Tunable Transmission Zeros Research Articles

High-Selectivity FA-FA-Based Frequency Selective Surfaces Using Magnetoelectronic Dipole Antennas

A new method for frequency selective surface (FSS) with high selectivity, namely, filtenna–filtenna (FA-FA)-based technique, is proposed, which consists of two filtering antennas and impedance matching network. Based on the reciprocity theorem, the radiation null of filtenna can provide a stopband of the proposed FSS. In this work, it is validated by using a periodic array of back-to-back magnetoelectronic (ME)-dipole antennas with split-ring resonators (SRRs), and a slotted GND plane is used to connect the two ME-dipole antennas for impedance matching. Bandpass response of the proposed FSS is realized by the intrinsic high-pass characteristic of ME-dipole antennas and the bandstop characteristic of SRRs. Four transmission zeros (TZs) and three transmission poles (TPs) are generated, exhibiting a high-order filtering response. TZs and TPs are analyzed by equivalent circuit model and current distributions. The 3 dB fractional bandwidth ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathrm {FBW}}_{\mathrm {3\,dB}}$ </tex-math></inline-formula> ) of 38.5% in the passband and the 20 dB fractional bandwidths <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$({\mathrm {FBW}}_{\mathrm {20\,dB}})$ </tex-math></inline-formula> of 8.7% in the lower rejection band and 13.8% in upper rejection band are obtained in this design. Finally, a prototype is fabricated and measured to further validate the proposed FA-FA-based technique. Simulated and measured results show that the proposed FA-FA-based FSS is with the distinct advantages, including wide passband, low profile, tunable TZs, and dual-polarized application, which verifies the proposed method of FSSs.

Constant Absolute Bandwidth Tunable Symmetric and Asymmetric Bandpass Responses Based on Reconfigurable Transmission Zeros and Bandwidth

A two-pole reconfigurable and tunable bandpass filter (BPF) with constant absolute bandwidth (CABW) and tunable transmission zeros (TZs) is presented in this paper. The BPF consists of asymmetrical spiral resonators having open ends on one side and capacitor loaded on the other side. The varactor loaded coupled-feed scheme is used at input and output. The two coupling feed-lines are also coupled to each other. It is shown that the control over TZs using the coupling-capacitor loaded between the two resonators leads to two different symmetric frequency responses – first with one TZ on each side of the passband, second with no TZ on either side of the passband. Further, asymmetrical frequency responses having single TZ either on lower or upper side of the passband are achieved by varying the capacitors loaded to the end of coupling-line. CABW while tuning as well as tunable bandwidth (BW) are achieved in all type of responses by using the coupling-capacitor. While tuning, quality factor is maintained by varying the capacitor loaded at the end of the coupled-feed. For the demonstration of the concept, a two-pole tunable BPF is designed and fabricated. The fabricated BPF having size of 13.1 mm × 9.4 mm, shows four different responses based on the position of TZs. All four CABW responses of the BPF have a tuning range of 300 MHz. Also, a tunable bandwidth is achieved in every response of the BPF.

Synthesis design of dual‐path coupled line circular ring resonator filter with tunable characteristic impedances and transmission zeros

A dual-path coupled-line circular ring resonator filter synthesis is presented. The topology comprises identical coupled lines in series duplicated to form dual-path coupled-line interconnections. A bandpass response of nth order equal to (n/2) + 1, corresponds to controllable transmission zeros (TZs) found on both sides of the passband by varying the values of the even- and odd-mode impedances of the coupled lines, denoted Zoe and Zoo, respectively. Concerning the efficiency of controllable electrical characteristics of the resonator filter, such as in-band matching level, high selective response, and wide bandwidth, and fixing the position of the TZ frequencies, the proposed method using asymmetrical quarter-wavelength coupled lines in a circular ring resonator filter is superior to existing methods. Ultimately, the proposed bandpass filter is implemented on contrast dielectric substrate FR-4 and Taconic TRF-45 substrates using a planar form of microstrip technology to verify the synthesis equation. The measurement results indicate that the filter achieved more than 40% fractional bandwidth with excellent in-band matching levels and a controllable filter order. Both measurement and simulation results are in good agreement regarding return and insertion loss values, thus validating the proposed designs.

A microstrip bandpass filter with tunable bandwidth and center frequency

AbstractThis 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.

A Compact Dual-Mode Bandpass Filter with High Out-of-Band Suppression Using a Stub-Loaded Resonator Based on the GaAs IPD Process

In this letter, a compact dual-mode bandpass filter (BPF) with an ultra-wide stopband that employs two folded open-loop resonators (FOLRs) and stub-loaded resonators (SLRs) is proposed. The dual-mode resonators are optimized by loading two SLRs onto the folded open-loop resonators, and this process is analyzed using the dual-mode theory. To miniaturize the device size and increase chip performance, the proposed BPF is fabricated by a III–V compound semiconductor-fabrication process using a high-performance GaAs substrate based on the integration passivation device (IPD) fabrication process. A compact dual-mode BPF with low insertion loss and high return loss is designed and fabricated. Two extra transmission zeros (TZs) located in the high-frequency range increase the wide stopband, and the two TZs near the passband result in a higher selectivity. A resonant frequency centered at 7.45 GHz with an insertion loss of −1.21 dB and a measured return loss of higher than −23.53 dB and 3 dB fractional bandwidths of 5.8% are achieved. The stopband can be suppressed up to 20 GHz owing to the two tunable TZs resulting in higher selectivity and wideband rejection. The size of the filter was drastically optimized using a simplified architecture of two FOLRs and SLRs.

A microstrip bandpass filter with 2 independently tunable transmission zeros

AbstractThis 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.

Bandpass Filter Using Three Pairs of Coupled Lines With Multiple Transmission Zeros

A novel but very simple bandpass filter using just three pairs of coupled lines with multiple transmission zeros (TZs) and transmission poles is proposed in this letter. The filter structure can be seen as the two one-wavelength ring resonators coupled each other with $\lambda _{g}/4$ coupled length and sandwiched between the $3\lambda _{g}/4$ input and output feedlines. Good out-of-band suppression and sharp roll-off skirts can be realized by multiple fixed and tunable TZs without complicated coupling scheme. For further demonstration, a filter example with center frequency at $f_{0}= 2.1$ GHz is implemented with generation of eight TZs at the frequency range from 0 to $2f_{0}$ for theoretically calculated certification. Good agreement between simulation and measured results validates the design method.

A compact tunable bandpass filter with tunable transmission zeros in the pass band adopting a nested open ring resonator

<scp>A</scp> compact tunable bandpass filter with tunable transmission zeros in the pass band adopting a nested open ring resonator

Compact Stepped Impedance Resonator Bandpass Filter with Tunable Transmission Zeros

This paper proposes a compact microstrip bandpass filter (BPF) with tunable transmission zeros, narrow bandwidth and low insertion loss. A λ/4 stepped impedance resonator (SIR) with two additional via holes has been adopted to obtain a compact size and a pair of transmission zero (TZ). Furthermore, three techniques have been developed to create a pair of controllable transmission zeros on both side of each passband. The TZ can be controlled by adjusting either magnetic or electric coupling. The measured return losses and insertion loss is larger than 18 dB and 2.2 dB respectively. The overall size of the proposed design filter is 5.3mm x 5.5mm without considering the feeding lines.

A novel compact Wilkinson power divider with controllable harmonic suppression and simple structure

In this paper, based on the equivalent circuit of spiral-shaped CPW resonators, a novel compact cascaded inhomogeneous coupled-line (CIHCL) with two tunable transmission zeros is proposed to replace the quarter-wavelength microstrip line of the conventional power divider (CPD). The modified power divider with CIHCL could achieve the same frequency responses of CPD and also realize controllable harmonic suppression with compact size. To verify the expected results, two power dividers with different harmonic suppression ratios are designed and one of them is fabricated to demonstrate the performances of the proposed power divider. The simulated and measured results illustrate that the return loss and isolation at center frequency 3 GHz are better than 25 dB and the insert loss is better than 0.4 dB. In addition, about 30 dB suppression for second and third harmonic is achieved.

Design of planar diplexers with improved isolation using the tunable transmission zeros of a dual‐mode cavity filter

A substrate integrated waveguide dual-mode single cavity filter with two inclined slots on top metal plane provides two transmission zeros. It is shown that the feed offset as well as the slot offset both can be used to tune the zeros over a wide range of frequencies. Leakage loss from the slot is negligibly small. Two bandpass filters are fabricated to validate the predicted zero variation. Further, tunability of the zeros is utilised to greatly improve the isolation characteristics of two X-band planar diplexers. Measurement results show that the isolation is better than 30 dB even if the channel separation is as close as 5% of the average frequency. The effect of packaging on the filter characteristics is also presented.

Folded Feedthrough Multilayer Ceramic Capacitor EMI Filter

A chip three-terminal multilayer ceramic capacitor (MLCC) electromagnetic interference (EMI) filter with a good stopband characteristic was designed. Parasitic effects on the internal signal electrode, including equivalent parallel capacitance and series inductance, were utilized to form a tunable transmission zero in stopband by folding the traditional chip three-terminal feedthrough MLCC. Based on a network theory, an equivalent circuit was constructed in ADS to explain the operating mechanism of the proposed structure. The ADS and HFSS were utilized to simulate the proposed structure, and the measured results corresponded to the simulation results. The structure of the proposed EMI filter presents a new method to improve the stopband characteristic of capacitor EMI filters.

Reconfigurable Planar Capacitive Coupling in Substrate-Integrated Coaxial-Cavity Filters

This paper expands our previous work on planar tunable capacitive coupling structures in substrate-integrated cavities using lumped components. We demonstrate both frequency and bandwidth tunable filters with adjustable transmission zeros (TZs). By the appropriate choice of the absolute and relative strength of magnetic and electric coupling coefficients, we demonstrate: 1) tunable bandwidth and the ability to maintain either a constant absolute bandwidth or a constant fractional bandwidth; 2) adjustable TZ location at a prescribed bandwidth; and 3) the ability to switch OFF the filter with high isolation. Filter design methodologies based on a dispersive coupling structure are presented using lumped circuit models, coupling matrix, and full-wave simulations. With this planar capacitive coupling, it is also convenient to realize cross-coupling in higher order filters to produce additional TZs for rejecting spurious resonances or interferes. Fabricated two-pole filters with one or two TZs and four-pole filters with three or four TZs validate the filter design. A two-pole filter with tunable center frequency and tunable bandwidth along with a four-pole filter with tunable center frequency and tunable TZs are also demonstrated.

Design of a Compact Ultra-Wideband Bandpass Filter With an Extremely Broad Stopband Region

This letter presents a design of a compact ultra-wideband (UWB) bandpass filter (BPF), which can exhibit an UWB response from 3.1 to 9.9 GHz with low insertion loss (IL) of 0.8 dB and an extremely broad stopband region from 10.9 to 25.1 GHz with high rejection level of 20 dB. This UWB BPF is simply constructed by using only one single-stage parallel-coupled line and two rectangular stub resonators. In this design, the rectangular stub resonators provide an exciting mode to enhance the desired UWB response and also create tunable transmission zeros to extend a wide stopband by controlling the impedance ratio $(R)$ properly. The filter was fabricated and measured to verify the design concept.

Design of a compact silicon-based integrated passive band-pass filter with two tunable finite transmission zeros

The thin film integrated passive device (IPD) has caused intensively attention due to its high integrated level and application in System in Package (SiP). Meanwhile, the IPD filter has shown great application value in modern wireless communication system. In this paper, the design method of a compact silicon-based lumped IPD band-pass filter with two tunable finite transmission zeros is shown. The passband of the designed filter is 2.4–2.5GHz. Specific schematic is used to introduce two independently tunable transmission zeros. The specific schematic contains mutual inductor structure, which is realized by making two planar spiral inductors to partially overlap with each other and is helpful to decrease the size of the filter. The mutual inductance is adjustable by changing the overlap degree. To design and optimize the layout of the filter more efficiently, the schematic-layout mutual feedback method is presented. Finally, the measurement results verify the validity of the whole design process and method. The size of the designed second-order band-pass filter is 0.85mm×0.85mm×0.35mm. To the best of our knowledge, the band-pass-filter developed in this paper has the smallest size while achieving equivalent electrical performance.

Efficient UWB filter design technique for integrated passive device implementation

A new approach is proposed to act as an efficient design technique for the implementation of ultra-wideband (UWB) bandpass filters in integrated passive device technology. The proposed design is based on two major original contributions. First, the use of hourglass filter theory which, to the best of authors knowledge, has not been applied yet to such technology. Then, the minimisation of the number of inductors involved in the filter circuit through the zigzag technique. Implemented in 0.18 µm CMOS technology, the designed filter exhibits excellent performances such as two finite tunable transmission zeros, enhanced selectivity, a bandwidth covering the entire UWB spectrum of 3.1–10.6 GHz, a stopband rejection better than 15 dB throughout frequencies below 2.7 GHz and above 11 GHz, a good matching better than 11 dB, a minimum mid-band insertion loss of 1.8 dB and, finally, a sufficiently constant group delay. With these features and only 10 lumped elements, the proposed filter is one of the most promising compact filters for UWB applications.

A Fully Tunable Two-Pole Bandpass Filter

In this letter, a two-pole tunable combline filter that allows for operational agility of the center frequency, passband bandwidth, and locations of upper and lower transmission zeros is proposed. To increase the tunable range of constant absolute-bandwidth for all frequency tuning states, a novel T-type bandwidth-control network was devised between resonators to flexibly tune the coupling coefficient without limitations imposed by the available varactor capacitance. By use of tunable source-load coupling, two tunable transmission zeros were produced on both sides of the passband to significantly improve filter selectivity and to dynamically afford interference suppression. A second-order 1.7–2.7 GHz filter with 1 dB constant bandwidth tuning from 50 to 110 MHz and transmission zero locations tuning from 300 to 700 MHz with respect to the passband was developed and fabricated. Good agreement was obtained between the simulated and experimental results.

Compact Bandpass Filter With Two Tunable Transmission Zeros Using Hybrid Resonators

This letter presents a novel type of compact, low insertion loss (IL) planar microstrip filter, which is mainly composed of two hybrid resonators with a mixed electric/magnetic (EM) coupling between them. Tunable EM coupling is constructed to realize a tunable transmission zero (TZ) very close to the passband. And two folded T-shaped feed lines are adopted to introduce another tunable TZ and obtain a desired external coupling. Advantages of the proposed filter are not only its compact size and low IL, but also its tunable TZs.

A Novel Cross-Coupled Filter Through Mixed Coupling Path with Multiple Tunable Transmission Zeros

A novel bandpass filter with multiple transmission zeros using a mixed-coupling microstrip stepped-impedance resonator is proposed and investigated in this article. A microstrip with a grounded via hole is added in the middle of the improved microstrip stepped-impedance resonator; the coupling gap and the grounded via hole act as the electric and magnetic coupling, respectively, by which additional transmission zeros can be generated near the passband. Based on the proposed filter topology, a fourth-order bandpass filter with five transmission zeros is fabricated and measured. Compared with a conventional cascaded-quadruplet filter, the novel designed filter has sharper rejection with a symmetrical response and more compact size. Moreover, the proposed idea leads to a new solution to decrease size as well as increase the capability of the conventional stepped-impedance resonator filters.

Cascaded triplet filter using mixed electric and magnetic coupling structure with wide stopband performance

ABSTRACTThis article proposes a novel type of microstrip cascaded triplet filter with controllable electric and magnetic coupling. The electric coupling can be tuned by adjusting the gap between the resonators, while the magnetic coupling is dominated by the diameter of the via‐hole. A transmission zero on the upper stopband is produced by the inductive cross‐coupling. To further improve the selectivity of the filter, the source‐load coupling is also proposed to introduce two tunable transmission zeros. To expand the stopband, proper feeding position is selected to suppress the third and fifth harmonics. Based on the proposed structure, a third‐order filter are designed and fabricated. Good agreement between the measured and simulated data is obtained. Measured results indicate that a rejection level of 20 dB up to more than six times the passband frequency can be obtained. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:2937–2940, 2014