Millimeter-wave Filter Research Articles

High-entropy processed high quality and low-temperature cofired LiMgPO4-based dielectric ceramics for low-loss packaged millimeter-wave filters

In this work, we successfully integrated the high-entropy concept into LiMgPO4-based ceramics by strategically substituting Mg2+ with various divalent ions. Through a low-temperature solid-phase reaction method, a series of high-entropy LiMg0.9R0.1PO4 (R = Ni, NiCo, NiCoZn, NiCoZnCu, NiCoZnCuMn) ceramics were prepared successfully, and their MW dielectric properties were found to be heavily influenced by the ionic character, lattice energy, bond energy, entropy, ion radius disorder, and electronegativity of chemical bonds. Remarkably, LiMg0.9(Ni0.2Co0.2Zn0.2Cu0.2Mn0.2)0.1PO4 (LM5RP) ceramics exhibited exceptional MW dielectric properties (εr = 7.43, τf = − 27.6 ppm/°C, and Q × f = 83,216 GHz@13 GHz and 105,889 GHz@28 GHz) when sintered at an extraordinarily low temperature of 900 °C for 2 h. To demonstrate the real-world applicability of the LM5RP high-entropy ceramic, we designed and fabricated a state-of-the-art millimeter-wave (mmWave) filter, operating at 28 GHz, using low-temperature co-fired ceramic (LTCC) packaging technology. The filter exhibited unparalleled performance, featuring a low insertion loss (0.9 dB) and wide stopband suppression (> 17 dB @ 2.14 f0). These groundbreaking findings underscore the immense potential of high-entropy ceramics in revolutionizing advanced MW applications, particularly in the domain of B5G/6 G mmWave communication systems.

A new electric coupling and its applications on millimeter-wave SIW filter with high selectivity and controllable TZs

In this paper, a novel simple horizontal electric coupling (SHEC) structure is proposed, which only requires a single elliptical ring slot to be etched at the top layer of the substrate between the two adjacent SIW cavities. The working mechanism of SHEC is revealed using the equivalent magnetic current theory. The results indicate that the ring slot is divided into two semi-elliptical slots by the via-hole, and the induced electric fields generated by equivalent magnetic current near the slots enhance the extremely weak electric field intensity at the edge of the SIW cavities, resulting in the formation of electric coupling between the adjacent two cavities. As a verification, a fourth-order SIW filter operating at 27.96 GHz was designed based on the SHEC structure. The filter exhibits high selectivity with sharp roll-off of 114/105 dB/GHz and obtains controllable transmission zeros (TZs) by changing the size of the elliptical slot. Additionally, the proposed SHEC design has a regular via-holes arrangement and does not require etching slot on the bottom layer of the substrate, making it easy to accurately process, integrate, and package, providing a new approach for the design of millimeter-wave filters.

Swarm Intelligence-Homotopy Hybrid Optimization-Based ANN Model for Tunable Bandpass Filter

High-performance tunable radio frequency (RF)/ microwave and millimeter-wave filter design is a challenging task due to the lack of a basic theory. The filtering characteristics are highly sensitive to the variation of tuning elements that are commonly modeled and achieved by optimization algorithms. However, those optimizations only provide satisfactory results with a good set of initial parameters. Such range-limited optimization algorithms generally have issues of falling into local optima, slow convergence, and cumbersome implementation. To mitigate this problem, for the first time, a topology-based local optimizer is integrated with metaheuristic global optimization algorithms in this work. We have hybridized the homotopy method with an improved whale optimization algorithm (WOA) and a gray wolf optimization (GWO) algorithm. In this work, an artificial neural network (ANN) is formulated and studied, which has twofold applications. First, ANN is used as a surrogate model to represent the time-consuming electromagnetic (EM) model in expediting the hybrid optimization process of tunable filters. Second, an ANN model is developed on data generated by the proposed optimization algorithm for predicting tunable circuit parameters at different tuning stages. The proposed ANN model-based algorithm is then applied to a fifth-order lumped-element tunable circuit and two fourth-order full-wave EM simulation models of two tunable bandpass filters (tBPFs). The calculated results out of the ANN model demonstrate a good agreement with simulation and measurement counterparts.

A method to improve the finite surface roughness of an LCP-based microstrip filter at Q-Band

In this paper, a 4-pole quasi-elliptic planar bandpass filter at Q-band on the liquid crystal polymer (LCP) is presented. The conventional design and development process of millimetre wave filters required to consider the surface roughness effects from the design stage for accurate development of filter at high frequency. Finite surface roughness of rolled copper and its impact on electrical performance are studied. Polishing is one of the methods to improve the conductor surface roughness. An improvement of 50% in surface roughness is reported for the rolled copper on the LCP, resulting in better than 1 dB insertion loss of the bandpass filter. Commensurate simulation and measurement results are presented.

3-D-Printing and High-Precision Milling of W-Band Filter Components With Admittance Inverter Sequences

This work presents the design of an additively manufactured W-band bandpass filter and a subtractively manufactured W-band diplexer to demonstrate the use of admittance inverter sequences for the ease of manufacture at millimeter-wave frequencies. Contrary to typical impedance inverters (E-plane and H-plane irises), the use of admittance inverters (E-plane and H-plane stubs) allows for larger dimensions to be specified and ultimately does not impede the general waveguide path. The proposed bandpass filter is designed with all E-plane stubs, while the diplexer is designed with one branch using both E-plane and H-plane stubs as an arbitrary sequence, and the second branch using all H-plane irises. The additively manufactured bandpass filter is fabricated using the most elementary-level stereolithography (SLA)-based methods, demonstrating that a hobbyist-type SLA printer and metallization method can procure exceptional results for millimeter-wave filter designs. The subtractively manufactured diplexer is fabricated using high-precision computer numerical control (CNC) milling to highlight the use of arbitrary inverter sequences in a more complex and robust design profile, while the dispersive transmission zeros that are caused by over-moding of the inverter stubs are used to demonstrate unique isolation characteristics in the upper W-band region. The design concepts, fabrication profiles, simulations, and measurements which are presented in this work highlight a viable option for overcoming miniaturized dimensions in millimeter and submillimeter-wave applications.

Comparative Study of Surrogate Modeling Methods for Signal Integrity and Microwave Circuit Applications

With a short product cycle as we see today, fast and accurate modeling methods are becoming crucial for the development of new generation of electronics devices. Furthermore, increased complexity in circuitry and integration compounds design iteration and the associated, high-dimensional sensitivity analysis and performance optimization studies. Therefore, black-box surrogate models replacing the actual circuitry offer an attractive alternative for more efficient design iteration, optimization, and even direct Monte Carlo analysis. In this article, surrogate models built using nonparametric Gaussian process (GP) are presented. A robust framework based on probabilistic programming is used for training GP models. Other methods, such as partial least-square regression, support vector regression, and polynomial chaos, are used to compare with the performance of GP. Three design applications, a high-speed channel, a millimeter-wave filter, and a low-noise amplifier are used to demonstrate the robustness of the proposed GP-based surrogate models.

Laminated Glass-Based, Compact Inline Stepped-Impedance Resonator Bandpass Filters for 5G New Radio Modules

Miniaturization of millimeter-wave (mm-wave) filters relies on innovative designs that require fine lines and spacings. These designs drive the need for precision fabrication that cannot be achieved with traditional laminates or cofired ceramics. This gives unique performance justification to glass substrate technologies for future integrated mm-wave components because of both superior performance and 3– $5\times $ reduction in volume. In this letter, laminated glass-based compact inline stepped-impedance resonator (SIR) filters are presented for 5G new radio (NR) modules. The frequency bands of interest are 5G NR bands: n257 and n260. The filters are designed using microstrip transmission lines with the fourth-order quasi-elliptic response with skew-symmetric feed. The enhanced transition responses of these filters provide transmission zeros (TZs) on either side of the passband. When these filters are designed for mm-wave applications, their dependence on the accuracy of metal patterns increases significantly due to narrow linewidth and spacing requirements to realize moderate fractional bandwidths. Two inline SIR filters are fabricated using a large-area, panel-compatible semiadditive patterning (SAP) process on precision redistribution layers (RDLs) of a laminated glass substrate stack up. The sensitivity of these filters to the dimensional variations of patterned copper is analyzed, and the discrepancy between simulated and measured response of the filters is studied by performing postfabrication simulations.

Low-loss and tunable millimeter-wave filters using spherical dielectric resonators

AbstractMillimeter-wave band-pass filters using spherical dielectric resonators are presented. The dielectric spheres are sandwiched between metal plates and are excited by a simple microstrip line structure on a thin-film circuit board. As such, these filters could also be implemented in the back-end-of-line layers of an integrated circuit. A single resonator, based on a diameter 0.6 mm alumina ceramic sphere, is shown to resonate with high unloaded Q-factor of 750 at 170 GHz. A three-sphere band-pass filter is measured showing <5 dB insertion loss and 0.4% bandwidth at 170 GHz. A concept for mechanically tuning of a two-sphere band-pass filter is demonstrated for a filter operating around 105 GHz. The measured filter shows approximately 5 dB insertion loss and <0.5% bandwidth and its passband can be varied over 3 GHz of frequency, or 3%. Technological challenges are discussed.

Miniaturised multi‐channel millimetre wave filter bank

This Letter presents a miniaturised multi-channel millimetre-wave filter bank implemented by stripline and substrate integrated waveguide (SIW). Since the orthogonal transverse electric mode and transverse electromagnetic mode can coexist in the same hybrid structure cavity and be transmitted almost without mutual interference, embedding stripline into SIW to miniaturise the size of the filter bank is achievable. A prototype composed of four bandpass filter units working at X-band is designed, fabricated and tested. Measurement results show that the proposed device has four independent channels with a good quality of bandpass filtering and the isolation among ports of different filter units are relatively high. The proposed prototype has 39.5% volume reduction than traditional stacking structure as well as good scalability, which has broad prospects in the application of next-generation wireless communication system.

3D Printing of Microwave and Millimeter-Wave Filters: Additive Manufacturing Technologies Applied in the Development of High-Performance Filters with Novel Topologies

Three-dimensional (3D) printing, additive manufacturing (AM), digital manufacturing, and free-form fabrication are some of the synonymous expressions commonly used to identify the new class of processes through which objects are built by selectively adding material, usually layer by layer, instead of subtracting material, as in conventional machining techniques like chemical etching, laser-cutting, milling, and electroerosion [1].

Homotopy Optimization of Microwave and Millimeter-Wave Filters Based on Neural Network Model

High-performance microwave and millimeter-wave filters’ design is a challenging task because the filter characteristic is rather sensitive to the variation of geometric dimensions and electrical sizes. A common practice in filter design is to optimize the design variables starting from a set of initial values. However, if the initial values are not sufficiently close to the optimal solution, the optimization often fails to provide any satisfactory result. To deal with this problem, for the first time, the homotopy method is introduced to microwave and millimeter-wave filters’ optimization problems in this article. The homotopy method formulates a series of intermediate optimization problems, which can guide the optimizer to approach the optimal solution for the target filter design. In this article, the artificial neural network (ANN) is adopted as the surrogate model to the time-consuming electromagnetic model to speed up the homotopy filter optimization process. Two design examples are given to demonstrate the homotopy optimization technique based on the ANN model, including an all-pole filter and a generalized Chebyshev filter with a frequency-dependent coupling. Both filters with optimized geometric dimensions are simulated, and the all-pole filter is fabricated and measured. The simulation and measurement results verify the accuracy of the ANN model and validate the homotopy optimization method.

SYNTHESIS OF CHAINED-ELLIPTIC FUNCTION WAVEGUIDE BANDPASS FILTER WITH HIGH REJECTION

This thesis presents the synthesis of a bandpass filter to achieve high selectivity and rejection properties using a new class of filter functions called chained-elliptic function filters. Chained-elliptic filters have higher selectivity compared to Chebyshev function filters and have the property of sensitivity to manufacturing tolerance reduction in chained-function filters. The proposed design has high selectivity and reduced sensitivity, enabling easier and faster filter fabrication, which greatly reduces the time and cost required to manufacture a millimeter-wave filter. The characteristic polynomials of chained-elliptic function filters are derived (through chaining elliptic filtering function) and extracted to form the coupling matrix of the bandpass filter.

CMOS Closed-Loop Control of MEMS Varactors for Robust Ka -Band Tunable Filters

A closed-loop capacitance sensing and control mixed-signal CMOS circuit with a dedicated sensor electrode and a proportional-integral controller was designed to have a sensitivity of 1 fF, a settling time of $200~\mu \text{s}$ , and a voltage capacity of 150 V. In a hybrid integration with a MEMS varactor, the control circuit could switch the varactor capacitance from 270 to 365 fF in 15 ms and hold it within ±2% between 25 °C and 80 °C. Furthermore, using the varactor as the tuning element of a substrate-integrated-waveguide (SIW) resonator, the control circuit could tune its resonance frequency from 33.2 to 33.9 GHz while keeping it within ±0.08% over a temperature range of 25 °C–90 °C. The control circuit occupied an area of 1.7 mm2 and consumed 6 mW. The present closed-loop-controlled MEMS-tunable resonators could be used in a quartz SIW multi-pole filter to realize tunable, small, light, low-cost, robust, and reliable millimeter-wave filters that are critical to 5G and other future-generation communication and radar systems.

Millimeter-Wave Ultra Wideband Multilayer Superconducting Filter

Ultra wideband (UWB) millimeter-wave filters are needed in the realization of analog front ends of superconductor digital receivers. In this paper, a multi-layer niobium-based superconducting bandpass filter operating over the frequency range 30-60 GHz is demonstrated for the first time. The filter is realized in a co-planar waveguide structure, where cross-overs in the multilayer structure are employed to reduce the filter size. The paper also illustrates how these cross-overs can be utilized to fine tune the filter performance. An 8-pole superconductor niobium-based filter with a size of 0.88 × 2.2 mm designed with a center frequency of 45 GHz and a bandwidth of 30 GHz has been developed, fabricated, and tested. The results demonstrate the feasibility of monolithically integrating UWB millimeter-wave filters with a superconductor receiver on single-chip.

Over-Moded Transverse Magnetic Cavity Filters for Narrowband Millimeter-Wave Applications

A new version of transverse magnetic (TM) cavity filters for high-frequency applications is presented in this letter. The basic building block of these filters is the over-moded transverse magnetic cavity (OTMC). The OTMC supports a pair of high-order resonant modes, namely TM320 and TM230, as well as various nonresonating modes that can be exploited to provide an additional input-to-output path (the propagating TM11 is the one primarily employed in the particular configuration here presented). Besides the possibility of realizing elliptic responses having the maximum number of transmission zeroes, a higher ${Q}$ -factor as well as a significantly reduced sensitivity to manufacturing imperfections is the main benefit making these structures especially suitable for the practical realization of very narrowband millimeter-wave filters. The experimental results of an elliptic fourth-order OTMC filter with 0.2% fractional bandwidth at 41.415 GHz validate the feasibility of these new transverse magnetic filters.

Novel compact millimeter‐wave gap waveguide filter with double‐sided selectivity

AbstractNovel compact gap waveguide (GW) filter with negative coupling structure is proposed. GW is an attractive technology to design accurate millimeter‐wave components without electrical connection. It takes advantages of high Q‐factor, low loss, capability of integration with MMICs and multilayer implementation. GW features with stacked and negative coupling topology are effectively employed here to bring a high performance interconnect, miniaturized and cost‐effective filter. A fourth‐order Ka‐band filter with two prescribed symmetric transmission zeros is designed and fabricated based on the groove GW platform. The simulation and measurement results verify the validity of the method and exhibit high double‐sided selectivity.

Millimiter-wave stripline bandpass filters

The possibility to use thin (1 mm or less) pseudocombline structures of λ/2 resonators made of sections of symmetric transmission strip lines from a dielectric material with e r = 2.2 as millimeter-wave bandpass filters is shown. It is established that the relative bandwidth of such filters can reach FBW = 0.1. Designs of stripline pseudocombline filters with alternating signs of the coupling coefficients, which are characterized by improved selectivity due to the attenuation poles located on both sides of the passband, are proposed; the filters contain stepped-impedance “half-wave” resonators with a new feature. The results of the computer simulation of the frequency characteristics of the filters in the frequency band of 31–122 GHz and the comparison of them with the characteristics of other filters are presented.

Design of Millimeter-Wave Resonant Cavity and Filter Using 3-D Substrate-Integrated Circular Waveguide

In this letter, a millimeter-wave filter constructed by 3-D substrate-integrated circular waveguide (SICW) resonant cavity is numerically and experimentally investigated. The 3-D SICW with the sidewall structure of via fences and circular pads is fully integrated in a low-temperature co-fired ceramic (LTCC) substrate. The fundamental mode (quasi-TE111 mode) of 3-D SICW cavity and the TE130, TE140 modes of overmoded substrate-integrated waveguide cavity are adopted to form the fourth order bandpass filter. Compared with the traditional planar structures, the spatial configuration can enhance the microwave performance of the device and brings more flexibility for the system design. The measurements of the fabricated filter show that the insertion loss is less than 1.9 dB at 174 GHz and the relative bandwidth is about 13.8%, which is in good agreement with electromagnetic simulations.

Design and study of frequency selective surface based on hybrid period metallic mesh

Frequency selective surface based on metallic mesh can realize the physical properties of both high infrared transmittance and millimeter-wave band-pass filter. In order to improve the optical transmittance, reduce surface resistance and suppress the effect of high order diffraction energy on the imaging quality of the optical system, a new design of frequency selective surface based on hybrid period metallic mesh is obtained. In this paper, the diffraction intensity distribution and surface current of frequency selective surface are analyzed based on metallic mesh. Simulation and experimental results show that frequency selective surface based on hybrid period metallic mesh realizes a stable millimeter-wave band-pass filter property, at the same time, it obtains 5% increase of infrared transmittance and 4 Ω reduce of surface resistance. New design of frequency selective surface based on hybrid period metallic mesh effectively suppresses the effect of high order diffraction energy on the imaging quality of the optical system.

Millimeter-wave reconfigurable bandpass filters

Millimeter-wave reconfigurable bandpass filters with the ability to operate between 60 GHz and the E-band, capable of providing good channel isolation, are presented. A fully integrated filter with all reconfigurable elements embedded for compactness and a switchable filter that uses radio frequency micro-electro-mechanical system (RF MEMS) single-pole double-throw switches are designed. A new method that increases fractional bandwidths is introduced. It uses inductively coupled inverters without requiring their tuning. New circuit models are offered for inverters, reconfigurable resonators, and reconfigurable bandstop stubs. Our compact bandpass filter achieved a footprint of only 4.75 mm × 3.75 mm. Measurements for our filters show good agreement with the results of simulations.