dB Stopband Research Articles

Design of highly selective ultrawideband FSS based on deep learning for EMI shielding in 5G bands

Abstract In this paper, a frequency selective surface (FSS) for n41, n78, n257, n258, n260, and n261 bands electromagnetic interference (EMI) shielding is proposed using a deep learning method. The size of the FSS unit cell is 4.8 mm (0.071 λ 0 ), where λ 0 is the wavelength corresponding to the cutoff frequency of the −10 dB stopband where the n41 and n78 bands are located. This FSS exhibits three key characteristics. Firstly, it possesses ultrawideband capability, with a stopband frequency range of 23.922–40.023 GHz and a relative bandwidth reaching 50.4%. Secondly, it effectively shields against electromagnetic radiation in multiple 5 G bands, including n41, n78, n257, n258, n260, and n261. Thirdly, it demonstrates high selectivity, with a very narrow transition band of only 0.405 GHz between the passband and the stopband. To validate the designed FSS, a prototype of this structure was first fabricated for experimentation. The experimental results show that the measured S21 is highly consistent with the simulated S21. Secondly, the unit cell array of FSS is placed above the monopole antenna for co-simulation. It can be observed that after placing the FSS, the intensity of electromagnetic radiation propagating in space is significantly reduced by up to more than 90%. Therefore, the proposed FSS can be effectively applied for EMI shielding in 5 G bands.

A novel narrowband MEMS filter with extensional mode resonators

This work proposes a novel electrostatic mechanically coupled narrowband filter based on width extensional mode (WEM) resonators. Rectangle plate WEM resonators have the potential for high quality factor (Q) and low motional resistance. The rectangle plate was slotted to reduce thermoelastic loss and optimize mode shape coefficient of WEMs. The Q values of coupled WEM resonators exceed 80,000, enabling a narrow bandwidth filter. A 3λ/4-length coupling beam filter (CBF) was demonstrated. The 3λ/4 coupling beam filter has a center frequency of 79.79 MHz and a bandwidth of 0.024 %. The CBF exhibits an impressive 26.39 dB stopband rejection and 6.97 dB insertion loss with 5.13 dB passband ripple. The MEMS filters presented in this work hold significant potential for applications in wireless communication systems.

A mechanically coupled MEMS filter with high-Q width extensional mode resonators

This work presents a novel radio frequency (RF) narrowband Si micro-electro-mechanical systems (MEMS) filter based on capacitively transduced slotted width extensional mode (WEM) resonators. The flexibility of the plate leads to multiple modes near the target frequency. The high Q-factor resonators of around 100 000 enable narrow bandwidth filters with small size and simplified design. The 1-wavelength and 2-wavelength WEMs were first developed as a pair of coupled modes to form a passband. To reduce bandwidth, two plates are coupled with a λ-length coupling beam. The 79.69 MHz coupled plate filter (CPF) achieved a narrow bandwidth of 8.8 kHz, corresponding to a tiny 0.011%. The CPF exhibits an impressive 34.84 dB stopband rejection and 7.82 dB insertion loss with near-zero passband ripple. In summary, the RF MEMS filter presented in this work shows promising potential for application in RF transceiver front-ends.

Design and fabrication of a Wilkinson power divider with harmonic suppression for LTE and GSM applications

Conventional Wilkinson power dividers (WPDs) can provide acceptable performance close to the nominal center frequency. However, these WPDs can also exhibit poor out-of-band performance while requiring a large footprint. In order to improve on the current state of the art, a modified microstrip WPD is proposed that exhibits a substantially improved stopband and high isolation. A lowpass filter (LPF) structure is utilized in both branches of the power divider to provide harmonic suppression. According to the obtained results, the input return loss (|S11|), output return loss (|S22|), output insertion loss (|S21|) and isolation (|S32|) are better than 34.2 dB, 26.2 dB, 3.52 dB and 31.2 dB, respectively. The proposed modified WPD has a wide 20 dB stopband (from 2.54 GHz to 13.48 GHz) and filters the second to seventh harmonics with attenuation levels of greater than 20 dB. The proposed WPD has a small size of 33.8 mm × 27 mm (0.42 λg × 0.33 λg), where λg is the guided wavelength at the operating frequency of 1.8 GHz. The WPD has been fabricated and tested and shows good agreement between simulated and measured results and the proposed design has desirable characteristics for LTE and GSM applications.

On Controlling the Passband and Stopband of UWB Band-Pass Filter

In this article, the passband, and stopband of quarter wavelength stubs-based Band Pass Filter (BPF) are controlled by a straightforward and new method. This method depends on inserting an attenuation pole and tuning the passband by a Circular Slot Ring Resonator (CSRR) and rectangular slot beneath the BPF’s stubs. Hence, controlling the passband width without any further area usage is achieved. The stop-band rejection level and bandwidth are controlled by inserting Band Stop Filter (BSF) after the BPF such that the used rectangular slots beneath the stubs control the stop-band bandwidth. For verification, third-order Chebyshev BPF and Butterworth BSF filters are used. The proposed filter passband is chosen to cover the sub-6 GHz different 5G bands. The proposed BPF has an ultra-wide 3 dB passband of 2.97 GHz (2.35 GHz-5.32 GHz) and a 20 dB stopband of 4.59 GHz (6.18-10.77 GHz). The proposed BPF filter is fabricated, measured, and the results are in good agreement with their simulated counterparts.

A micromachined ultra‐wide stopband lowpass filter based on stepped impedance resonator loaded T‐shaped structure

This article presents an additively manufactured micro-coaxial lowpass filter with ultra-wide stopband suppression. To achieve a wide stopband response, two-stage stepped impedance resonator loaded T-shaped structures are used to substitute the low impedance transmission lines in the lowpass filter prototype to generate extra transmission zeros in the stopband. Shunt open-ended stubs are introduced to the structure in flexible positions to further strengthen the suppression at specific frequency points. The filter is designed with 22 GHz passband and the 20 dB stopband suppression bandwidth over 300 GHz. The micro-coaxial lowpass filter structure was fabricated based on additive metal micro-manufacturing process with the rectangular cross-section dimensions of 0.5 mm × 0.5 mm (300 GHz cutoff frequency). To facilitate the measurement, pair of coax-to-CPW transitions are added at the input/output ports. The measurement results show excellent agreement from DC-110 GHz (limitation of the measurement device), with insertion loss of 1.05 dB at the cutoff frequency fc, which validates the design.

Ultra-wide stopband waveguide bandpass filter based on out-of-band staggered tuning coupling technology

ABSTRACT A design method of greatly widening the stopband of waveguide bandpass filter is proposed in this paper. The parasitic passband of the filter mainly comes from the high-order oscillation mode of the resonant unit. Based on the theory of capacitance loading shortening of the transmission line and the transmission characteristics of each mode in the resonant cavity, a pair of transverse one-dimensional uniform capacitive diaphragms are placed in the center of the resonant cavity to shorten the resonant cavity and suppress the high-order modes. By changing the width of the waveguide and the height of the waveguide on the distribution of out-of-band transmission poles, the out-of-band transmission poles of each resonator can be staggered, which destroys or weakens the high-order oscillation coupling transmission conditions between the resonant units of the filter and further expands the stopband width of the filter. The results show that the length of the X-band waveguide bandpass filter (center frequency 9.5 GHz, bandwidth 400 MHz) is reduced by 54%, and the 40 dB stopband rejection bandwidth is 5.7 times the cutoff frequency of the transmission waveguide, compared with the prototype half-wavelength direct-coupled filter of the conventional Chebyshev filter.

Compact SIDGS Filtering Power Divider With Three-Port 10-GHz Reflectionless Range

In this brief, a compact high isolation filtering power divider (FPD) with wide stopband and wide-band three-port reflectionless operation is proposed. Such FPD consists of a trigonometric substrate-integrated defected ground structure (SIDGS) based resonator and three absorptive networks. The good stopband performance is achieved by using the trigonometric SIDGS. Meanwhile, the absorptive networks based on common-gate topology capable of absorbing the out-of-band reflected signals are connected to the SIDGS resonator, which provides the wideband reflectionless operation in all three ports of the FPD. To verify the mechanisms mentioned above, a FPD operated at 2.10 GHz (i.e., <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\boldsymbol {f_{0}}$ </tex-math></inline-formula> ) is fabricated, which exhibits a 42 dB stopband rejection level up to 20 GHz (i.e., <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$9.5\boldsymbol {f_{0}}$ </tex-math></inline-formula> ). The measured reflectionless range with return loss lower than −10 dB is up to 10 GHz in all three ports. The core size of the FPD is about <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.36\,\,\boldsymbol {\lambda _{g}} \times 0.38\,\,\boldsymbol {\lambda _{g}}$ </tex-math></inline-formula> , where <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\boldsymbol {\lambda _{g}}$ </tex-math></inline-formula> is the microstrip guided wavelength at the center frequency.

Conformal Multifunction FSS With Enhanced Capacitance Loading for High Angle Stable Stopband Filtering and Microwave Absorption

This article presents a new method of enhancing the capacitance of a single-layer conformal frequency selective surface (FSS) for multifunctional operation. The unit cell of the FSS consists of gridded square loop-loaded modified Jerusalem cross (JC) element. End capacitance of the conventional JC FSS is enhanced by placing parallel diagonal strips, resulting in a pair of reflection zeros near the sidebands. The modified geometry provides high selective stopband filtering that reflects the signal at 20.16 GHz with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sub> < −10 dB stopband bandwidth of 20.52%. The FSS shows stable stopband reflection up to a high incidence angle of 85°. The stability in the filter response is also achieved at various conformal angles. The proposed FSS is optimized for dual-band near-perfect absorption. At two resonances 12.76 and 22.54 GHz, the absorption are 99.88% and 99.1%, respectively. Prototypes of the stopband FSS and the dual-band absorber are fabricated to verify the feasibility of the proposed concept. The measured filter and absorber responses are found consistent with the simulated results. As a conformal multifunctional structure, the proposed FSS has the merits of electromagnetic interference shielding and radar cross section reduction in stealth application.

Calibration of optimized minimum inductor bandpass filter with controllable bandwidth and stopband rejection

A calibration methodology for the optimized minimum inductor (OMI) bandpass filter (BPF) to compensate passive components' inherent loss, such as resistances and reactances, is presented. OMI BPF prevails conventional elliptic and Chebyshev BPFs by introducing fewer inductors for the same stopband rejection requirement. Given design specifications (bandwidth, stopband rejection) at a specific center frequency, the calibration flow optimizes the approach to offset the inaccuracy of center frequency, bandwidth, and stopband rejection due to the discrepancy between the actual and ideal prototype passive components. Two OMI BPF designs before and after calibration are presented for demonstration and comparison. They are 1) a 3rd order centered at 2.388 GHz, 35.54% fractional bandwidth (FBW), 29.97 dB stopband rejection, and 2) a 7th order centered at 2.333 GHz, 17.40% FBW, 62.29 dB stopband rejection.

Miniaturized dual stop band frequency selective surface with broadband linear co to cross polarization conversion ability

This work presents a miniaturized angularly stable frequency selective surface (FSS) on a single layer substrate. Unit cell comprises of convoluted circular rings connected cross dipole and linear arrow headed dipole printed on both sides of the substrate. A closely spaced dual stop band resonances are obtained at 10 and 14.3 GHz with frequency ratio of 1.43. FSS shows a − 10 dB stopband bandwidth (BW) from 7.32 to 11.05 GHz (40.61%) and 12.68 to 15.97 GHz (22.97%) with >30 dB stop band rejection at center frequencies. FSS poses the features of stable performance under change in polarization and angularly stable response up to 60° incident angle. Later the proposed FSS is printed on grounded thick dielectric substrate and it shows broadband reflection type co to cross polarization conversion from 7.2 to 17.32 GHz (82.54%) with polarization conversion ratio >80% and oblique angle stability up to 35°. To the best of authors' knowledge, this is the first time in literature a single unit cell geometry with two different properties, spatial filtering on transmission and polarization conversion on reflection have been designed and analyzed in details. A finite array with size 25 × 25 of the proposed FSS is fabricated and the measured filter response shows a good agreement with simulated performance at normal and oblique incidences. The proposed FSS is used as a reflector of a dual band monopole antenna and the simulation results shows gain improvement of 5.51 and 3.8 dB at the two stopband frequencies.

A Novel Miniaturized Ultra-Wideband Frequency Selective Surface With Rapid Band Edge

This paper presents a novel frequency selective surface (FSS) structure with miniaturized, rapid band edge, ultra-wideband passband, and ultra-wideband out of band shielding. The proposed FSS is formed by using two air foam layers to cascade three single layers with the unit cell dimension in 6mm×6mm(0.0234λ×0.0234λ). The corresponding equivalent circuit models are developed for interpretation on the physical mechanism and formulate the relevant design equations. Finally, a prototype of the proposed structure is fabricated and measured to verify the full-wave simulation, and theoretical analysis results. The measured results show the proposed FSS has a -3 dB passband from 1.17 GHz to 7.08 GHz and a -10 dB stopband from 7.34 GHz to 15.0 GHz. The calculated relative bandwidth is 143.3% and 68.6%, respectively, which exhibits the stable performance against the variation of the incident angles from 0° to 45° for both TE and TM polarizations. All these results demonstrate that the proposed FSS is suitable for the actual miniatur-ized EMI shielding scene application.

Novel Selective Feeding Scheme Integrated With SPDT Switches for a Reconfigurable Bandpass-to-Bandstop Filter

This paper demonstrates a new technique for designing high performance reconfigurable bandpass-to-bandstop filters by employing a ring resonator and a selective feeding scheme integrated with single-pole double-throw switches (SPDT). The transformation from bandpass-to-bandstop mode and vice-versa is achieved by connecting or disconnecting two λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> /4 open-circuited stubs on the ring using PIN diodes. SPDT switches are employed for electronic switching between two different feeding line sections. In the bandpass state the resonator presents two transmission zeros near the edges of the passband and four attenuation poles inside the passband, enhancing the filter's performance, thus achieving excellent sharp rejection with high roll-off-rate (ROR <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20dB</sub> ). On the other hand, high stopband rejection with wide bandwidth, good return loss and good skirt-band attenuation rates are achieved in the bandstop state. Even- and odd-mode analysis is adopted and closed-form expressions are derived to describe the filter's behaviour. To verify the validity of the proposed design, a prototype filter was fabricated and measured. In measurement, a 65% 3-dB bandwidth bandpass filter (BPF) with an insertion loss of 0.86 dB was switched to a 70% 20-dB bandwidth bandstop filter (BSF) with more than 40 dB stopband rejection.

Compact tunable U-Hi-Lo low pass filter using quasi C-shaped stubs

This paper introduces a tunable U-form low pass filter composed of Hi-Lo impedance lines with a pair of quasi C-shaped inductors for covering L-band, S-band, and C-band. The quasi- C-shaped inductors are integrated for the purpose of widening pass-band, minimizing the insertion loss, and improving the roll-off rate. The constructed topology has been printed on RO4003 with permittivity of 3.38, loss tangent of 0.0027, and height of 0.813 mm. The filter is distinguished with a compact size of 15.5 × 10.5 mm2, in addition to its low insertion loss of 0.6 dB in L-band and S-band, sharpness factor of 28.8 dB/GHz, and -20 dB stop-band of 4.85 GHz. A pair of lumped capacitors are both embedded in the substrate between the open ends of the C-shaped inductors and the ground plane for achieving a tuning range from 4.7 GHz till 7.56 GHz when using 0.2 pF lumped capacitors. Consequently, it will lead to a reduction in size of 37.5 % when compared to the size of the conventional LPF. Good consistency between simulated and measured characteristics of the fabricated LPF is achieved with/without lumped capacitors.

Double-Layer Microstrip Band Stop Filters Etching Periodic Ring Electromagnetic Band Gap Structures

The electromagnetic band gap structure (EBGs) is widely used in microwave engineering, such as amplifiers, waveguides, microstrip filters, due to the fact of its excellent band stop characteristics. In this paper, three kinds of microstrip band stop filters were proposed which were etched with a hexagonal ring EBGs, octagonal ring EBGs and elliptical ring EBGs. Firstly, the etching coefficient of a band stop filter is proposed, and the performance of filters with different etching coefficient was analyzed. Secondly, the equivalent circuit of an EBGs band stop filter is proposed. By comparing the simulation results using advanced design system (ADS) and high frequency structure simulator (HFSS), it was found that the simulation results had the same −10 dB stopband width which verifies the correctness of the equivalent circuit model. Finally, three kinds of microstrip stopband filters were fabricated and measured. The experimental results of the −10 dB stopband width and resonant frequency were in good agreement with the simulation results. The −10 dB stopband fractional bandwidth of the three kinds of microstrip stopband filters was more than 63%. The proposed microstrip band stop filters can be widely used in microwave devices with a wide stopband.

Compact Ultra-Wide Band Frequency Selective Surface With High Selectivity

In this communication, an ultra-wide band frequency selective surface (FSS) with excellent transmission performance in the desired passband and reflection performance in the out-of-band is proposed. In order to simultaneously achieve a wide bandwidth, as well as the better band-stop characteristic, a cascaded hybrid resonant element is employed to construct the FSS, which is composed of resonant circular rings and inductive meandering grid arranged on different layers. In addition, a fifth-order equivalent circuit model (ECM) is established over the entire band from 0.1 to 8 GHz, which can accurately describe the frequency responses. Finally, a prototype of the proposed design is fabricated and measured. The results show that a wide −3 dB bandwidth of 154.4% from 0.5 to 3.88 GHz is achieved. The −10 dB stopband is from 4.04 to 7.68 GHz and a sharp transition within 160 MHz from passband to stopband is obtained. The dimensions of the element are as compact as $0.0263\lambda \times 0.0263\lambda $ at the starting frequency of the passband. All the results demonstrate the ultra-wide bandwidth advantage and excellent out-of-band reflection, steep roll-off, and miniaturization performances.

Dual-Band Bandpass Filter and Filtering Power Divider With Ultra-Wide Upper Stopband Using Hybrid Microstrip/DGS Dual-Resonance Cells

In this paper, two types (i.e., type-A and type-B) of hybrid microstrip/defected ground structure (DGS) cells are proposed for passive circuit implementation with ultra-wide stopband. Both cells consist of the stepped-impedance DGS and embedded folded slotline on the ground, which could obtain the dual-resonances. In type-A cell, a microstrip T-stub on the top side is introduced, which can not only allocate a strong coupling to the DGS with slotline on the bottom side, but also act as the input/output port. To finely adjust the dual-resonances of the type-B cell, a grounded microstrip patch is used. Meanwhile, such compact cells could feature an ultra-wide upper stopband, due to their own slow-wave effect. Based on the aforementioned hybrid microstrip/DGS cells, two dual-band bandpass filters (BPFs) and a dual-band filtering power divider (FPD) are proposed and fabricated. Measured and simulated results are in a fairly-close agreement. Both dual-band BPFs exhibit the ultra-wide upper stopband, which extends up to 40 GHz with a high rejection level about 30 dB. Besides, the dual-band FPD has merits of more than 18.7 dB of in-band isolation and 28 dB stopband rejection levels up to 40 GHz.

Synthesis and Design of LTCC Filtering Balun With Wide Stopband

This brief presents a synthesis method to design compact low temperature co-fired ceramic (LTCC) filtering balun with harmonic suppression. To achieve wide upper stopband, the design equations are driven based on the schematic. Benefiting from the three-port synthesis method and the flexible 3-D layout on LTCC technology, the coupling matrices at the second and third harmonics are manipulated to achieve the harmonics suppression without extra circuits. Meanwhile, the coupling elements of the passband can keep their desired values. A LTCC filtering balun is designed and implemented for demonstration. A 20 dB stopband rejection from $1.2{f} _{0}$ to $4.1{f} _{0}$ is observed. Good agreement between simulated and measured results verifies that the proposed method is suitable and the LTCC technology is reliable for wide stopband filtering balun designs.

Metamaterial cell inspired high gain multiband antenna for wireless applications

In this paper, a novel metamaterial inspired high gain dual band antenna has been designed. The unit cell is highly reflective, resonating at 5.9 GHz and 9.9 GHz with 3 dB stopband characteristic from 2 to 4.5 GHz. The unit cell structure has been characterized by deriving its equivalent circuit model in ADS, where the circuit simulation shows good matching with the full-wave analysis. The unit cell is placed at a height of 0.152λo above the patch antenna forming a resonant cavity which causes gain enhancement of 4 dBi and 3.1 dBi along with an increase in bandwidth of 90.63% and 179.63% respectively at the operating frequency of 5.7 GHz and 10.3 GHz. Furthermore, the prototype antenna has been fabricated and measured where the experimental results are in good agreement with the simulated responses.

A New Lowpass Filter Unit Cell with Sharp Roll-off and Improved Stopband Performance in Coplanar Waveguide Technology

In this paper, a new compact unit cell for a coplanar waveguide (CPW) lowpass filter (LPF) is proposed. By combining a pair of coupled parallel stepped impedance resonators (SIRs) and high impedance short stubs in the CPW line, a fifth-order elliptic lowpass filter unit cell (LUC) is designed. The extra transmission zero introduced by parallel coupled SIRs is used to extend the stopband and increase the roll-off rate. The characteristics of the proposed LUC is investigated to achieve a sharp roll-off and a wide stopband. The measured results are in accordance with the simulated results. It has an insertion loss less than 0.9 dB from dc to 6 GHz, and a wide -15 dB stopband from 7.5 to 18 GHz. In addition, the filter dimensions are as small as 4.9 mm × 8.7 mm, that is, 0.046λg2, where λg is the guided wavelength at the cut-off frequency. The filter structure is simple and easy to fabricate as well.