Tunable Center Frequency Research Articles

Graphene based THz filter design using open-loop resonator for space applications

This paper introduces a graphene-based tunable terahertz bandpass filter with a dual-mode open-loop resonator, designed to operate at a resonant frequency of 6.35 THz. Leveraging a graphene layer between the conductor and dielectric layers, the filter enables the propagation of surface plasmons, allowing dynamic tunability essential for terahertz applications. Key advantages of this design include precise bandwidth control, achieved by manipulating the resonator's physical parameters, and adaptable center frequency tuning through variations in graphene's chemical potential, providing a tuning range of approximately 0.16 THz. These reconfigurable properties make the filter highly suitable for integration into terahertz space communication systems, where adaptability and high performance are critical. The proposed design's ability to fine-tune both bandwidth and center frequency establishes it as a versatile and efficient solution for emerging terahertz-based applications, demonstrating significant potential for enhancing the flexibility and functionality of future communication technologies.

High-selectivity fully tunable wideband bandpass filter with adaptive transmission zeros

This paper presents a compact fully tunable wide-band bandpass filter based on the varactors loaded quadruple mode coupled resonator. Triple adaptive transmission zeros (TZs) have achieved by reasonably setting the identical shifting mechanism architecture of transmission poles and adjacent TZs, which dramatically improve the selectivity. Meanwhile, it realized the tunable center frequency (CF) with a constant absolute bandwidth, resulting in the CF tuning range of 26.7 %, 25.8 %, and 15.6 % from 1.5 to 1.95 GHz. The bandwidth tuning range of 42.7 % can be achieved from 180 to 920 MHz with a fixed CF. The test results match well with the theoretical simulation so as to successfully validate the correctness of the proposed design approach.

Multifunctional and tunable bandpass filters with RF codesigned isolator and impedance matching capabilities

Abstract This paper presents the radio frequency (RF) design and experimental validation of a multifunctional bandpass filtering (BPF) concept with center frequency tunability and RF codesigned isolator and impedance matching functionality. The multifunctional bandpass filter/isolator (BPFI) concept combines frequency-tunable reciprocal resonators and nonreciprocal frequency-selective stages (NFSs) to realize center frequency tunability and fully directional transfer characteristics. The NFS, as the core component of the BPFI concept, exhibits frequency-selective transmission response in the forward direction and signal cancellation in the reverse direction. Its tunability is achieved by combining a transistor-based network with a tunable capacitively loaded coupled-line section. Furthermore, the NFS facilitates matching of different source loads allowing for the BPFIs to be used as reconfigurable matching networks. For experimental validation, an NFS and two BPFIs were designed, manufactured, and measured at L band. Their features include (i) NFS: center frequency tuning from 1.55 to 1.9 GHz with maximum directivity from 20 to 52 dB and gain from 0.3 to 1.3 dB. (ii) BPFI (topology A): center frequency tuning from 1.52 to 1.9 GHz with maximum directivity from 20 to 44 dB and gain from −1.5 to −0.5 dB. (iii) BPFI (topology C): ability to match complex loads with 26 + j18 Ω and 26 − j14 Ω.

An edge-coupled magnetostatic bandpass filter

The further development of 5G and 6G communication systems introduced new frequency allocations beyond 6 GHz, necessitating the development of compact bandpass filters that can operate over wide gigahertz frequency ranges. Herein, we report on the design, fabrication, and characterization of an edge-coupled magnetostatic forward volume wave bandpass filter (MSFVW). Using micromachining techniques, we fabricate both 2-pole and 4-pole filters from a yttrium iron garnet (YIG) film grown on a gadolinium gallium garnet (GGG) substrate with inductive transducers. By adjusting an out-of-plane magnetic field, we demonstrate linear center frequency tuning for a 4th-order filter from 4.5 GHz to 10.1 GHz while retaining a fractional bandwidth of 0.3%, an insertion loss of 6.94 dB, and a − 35 dB rejection level. We characterize the filter nonlinearity in the passband and stopband with IIP3 measurements of − 4.85 dBm and 25.84 dBm, respectively. In this work, we demonstrate a compact octave tunable narrowband channel-select filter with a significant degree of design flexibility and performance comparable to the state-of-the-art.

Multi-stopband filter based on frequency selective surface

This paper presents the design of a terahertz multi-stopband filter based on a metasurface structure which consisting of multiple metallic strips. By adjusting the dimensions and arrangement of these metallic strips placed on a dielectric substrate, the precise control of the filter performance is achieved. This structure exploits the electromagnetic resonances of metallic strips at terahertz frequencies to generate stopbands, thereby enabling selective filtering. Numerical studies demonstrates that these configurations can produce multiple stopbands within the 4.2 to 15.5 THz frequency range, with tunable center frequencies and bandwidths. Through the research of the transverse and longitudinal filter model structure, the band-stop filter with more stopbands can be realized, and the transmission valley value is kept within 5 %, which is crucial for enhancing filter performance. Moreover, the study reveals that tuning the dimensions of the metallic strips can modulate the resonances, resulting in a redshift of the filter's central frequency and an expansion of the passband width. These findings provide a new method for the performance improvement of terahertz filters, advanced filtering technology and electromagnetic wave control technology.

Design of a tunable center frequency and small size cavity bandpass filter by separating capacitor-loaded resonators

This paper presents a tunable center frequency and small size cavity bandpass filter design method. In this method, a capacitor-loaded open terminal coaxial resonator is employed to reduce the size of cavity filters. The resonator is designed and fabricated separately into two parts to achieve the flexible operating frequency purpose. The first part is called the base of the resonator which is simply a pillar and directly fabricated integrally with the cavity housing. The second part called the hat of the resonator is the main part causing the load capacitance in cavity filters. By using different heights of the base part or/and different shapes and sizes of the hat, the operating frequency of cavity filters can be changed flexibly. This method not only reduces the difficulty and cost of cavity filter processing but also makes cavity filters reconfigurable. To demonstrate the effectiveness of the method, a cavity filter sample with a center frequency of 3.45 GHz and a bandwidth of 80 MHz was designed, fabricated, and measured. The measured results show that the insertion loss was smaller than 1.33 dB in the whole bandwidth, one zero-point at 3.350 GHz reaching -68 dB, the rejection at 3.550 GHz was -41 dB, unloaded Q was 5,898, and the dimension of the filter was 128 mm×86 mm×23 mm.

Design of high selectivity tunable bandpass filter with switchable single‐/dual‐band responses

AbstractThis article proposes a switchable filter with four operating states, namely dual‐band mode, lower band mode, upper band mode, and all‐stop mode. The transition between these states is achieved by using positive intrinsic negative diodes, while the center frequency tuning is done by using varactors. In order to verify the design concept, a prototype has been fabricated on an F4BME substrate with a dielectric constant of 2.25 and a thickness of 1 mm, and the measured results were in agreement with the simulations. Notably, the dual‐band mode yields four transmission zeros (TZs), while the single‐band mode produces two TZs. The final circuit size of the designed filter is 0.18 × 0.14 λg, making it ideally suited for reconfigurable multi‐band communication systems.

Non-Magnetic Non-reciprocal Bandpass Filter with Quasi-Elliptic Response and Tunable Center Frequency

This paper presents a detailed analytical numerical design methodology and experimental validation of non-magnetic non-reciprocal bandpass filter (NBPF) that exhibits a highly selective quasi-elliptic response in forward direction of propagation. By modulating resonators with progressive phase shift AC modulation signal, unidirectional RF signal propagation (|S21| ≠ |S12|) is achieved. Analytical spectral S-parameters of the proposed quasi-elliptic NBPF have been derived to gain insight nonreciprocal response. Furthermore, empirical relationships between modulation parameters and filter specifications have also been established through numerical simulations, allowing for simplified design process of the NBPF. By varying the resonant frequency of the resonators, a frequency reconfigurable transfer function can be achieved, providing center frequency tunability of NBPF. For experimental validation purposes, a prototype of microstrip line NBPF is designed and manufactured using three time-modulated resonators, which are implemented using transmission line loaded with varactor diodes. The passband frequency can be continuously tuned by changing DC-bias voltage of varactor diodes. The measurement results revealed that passband center frequency is tuned from 1.65 GHz to 1.95 GHz achieving at tuning ratio of 1.182:1, while maintaining the nonreciprocal behavior with two transmission zeros. For all tuning states, in-band minimum insertion loss in forward direction was measured between 3.20 and 4.8 dB, whereas isolation in reverse direction was measured up to 34.5 dB.

Unequal Power Division Ratio Nonreciprocal Filtering Power Divider With Arbitrary Termination Impedance and Center Frequency Tunability

Unequal Power Division Ratio Nonreciprocal Filtering Power Divider With Arbitrary Termination Impedance and Center Frequency Tunability

A New Two-Port MIMO Filtenna With Tunable Bandwidth and Center Frequency

A New Two-Port MIMO Filtenna With Tunable Bandwidth and Center Frequency

Reconfigurable bandpass filter with flexible tuning of both center frequency and bandwidth

This paper proposes a novel reconfigurable filter that can flexibly tune both the center frequency and bandwidth simultaneously. This reconfigurable filter consists of a filter network with Chebyshev properties and a parallel resonator. These two structures are symmetrical and can be analyzed using the odd-even mode analysis method for the filtering network. With the addition of a tuning factor, the Chebyshev filter network provides a shift in passband and coupling coefficients that can be adjusted to maintain a stable bandwidth; the parallel resonators will produce adjustable transmission zeros on either side of the passband, providing good out-of-band rejection when adjusting the bandwidth and center frequency. By using the transformation of the resonant frequency of two filtering networks, filters of different orders can also be achieved. This reconfigurable filter can achieve continuous bandwidth and center frequency adjustment and can maintain a constant absolute bandwidth during the center frequency adjustment process, with flexible tuning methods. Through simulation and experiments, it has been proven that this reconfigurable filter has a tunable frequency coverage range of 0.4GHz to 2GHz, with a large tunable range.

Microstrip multifunctional reconfigurable wideband filtering power divider with tunable centre frequency, bandwidth, and power division ratio based on three‐parallel‐coupled line section

AbstractA compact microstrip multifunctional reconfigurable filtering power divider (FPD) is proposed based on varactor‐loaded three‐parallel‐coupled line section (TLCS) in this letter. Continuously tunable third‐order filtering power division response with two transmission zeros (TZs) including centre frequency (CF), bandwidth (BW) and power division ration (PDR) is successfully attained by varying the external and internal coupling through loading varactors into a three‐parallel‐coupled line topology. For validation, a prototype is implemented. The results indicate that the CF can be tuned from 0.46 to 0.8 GHz, a tunable 3‐dB BW of 80 to 190 MHz is exhibited with better than 17‐dB return loss and higher than 15‐dB in‐band isolation. In addition to the CF and BW tunability, the power division ratio (PDR) for the presented FPD can also be adjusted from 1:1 to 1:5. Additionally, a compact size is realized with only 0.12λg × 0.16λg.

Chirality-selective electromagnetically induced transparency in a dielectric metasurface based on chirality transfer between bright and dark modes

Chiral metasurfaces have wide applications in chiral sensing and functional devices, such as ultrathin circular polarizers. By analytical coupled mode theory and finite-difference time domain simulation, we investigate the chiroptical properties of designed dielectric metasurface with unit cell of corner-stacked nanorods and stacked nanorings, paying attention to the bright-dark-mode coupling effects. With the help of phase modulation and mode hybridization, we can realize chirality transfer from bright modes of chiral nanorods to dark modes of achiral nanorings, which results in chirality-selective transparency due to chirality-selective excitation of binding/antibonding dark modes. Moreover, one can switch between different coupling regimes with a distinct physical effect (Fano effect vs Rabi splitting) by changing only the chirality of the incident field without varying the structure of the metasurface. Based on the mechanisms of chirality transfer and mode hybridization, our designed metasurface has achieved chirality-selective transparent window with tunable central frequency and bandwidth, which provides insight and guidance for the optoelectronic device design.

On-Chip Photonic Generation of Tunable Wideband Phase-Coded Linearly-Chirped Microwave Waveforms

On-chip photonic generation of tunable wideband phase-coded linearly-chirped microwave waveforms (PC-LCMWs) is proposed and experimentally demonstrated. The on-chip generation system consists of two main parts. One part is a hybrid Fourier-domain mode-locked opto-electronic oscillator (FDML-OEO) for a wideband linearly-chirped microwave waveform (LCMW) generation; the other part is a high-speed microwave photonic phase shifter (MPPS) to perform the phase-coding. By synchronizing the two parts, a PC-LCMW is finally generated. In the proposed chip, a thermally-tunable ultrahigh-Q micro-ring resonator (MRR) is a key component which is incorporated in the FDML-OEO loop to select the oscillation frequency, and an electrically-tunable micro-disk resonator (MDR), as another key component, is used to code the phase of the generated microwave signal while maintaining the amplitude by controlling the free carrier concentration in the lateral PN junction of the disk. With the use of the two key component chips, an experiment is performed and different PC-LCMWs are generated with a tunable center frequency from 12 to 16 GHz, a bandwidth from 2 to 6 GHz, and phase code of 7-bit and 13-bit Barker code. In particular, a PC-LCMW with a maximum bandwidth as wide as 6 GHz and an ultra-large time-bandwidth product (TBWP) as large as 1.96×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> is experimentally demonstrated. The proposed integrated PC-LCMW generation system holds great advantages including ultracompact configuration and fast tunability, which holds a high potential for applications in modern high-resolution multi-functional radar system.

A 1.65 to 1.84 GHz Filtering SPDT Switch With Fully Tunable Insertion Phase Using NRN and Mixed Electromagnetic Coupling Techniques

This paper presents a novel filtering single-poledouble-throw (SPDT) switch with continuously tunable center frequency and insertion phase. It is comprised of six varactorloaded microstrip resonators and three tunable non-resonating nodes (NRNs) at source and loads. The controllable mixed electromagnetic couplings are introduced on the two main paths of each channel of the switch. At ON-state, the designed filtering switch can generate a 4th-order bandpass filtering performance with high selectivity, and the insertion phase can be continuously tuned with a full 3600 tuning range by changing the reactances of NRNs and switching the dominated modes of the mixed couplings. At OFF-state, the mixed electromagnetic couplings are set to zero, and hence high isolation can be realized. To validate the proposed technique, a circuit prototype is designed and fabricated. The measured results suggest that the designed filtering SPDT switch can operate with a tunable center frequency ranging from 1.65 GHz to 1.84 GHz, and its insertion phase at ON-state channel can be continuously tuned from -1800 to 1800. During the whole tuning process, the measured in-band isolation for OFF-state channel keeps better than 40 dB.

Design of Orthogonal Pulse Waveforms With Tunable Frequency and Bandwidth for Carrier-Free THz Communications

Carrier-free pulse-based waveforms are desired in terahertz (THz) communications since pulse-based systems have simpler transceiver architectures than carrier-based systems. For such a pulse-based THz communication system, there still lacks pulse waveforms that can support high-order modulation and flexible multiple access. In this paper, a pulse-based waveform with continuously tunable center frequencies and bandwidths is designed for carrier-free THz communications. More specifically, a Gaussian pulse is utilized as the basic pulse, and then the weighted sum of its high-order derivatives is used to generate waveforms with tunable frequencies and bandwidths according to the probability density function of Rice distribution. Such a pulse-based waveform is called frequency and bandwidth continuously tunable (FBCT) pulse. By making the derivative orders of all weighted terms odd or even, a pair of orthogonal FBCT pulses can be generated. Based on the pair of orthogonal FBCT pulses, a basic pulse-based <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> -ary quadrature amplitude modulation (P <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> QAM) scheme can be readily achieved. Moreover, with frequency and bandwidth tunability, FBCT pulse can support pulse division multiple access (PDMA) with tunable bandwidth, through which frequencies with high molecular absorption loss can be avoided. Numerical results demonstrate that FBCT pulse is significantly effective and flexible in supporting carrier-free THz communications.

Ultra-Narrow Bandwidth Microwave Photonic Filter Implemented by Single Longitudinal Mode Parity Time Symmetry Brillouin Fiber Laser.

In this paper, a novel microwave photonic filter (MPF) based on a single longitudinal mode Brillouin laser achieved by parity time (PT) symmetry mode selection is proposed, and its unparalleled ultra-narrow bandwidth as low as to sub-kHz together with simple and agile tuning performance is experimentally verified. The Brillouin fiber laser ring resonator is cascaded with a PT symmetric system to achieve this MPF. Wherein, the Brillouin laser resonator is excited by a 5 km single mode fiber to generate Brillouin gain, and the PT symmetric system is configured with Polarization Beam Splitter (PBS) and polarization controller (PC) to achieve PT symmetry. Thanks to the significant enhancement of the gain difference between the main mode and the edge mode when the polarization state PT symmetry system breaks, a single mode oscillating Brillouin laser is generated. Through the selective amplification of sideband modulated signals by ultra-narrow linewidth Brillouin single mode laser gain, the MPF with ultra-narrow single passband performance is obtained. By simply tuning the central wavelength of the stimulated Brillouin scattering (SBS) pumped laser to adjust the Brillouin oscillation frequency, the gain position of the Brillouin laser can be shifted, thereby achieving flexible tunability. The experimental results indicate that the MPF proposed in this paper achieves a single pass band narrow to 72 Hz and the side mode rejection ratio of more than 18 dB, with a center frequency tuning range of 0-20 GHz in the testing range of vector network analysis, which means that the MPF possesses ultra high spectral resolution and enormous potential application value in the domain of ultra fine microwave spectrum filtering such as radar imaging and electronic countermeasures.

High‐Speed, microwave ignition: Antenna igniters with frequency and bandwidth selectivity

AbstractThis effort demonstrates the development of (1) dielectric breakdown igniters and (2) bridgewire igniters powered by half‐wavelength dipole microwave microstrip antennas with bandwidth selectivity for use in standoff ignition applications. Dielectric igniters utilized a dielectric epoxy filled with up to 30 wt. % nanothermite cast between the gap of two quarter wavelength microstrips. Bridgewire igniters employed a wire soldered across the gap of two quarter wavelength microstrips. A variety of different bridgewire materials were tested in diameters ranging from 76 μm to 203 μm. The ignition delay of resonant microwave igniters is measured from high‐speed video observation of the microwave illumination process within a free‐space anechoic microwave cavity driven by a 2 kW continuous magnetron source. Dielectric breakdown igniters produced ignition delays ranging from 400 ms to 850 ms. Ignition delay decreased with higher loading of thermite. Bridgewires were found to be highly efficient and measured ignition delays ranged from 17 ms to 271 ms (24.4 J to 457.1 J ignition energy). Smaller diameter bridgewires and high resistivity metals were found to produce the shortest ignition delays. We demonstrate through S‐parameter measurements the bandwidth selectivity of bridgewire igniters and show that control of dipole geometry enables tuning of igniter center frequency and bandwidth and demonstrate experimentally the ability of bridgewire igniters to reject accidental ignition from an off frequency high power microwave field. The tunability, bandwidth selectivity, and low energy requirements, in particular of bridgewires, make them attractive for a number of new and challenging ignition applications.

Tunable topological phase transition in the telecommunication wavelength

Recent progress in the Valley Hall insulator has demonstrated a nontrivial topology property due to the distinct valley index in 2D semiconductor systems. In this work, we propose a highly tunable topological phase transition based on valley photonic crystals. The topological phase transition is realized by the inversion symmetry broken due to the refractive index change of structures consisting of optical phase change material (OPCM) with thermal excitation of different sites in a honeycomb lattice structure. Besides, simulations of light propagation at sharp corners and pseudo-spin photon coupling are conducted to quantitatively examine the topological protection. Compared with other electro-optical materials based on reconfigurable topological photonics, a wider bandwidth and greater tunability of both central bandgap frequency and topological phase transition can happen in the proposed scheme. Our platform has great potential in practical applications in lasing, light sensing, and high-contrast tunable optical filters.

Reconfigurable triple‐band filtering power divider with continuously tunable center frequency using multimode resonators

AbstractIn this paper, a reconfigurable microstrip triple‐band filtering power divider (FPD) is proposed with continuously independent tunable center frequency (CF) by using varactor‐loaded triple‐mode resonators. The proposed tunable FPD is basically composed of a pair of varactor‐loaded triple‐mode resonators, a common input line, a pair of coupled output lines, and an isolation resistor. Based on the proposed topology, the employed triple‐mode resonators and the input/output are reasonably arranged to achieve desired triple‐passband filtering power division response. To demonstrate the feasibility of the design concept and method, a reconfigurable triple‐band FPD is designed and fabricated. The CFs of the three passbands can be adjusted independently, the frequency tuning ranges of triple‐bands are 1.63–1.79, 2.0–2.11, and 2.26–2.33 GHz with good return loss and high port‐to‐port isolation, respectively.