion loss and good selectivity can also be observed. [ABSTRACT FROM AUTHOR]

This article presents a dual‐band bandpass (BP) to bandstop (BS) switchable filter with independently tunable center frequency and bandwidth. Two different length half‐wavelength resonators are coupled with two separate coupling lines. Each resonator consists of a varactor‐loaded stub at the center to tune the even mode frequencies, whereas two identical varactors are connected at the ends to tune odd and even mode frequencies simultaneously. The center frequency of the first passband is tuned from 1.03 to 1.4 GHz with constant fractional bandwidth (CFBW) of 3.4%, and second passband from 1.35 to 1.74 GHz with CFBW of 2%. BP to BS switching is achieved by connecting a p‐i‐n diode between input and output feedlines. [ABSTRACT FROM AUTHOR]

A compact differential tunable dual-band bandpass filter is achieved by using cascaded capacitor-loaded coupled microstrip lines and voltage-controlled varactors. Six differential-mode (DM) transmission zeros (TZs), four DM transmission poles (TPs), and four common-mode (CM) TZs can be obtained by controlling values and positions of capacitors and the even-odd mode impedances of coupled microstrip lines. What's more, the first three DM TZs, two TPs and three CM TZs will change with the capacitance of varactors. Thus, the proposed filter possesses a tunable lower passband and a fixed upper passband. Compared with the state-of-the-art differential tunable DBBFs, the proposed design has the advantages of wide tuning range, compact size, low number of control voltages, and simple structure. Theoretic analysis, parametric study and design procedure are carried out completely. A prototype with the size of 58.2 × 10.1 mm (1.13 λg × 0.20 λg, λg is the microstrip guided wavelength at the center frequency of the upper passband) is designed for demonstration, which exhibits the center frequency of the lower passband can be tuned from 1.55 to 2.42 GHz. Meanwhile, the center frequency and 3-dB bandwidth of the upper passband keep constant at 3.79 GHz and 8.5%, respectively. [ABSTRACT FROM AUTHOR]

This paper presents a bandwidth and center frequency tunable bandpass filter with wide tuning range. The proposed filter employs two cascade cross‐shaped resonators. The cross‐shaped resonator can generate two independent tunable transmission zeros by introducing two microwave varactors. The center frequency and bandwidth can be tuned by adjusting the DC bias voltages applied to the varactors. For demonstration, a filter example was designed, fabricated and measured. The tuning 3‐dB bandwidth from 20% to 30% is achieved when the center frequency is tuned from 3.5 to 5 GHz. Meanwhile the insertion loss is less than 4 dB with a high selectivity. The size of the filter is 0.32λg × 0.36λg where λg is the guided‐wavelength at the center frequency. [ABSTRACT FROM AUTHOR]

In this article, a reconfigurable frequency selective surface (RFSS) with a wide reconfigurable passband and angular stability property is proposed. In this design, The RFSS consists of three metallic layers and two thin dielectric substrates with metallic via holes. Tunable electromagnetic characteristics are achieved by loading varactor diodes. Because of the symmetry of the structure and the insertion of metal via holes, the RFSS is insensitive to the incidence of TE and TM polarization, thus improving angular stability. The proposed RFSS can be tuned from 5.4 to 15.96 GHz when the incident angle ranges from 0° to 60°. The RFSS was made and tested to verify the proposed design. The proposed RFSS has good performances, which are verified by simulation and measurement results. [ABSTRACT FROM AUTHOR]

This letter presents a new design method for a tunable bandpass filter with independently and widely tunable transmission zeros (TZs). Varactor‐loaded microstrip resonators are introduced to a fixed bandpass filter to realize the continuously tunable TZs. Although the tunable resonators have unloaded Q lower than 100, the insertion loss of the filter is not increased. The coupling matrix with the lossy resonators is synthesized and used to analyze the characteristic of the TZ‐tunable filter. A bandpass filter centered at 2 GHz with a 3‐dB bandwidth of 230 MHz is designed to validate the design concept. The measured results show good agreement with the simulated results. The fabricated filter has 2 TZs ranging from 1.4 to 1.8 GHz and from 2.28 to 3.08 GHz, respectively. The TZ‐tunable filter exhibits insertion losses ranging from 1.62 dB to 1.93 dB at the center frequency. The center frequency and bandwidth keep nearly unchanged when the TZs are tuned. [ABSTRACT FROM AUTHOR]

Summary: 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. [ABSTRACT FROM AUTHOR]

This paper demonstrates a tunable synthetic fourth-order bandpass filter (BPF) at microwave frequencies. Two parallel second-order Q-enhanced LC BPFs responses are added with the out of phase to synthesize a fourth-order BPF response. The filter is implemented in a 130-nm SiGe BiCMOS technology with a core die area of $0.53 \times 0.7$ mm2. The filter center frequency can be tuned from 4 to 8 GHz (C-band). The filter also achieves a wide 3-dB fractional bandwidth (BW) tuning range of 2%–25%, with a passband ripple of less than 0.5 dB. The corresponding normalized dynamic range (DR) is 151–166 $\text {dB}\cdot \text {Hz}$ owing to a switched varactor control scheme to realize a large effective tuning range with high linearity. Using the parallel synthesis approach, the filter can maintain the DR of a second-order BPF while achieving a fourth-order frequency selectivity, which is favorable compared to cascading resonators. On the lower side of the band, the filter achieves more than 65 dB of ultimate rejection. On the upper side, the rejection is more than 52 dB. The filter also employs a variable transconductor for noise-linearity tradeoff flexibility. The power consumption of the filter is 112–125 mW over the above fractional BW tuning range at the target C-band. [ABSTRACT FROM AUTHOR]

A 2‐pole fully tunable bandpass filter (BPF) with only two varactors implemented is proposed in this paper. The ultra‐wide center frequency (CF) tuning range is built on short‐ended stepped‐impedance resonators (SIRs). Two varactors are placed at the symmetrical plane of the filter in series and shunt configurations, according to the odd‐even mode analysis the CF and bandwidth (BW) of the passband can be tuned independently without adding any redundant tuning elements. The CF of the proposed filter can be tuned from 0.61 to 2.39 GHz. The 3 dB BW tuning range is 150‐260 MHz at the lower side and 290‐520 MHz at the upper side of the entire passband tuning range. Good agreement is observed between simulated and measured results. [ABSTRACT FROM AUTHOR]

This article proposes a new dual-band tunable bandstop filter. This tunable filter is composed of two separate parts, the transmission line, and coupled stubs that each connect to three varactors. The center frequency and bandwidth in each band can be controlled individually. Moreover, the LC equivalent circuit of the proposed filter is calculated, and the results are simulated. In the proposed filter, the simulations show that the first stopband can be tuned in a frequency range from 1.2 to 1.9 GHz with 10-dB absolute bandwidth 310 ± 30 MHz, whereas the second stopband varies from 2.5 to 3.3 GHz with the 10-dB absolute bandwidth 720 ± 20 MHz. These two stopbands can also be tuned independently. Furthermore, the bandwidth of each fixed center frequency can be changed easily. The fabricated proposed filter validates the simulation results. The compact filter has an effective size of 9 × 27.5 mm 2 . [ABSTRACT FROM AUTHOR]

In this paper, an electronically switchable quad-band microstrip bandpass filter is designed using varactor perturbed dual-mode resonators. For this purpose, four meandered dual-mode square loop resonators (DMSLRs) having reference patches are nested and varactor diodes are used to serve as perturbation elements. The designed resonator is analyzed by deriving even/odd mode resonance conditions. S-parameters are also calculated according to the even and odd mode input impedances. Stub loaded feedlines are used to couple the DMSLRs to Input/Output (I/O) ports. According to the capacitances of varactor diodes, each passband can be independently turned on and off. Coupling scheme and coupling matrix of the designed filter are introduced to realize the coupling descriptions. Switching operations are also investigated over the frequency responses obtained from coupling matrix. Passbands of the filter are adjusted to 1.4, 1.7, 2.25, and 2.7 GHz. The designed filter was also fabricated and measured results show a good agreement with the simulated and theoretical results. [ABSTRACT FROM AUTHOR]

Abstract: A two‐pole tunable bandpass filter (BPF) which allows for the operational agility of both center frequency and bandwidth is proposed in this article. The fundamental structure is based on half mode substrate integrated waveguide (HMSIW) with different dimensions. Two sets of varactors placed at various nodes separately are utilized for adjusting center frequency and bandwidth over a wide tuning range. By adding a source‐load coupling path, two transmission zeros are produced on both sides of the passband to improve filter selectivity significantly. A second‐order 1.52–2.76 GHz filter with 3 dB constant bandwidth tuning from 240 to 350 MHz is developed, analyzed, simulated, and fabricated. Good agreement is obtained between simulated and experimental results. [ABSTRACT FROM AUTHOR]

We report a varactor-loaded tunable microstrip bandpass filter (BPF) with reconfigurable center frequency and bandwidth. The effects of loading varactors at intercoupled stepped-impedance resonator (SIR) resonator and at tapped input/output port coupled-lines are studied. Tuning elements are properly loaded at the middle of the coupled-line sections for achieving tunable center frequency and bandwidth performance. Initial BPF (without loading varactors) is based on a half-wavelength (λg/2-) coupled stepped-impedance resonator (CSIR) structure. For the validation of the proposed concept, a hardware prototype is fabricated and characterized. A total of three varactors are used as the tuning elements. Only two dc control voltages are used for tuning the center frequency and bandwidth. [ABSTRACT FROM AUTHOR]

In this paper, a novel tunable coupling mechanism with bandwidth-tunable capability is presented. Metallic via has been utilized to construct the coupling structure, namely via-coupling. Studies show that the via-coupling mechanism provides more flexibility and owns better tolerance to fabrication errors than the traditional gap-coupling. By employing varactor in parallel with the metallic via, the coupling coefficient between each resonator pairs can be effectively tuned. It is worth noting that only one varactor is required for each resonator pair. The lumped-circuit model of the tunable coupling mechanism is investigated and extracted by classical theory derivation and simple programming. For verification, a fourth-order quasi-elliptic bandpass filter (BPF) and a bandwidth-tunable one are designed, fabricated, and measured. For the bandwidth-tunable BPF, the factional bandwidth varies from 2.52% to 5.04%, meaning a double tuning range. The in-band insertion loss varies from 2.8 to 4.8 dB, and the return loss is better than 17 dB. If needed, BPFs with $N$ th-order ( $N > 4$ ) could be designed based on the proposed tunable coupling mechanism and only ( $N + 1$ ) varactors are required. [ABSTRACT FROM AUTHOR]

ABSTRACT A tunable bandpass-to-bandstop filter with controllable bandwidth is proposed in this article. The bandpass-to-bandstop transformation can be achieved by adjusting the PIN diode, and bandwidth of the bandpass filter can be controlled by adjusting the voltage of varactors, as the coupling coefficients and the external quality factors (Qe) can be tuned by changing the ratios of the loaded capacitances. In the bandpass mode, a tuning range of 805-1032 MHz (24.7%) with return loss better than 11.5 dB can be achieved. In the bandstop mode, a tuning range of 760-1228 MHz (47.1%) can be obtained. The rejection level of the bandstop mode is higher than 40 dB. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 59:110-113, 2017 [ABSTRACT FROM AUTHOR]

This paper presents an original and simple approach to the design of dual-band bandpass filters (BPFs) using the proposed stub-capacitor-loaded half-mode substrate-integrated waveguide (HMSIW) structure. The stub-capacitor-loaded HMSIW structure is investigated and a set of analytical equations is provided for the determination of center frequency, transmission zero, coupling coefficient, and external quality factor. Dual-band BPFs can be achieved by loading stub capacitor on the two sides of the HMSIW structure. The two passbands can be designed separately and controlled independently. To validate this method, a class of dual-band filters with a wide range of fractional bandwidth ratios of the two passbands is designed. Finally, a dual-band filter, with each passband tuned only by one control voltage, is fabricated and measured. The results show that the first passband can be tuned in a frequency range from 1.85 to 2.67 GHz (36.4%) without any effect on the second passband. In the meantime, the second passband varies from 3.84 to 5.34 GHz (32.7%) without any effect on the first passband. Good return loss is observed within a wide tuning range. [ABSTRACT FROM PUBLISHER]

This paper presents a second-order Chebyshev constant fractional bandwidth (CFBW) tunable bandpass filter with compact size and frequency invariant-passband characteristics using tunable external quality factors. The filter is designed using a stub loaded dual-mode resonator and two sets of varactors for tuning the center frequency and the return loss level. The design is featured with the net-type filter of a short grounded-stub so that the CFBW property can be assured. In addition to the conventional design, a pair of varactors is included at the ends of feeding transmission lines to keep the external quality factors constant while tuning. Within the tunable range of the varactors, the proposed filter is tunable from 1.7 to 2.1 GHz with a 5% CFBW. The measurement results show a 20-dB return loss and insertion loss below 2.9 dB over the entire tuning range. [ABSTRACT FROM PUBLISHER]

In this paper, tunable microstrip bandpass filters with two adjustable transmission poles and compensable coupling are proposed. The fundamental structure is based on a half-wavelength (\lambda/2) resonator with a center-tapped open-stub. Microwave varactors placed at various internal nodes separately adjust the filter's center frequency and bandwidth over a wide tuning range. The constant absolute bandwidth is achieved at different center frequencies by maintaining the distance between the in-band transmission poles. Meanwhile, the coupling strength could be compensable by tuning varactors that are side and embedding loaded in the parallel coupled microstrip lines (PCMLs). As a demonstrator, a second-order filter with seven tuning varactors is implemented and verified. A frequency range of 0.58–0.91 GHz with a 1-dB bandwidth tuning from 115 to 315 MHz (i.e., 12.6%–54.3% fractional bandwidth) is demonstrated. Specifically, the return loss of passbands with different operating center frequencies can be achieved with same level, i.e., about 13.1 and 11.6 dB for narrow and wide passband responses, respectively. To further verify the etch-tolerance characteristics of the proposed prototype fitler, another second-order filter with nine tuning varactors is proposed and fabricated. The measured results exhibit that the tunable fitler with the embedded varactor-loaded PCML has less sensitivity to fabrication tolerances. Meanwhile, the passband return loss can be achieved with same level of 20 dB for narrow and wide passband responses, respectively. [ABSTRACT FROM AUTHOR]

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 \lambda/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 \lambda/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 \pm 8 MHz. [ABSTRACT FROM PUBLISHER]

This article presents the design of a microwave bandpass filter with tunable center frequencies and reconfigurable transmission zeros. The tuning of the center frequencies is realized by loading varactors at the ends of the λ/2 resonator. The reconfiguration of transmission zero is implemented through varactors in combination with tapped λ/4 stubs. The equivalent circuit models of the tunable filters are provided. The transmission zeros can be set to be at the lower side, the upper side, or both sides of the passband. The design concept is demonstrated by measurement results for both one transmission zero case and two transmission zeros case. The tuning frequency ranges are 680-890 MHz and 680-1020 MHz for one zero case and two zeros case, respectively. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:1526-1531, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27628 [ABSTRACT FROM AUTHOR]

In this paper, a novel tunable bandpass filter with an improved J-inverter is suggested. In this resonator, as the loading capacitance is increased, the resonant frequency is decreased. Also, the phase of the admittance inverter has characteristics that are variable by series capacitors and parallel varactor diodes. The electrical length of the inverter always maintains 90° at a given physical length of the inverter, which should be changed for different operating frequencies. This new bandpass filter has a tuning range of 1.8 GHz ∼ 2.45 GHz and an insertion loss of less than 2.5 dB. Also, the bandwidth is almost constant within the tuning range. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:852-855, 2013; View this article online at wileyonlinelibrary.com. DOI: 10.1002/mop.27467 [ABSTRACT FROM AUTHOR]

This paper presents a study on reconfigurable transmit-array (TA) antennas based on multilayer tunable elements. The proposed configuration can be referred to as a multilayer antenna–filter antenna (MAFA) and it can be viewed as an extension of the antenna-filter antenna concept. It consists of multiple antenna layers cascaded to form a spatially distributed bandpass filter. Phase agility is achieved by tuning the resonant frequency of each layer. The main objective of this paper is to introduce a new design technique for MAFA TA antennas. As will be shown, the key feature of the proposed approach is that it allows to simultaneously take into account three key design parameters: the phase tuning range, the maximum acceptable losses per unit cell, and the operating bandwidth. Simulations and experimental results for MAFAs based on varactor-loaded slot antennas will be illustrated to prove the validity of the proposed method. In particular, numerical results will demonstrate that MAFA cells can be designed to provide full phase agility over operating bandwidths of up to 10% with maximum insertion losses of 3 dB. Furthermore, a specific example will show how introducing an inter-layer medium with a higher dielectric permittivity it is possible to reduce the array longitudinal size. Finally, experimental results obtained using a waveguide simulator will be presented for a 1\,\times\, 3 five-layer MAFA cell. [ABSTRACT FROM AUTHOR]

A varactor and FMR-tuned wideband band-pass filter (BPF) module with versatile frequency tunablity has been devised and realized on RT/Duroid substrate. By connecting four variable capacitance diodes (varactors) to the four open-circuited stubs of an X-band passive composite-BPF and adjusting the reverse bias voltage from 2 to 22 V, the 3 dB passband of the passive composite-BPF was continuously tuned from 7.7–12.4 GHz to 11.4–17.5 GHz. The corresponding bandwidth (BW) was enlarged from 4.7 to 6.1 GHz and the upper frequency of the passband was extended from 12.4 to 17.5 GHz, namely, a total frequency span of 9.8 GHz. The BW of the passive composite-BPF was further FMR-tuned using a pair of Band-Stop Filter (BSF). By setting the reverse bias voltage at 22 V together with the bias magnetic fields at 3250 and 4200 Oe, respectively, to the two BSFs, the 6.1 GHz BW and 9.8 GHz frequency span were compressed to 1.4 GHz toward the high-end frequency. The 4.7 GHz BW and 9.8 GHz frequency span were also compressed to 1.0 GHz toward the low-end frequency with 2650 and 3500 Oe bias magnetic fields and 2 V reverse bias voltage. Thus, the combination of the varactor and FMR-tuning has facilitated versatile passband and its BW tuning covering the X-and Ku-bands with both large frequency tunability and greatly improved frequency selectivity. [ABSTRACT FROM AUTHOR]

A new tunable dual-mode dual bandpass filter (DBPF) is presented based on coupled-line resonators structure. The center frequency and bandwidth can be individually tuned in each band with varactors, which are loaded at the end and middle of the coupled resonators. Varactor diodes can adjust the lower band from GPS to GSM standards (from 1.57 to 1.85 GHz), and the higher band for GSM, UMTS, WiFi and WiMAX applications (from 1.85 to 2.5 GHz). The LC equivalent model of the proposed filter is presented to analyze the resonator at odd- and even-mode frequencies to prove the favorable role of each varactor in tuning the center frequency and bandwidth. To validate the simulation results, the proposed filter is fabricated and measured. Measurement results are in good agreement with LC equivalent model and simulation results. [ABSTRACT FROM AUTHOR]