Microwave Circuit Applications Research Articles

Butyl rubber dispersed with low-loss Ca0.61Nd0.26TiO3 ceramics for miniaturized flexible microwave substrate applications

Ca0.61Nd0.26TiO3 (CNT)-filled butyl rubber (BR) composites were fabricated via Banbury mixing by changing the ceramic volume fraction from 0 to 0.5. The phase purity of the CNT filler was confirmed using X-ray diffraction, and the morphology of the composite samples was analyzed using scanning electron microscopy and atomic force microscopy. BR/CNT composites filled with 50 vol% filler possessed excellent dielectric properties with a dielectric constant of 11.12 and a very low dielectric loss of 0.0023 in the X-band (8–12 GHz) frequencies. Moreover, the material displayed an appreciable thermal conductivity of 0.883 W/mK, low water absorption of 0.0574 %, and an acceptable CTE of 37.08 ppm/ °C. Thus, the developed BR/CNT composites can be used for flexible microwave circuit applications that require miniaturization.

Electronic, Elastic, Optical, and Thermodynamic Properties Study of Ytterbium Chalcogenides Using Density Functional Theory

In this study, the structural, electronic, optical, elastic, and thermodynamic properties of Ytterbium chalcogenides YbX (X = S, Se and Te) were computed within the first principles using generalized gradient approximation (GGA) as implemented in the pseudopotential plane wave approach. The equilibrium total energy for YbX (X = S, Se, and Te) was calculated as a function of the energy cutoff, k-point grid, and lattice parameter. An optimized lattice parameter of 5.6, 5.66, and 6.136 Å were calculated for YbS, YbSe, and YbTe, respectively. The energy band gaps of YbS, YbSe, and YbTe computed are 1.14, 1.32, and 1.48 eV, respectively. In addition, the low band gap (less than 3 eV) for ytterbium chalcogenides indicated that they may have potential applications in photovoltaic cells and laser diodes. Moreover, the negative dielectric function value for a certain frequency range indicates that these compounds are suitable for specific optical and microwave circuit applications. The result of elastic and thermodynamic property computation reveals that ytterbium chalcogenides are mechanically and thermodynamically stable, which can be useful in a variety of electronic device applications.

Fractance with Tunable Fractor’s Order for Microwave Circuit Applications

There is an increasing number of studies in the literature to implement fractional order components by means of equivalent circuits based on integer order components. Such implementations aim to realize laboratory equipment that can exhibit a fractional behavior in a certain range of frequencies. One of the main limitations of the existing implementation is the fixed fractor’s order. In practice, every time the experimenter wants to change fractor’s order, it is necessary to calculate again the equivalent circuit and implement it again. In order to overcome this limitation, in this paper we propose a new implementation of a fractional order component that enables to tune the fractor’s order. This is achieved by means of variable resistors and a proper control methodology. The methodology can be applied in microwave circuits, for instance for the implementation of filters: a low-pass one is discussed in this paper.

A Windmill-Shaped SSPP Waveguide for High-Efficiency Microwave and Terahertz Propagation

We propose a novel type of spoof surface plasmon polariton (SSPP) waveguide based on windmill-shaped units for high-efficiency microwave and terahertz propagation. The dependence of terahertz dispersion characteristics on geometrical parameters of the proposed waveguide is detailed and investigated. Compared with the conventional comb-shaped and T-shaped SSPP waveguide units, the proposed windmill-shaped unit shows a lower asymptotic frequency and stronger field-confinement characteristics for the supported fundamental SSPP mode. To demonstrate the properties of the windmill-shaped SSPP waveguide, a tapered conversion is designed to connect the windmill-shaped SSPP waveguide and the microstrip for smooth momentum and impedance matching. The simulated results show that the whole waveguide has excellent transmission performance with S11 < −10 dB and S21 > −1 dB from 0 THz to 5.68 THz, as well as a large out-of-band rejection response (S21 < −80 dB). Then, a scaled microwave windmill-shaped waveguide prototype is fabricated and measured. The numerical and experimental results are in good agreement, which further validates the proposed SSPP waveguide design. The proposed waveguide has excellent microwave and terahertz propagation and rejection characteristics, which may have great potential applications in various microwave and terahertz devices and circuits.

Ultra-low permittivity HSM/PTFE composites for high-frequency microwave circuit application

Ultra-low permittivity HSM/PTFE composites for high-frequency microwave circuit application

Laser-Scribed Graphene (LSG) Fabrication Method for Microwave Circuit Applications

In this letter, a laser-scribed graphene (LSG) fabrication method is used to create graphene to be used in a microwave application, for example, a voltage-tunable microstrip attenuator. Graphene used in microwave circuits, such as tunable attenuators, usually focuses on using exfoliated graphene, deposited graphene nanoplatelets, or microwave irradiation. It will be demonstrated that the fabrication of LSG from graphene oxide on standard double-sided FR-4 copper-clad substrate can be used as a voltage-controlled resistor that results in a tunable microstrip attenuator across the microwave frequencies from 50 MHz to 5.5 GHz.

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.

Optical and electrical studies on MgTiO3– Ba5Nb4O15 composite thin films for integrated microwave circuit applications

A correlative study on optical and electrical properties of MgTiO3 (MTO) -Ba5Nb4O15 (BNO) composite thin films (MTBN) with respect to film thickness (260–450 nm) is reported. The optical bandgap energy of MTBN composite thin films (3.76 eV–3.65 eV) are found to be intermediate to MTO (3.44 eV) and BNO (3.85 eV) monoliths. Further, the optical dispersion in MTBN composite thin films can be defined by Cody-Lorentz dispersion law. Higher photoluminescence (PL) emission intensity and shorter carrier lifetime (0.8376 ns) ascribed to local symmetry break is observed for 450 nm film. Moreover, the Mott's variable range hopping mechanism was dominant conduction process in the MTBN composite films. Further, density of states near the Fermi level were found to be around 7.85×1018 eV−1cm−3 - 3.478×1019 eV−1cm−3 for 260 nm–450 nm MTBN films. The obtained optical and electrical properties prospect a potential application of the MTBN composite thin films in integrated microwave circuits.

UV Flash Sintering of Aerosol Jet Printed Silver Conductors for Microwave Circuit Applications

Recently, there has been significant research focus on the design of high functional density electronic systems, especially for 5G applications. In parallel, to address accurate, high-quality, fine-resolution circuit geometry requirements, increased research into additive manufacturing (AM) printing techniques, such as aerosol jet printing (AJP), has emerged. The issues are complicated by low-loss flexible substrates, such as liquid crystal polymers (LCPs), which are commonly used in the design of high-frequency circuits and systems. These substrates cannot be taken through high sintering temperature processes that are commonly required for silver inks. To avoid permanent damage, it is essential that the sintering temperature remains below the glass transition temperature ( ${\text{T}}_{g}$ ) of the substrates. Recent work on LCP substrates has demonstrated 10%–20% bulk silver (Ag) conductivity by using low-temperature thermal sintering techniques. This article investigates the use of a rapid ultraviolet (UV) flash annealing technique to achieve high conductivity of Ag on LCP for use in high-frequency circuits while maintaining the integrity of the substrate. Results are compared against thermally sintered films. The resulting conductivity was studied as a function of UV exposure time and energy, printed film thickness, surface roughness of the substrate, and multilayer deposition.

Design of high-performance microstrip and coplanar low-pass filters based on electromagnetic bandgap (EBG) structures

In this paper, a rigorous method for the design of high-performance n-order Chebyshev low-pass filters based on electromagnetic bandgap (EBG) stepped impedances in microstrip and coplanar (CPW) technologies is proposed. Fifth-order low-pass filters implemented by means of circular patterns etched in the ground plane of microstrip and CPW lines have been designed, simulated and measured on an RO3010 substrate (∊r= 10.2, tanδ = 0.0023 at 10 GHz, substrate thickness h = 0.635 mm and copper thickness t = 0.018 mm). The designed filters have shown a good agreement between EM simulation and measurement results. The proposed EBG-based low pass filters, with six etched circles and a size of 42 × 20 mm2, achieve a passband with a cut-off frequency fc = 6 GHz measured at |S11| = −20 dB, return loss RL = 20 dB, 5 reflection zeros in the frequency axis, a ripple lower than 1.1 dB, and a stopband greater than 5 GHz with a rejection level higher than 30 dB. Therefore, the designed low-pass filters have an ultra-wide passband with high return loss, low insertion losses and a low ripple. In addition, they exhibit a large stopband with high rejection level compared to other reported wide stop-band low-pass filters based on EBG, defected ground structures (DGS), or modified complementary split ring resonators (CSRR). This technique can be used to design high-performance Chebyshev EBG-based low-pass filters with a high degree of control over the characteristics of their frequency responses. This allows to replace the EBG-, DGS- and CSRR-based wide stop-band low-pass filters in microwave circuit and antenna applications, with filter structures exhibiting high return loss, equiripple responses in the passband, and high rejection level in the stopband.

Caracterización en frecuencia de microondas de películas de titanato de bario obtenidas vía Sol-Gel

El presente trabajo se centra en la caracterización estructural, morfológica y dieléctrica de películas de titanato de bario (BTO o BaTiO3 por su fórmula química) depositados mediante la técnica que proporciona recubrimiento por medio de un sistema de rotación (spin coating) sobre substratos de silicio cristalino (Si) y resonadores CPW mediante la técnica Sol-Gel, utilizando una relación molar Ba/Ti de 0.5/0.5. Las guías de ondas se fabricaron sobre substratos de alúmina (Al2O3) con 3 mm de metalización en oro (Au) empleando la técnica de ablación láser. La microscopia electrónica de barrido (SEM) con espectrometría de dispersión de energía de rayos X (EDS) permitió evidenciar la existencia de una película de BTO con una composición elemental de 14.62 % de bario y 5.65 % de titanio, además de un espesor de 0.77 mm medido utilizando la modalidad perfilométrica de la microscopia de fuerza atómica (AFM). La caracterización dieléctrica se llevó a cabo mediante la comparación de la respuesta en frecuencia (parámetro S21) de un resonador CPW con película de BTO depositada y otro resonador de referencia (sin película) usando un analizador vectorial de red (VNA). Estas medidas se comparan a su vez con simulaciones computacionales para obtener las propiedades dieléctricas. Para la película de BTO se determinó una constante dieléctrica relativa (er) de 160 con tangente de pérdida (Tand) de 0.012 para una frecuencia de 3.60 GHz. La constante dieléctrica y la propiedad ferroeléctrica del material elaborado son características bastante promisorias para aplicaciones en circuitos de microondas, tales como miniaturización y sintonizabilidad.

System-Combined ADI-FDTD Method and Its Electromagnetic Applications in Microwave Circuits and Antennas

A system-combined alternating-direction-implicit finite-difference time-domain (SC-ADI-FDTD) is fully presented and well corroborated for 3-D electromagnetic applications. Spatial differential operators of Maxwell’s equations can be expressed as two matrices with the different time-steps in original ADI-FDTD method. From this matrix form, the system chart can be drawn by combining with digital signal processing (DSP) techniques. Based on the intuitive process of the original ADI-FDTD method, we can avoid the repeated computation. Due to the limited computer memory, we employ the unsplited-field convolution perfectly matched layer (CPML) to terminate the FDTD computational domain. Compared with HFSS commercial software, conventional FDTD and original ADI-FDTD methods, the proposed SC-ADI-FDTD method can be well demonstrated and further validated by adopting electromagnetic applications, such as microwave circuits and antennas.

Improved Characterization and Accurate Modelling of Drain Current Derivatives of InP Based High Electron Mobility Transistors Devices.

InP based HEMTs are of great importance, due to their enormous potential in a high-speed modern microwave circuit, power amplifier, and low noise amplifier applications. Therefore, an accurate non-linear equivalent circuit model of HEMTs is very important for an accurate circuit design. This paper presents improved characterization and accurate modelling of drain current derivatives of InP based HEMTs devices. The proposed model is simple, easy to extract, and suitable for implementation in simulation tools. The Ψ function is extended to increase the flexibility of the proposed model. A gate-drain dependent current source is added to increase the S-parameter fitting. A one-dimensional intrinsic multi-bias capacitances model is introduced to avoid convergence failure. The fitting results of the I-V characteristics and its high order derivatives show high accuracy. In addition, S-parameters and Pout for the proposed model are compared with the original Angelov model. The proposed model shows better accuracy.

Comparative investigation of reflection and band gap properties of finite periodic wideband artificial magnetic conductor surfaces for microwave circuits applications in X‐band

In this study, novel designs of artificial magnetic conductors (AMCs) for wideband operation are introduced. By using two techniques of the planar parasitic patches and stacked elements, AMC unit cells are achieved. The first design of the AMC unit cell is composed of four symmetric patches with two arms, which are coupled to a rhombic patch at the center. The second AMC design is configured of two stacked patches, which are connected together through the air spacing. The first and second AMC designs operate at the frequencies of 10.15 and 10.65 GHz with ±90° reflection phase bandwidths of 8 to 12.38 GHz (43.15%) and 8 to 12.90 GHz (46%), respectively, for X-band operation. The proposed AMC unit cells present prominent features such as symmetric structures and wide bandwidths with distinct capabilities. To achieve an accurate evaluation of AMC performance, the reflection and band gap properties of the finite periodic AMC surfaces are investigated. The finite periodic arrays of the AMC reflectors are implemented and tested. The measured results show that they can be applied for wideband applications in microwave circuits and low profile antennas. The 15 × 15 AMC planar and stacked arrays under normal incidence operate at the frequencies of 9.89 and 10.25 GHz with the measured bandwidths of 7.85 to 12.02 GHz and 7.77 to 12.58 GHz, respectively. In addition, to identify the band gap properties of the finite periodic planar and stacked AMC arrays, the method of the suspended microstrip line is applied. For this purpose, the measured band gaps of the planar and stacked arrays include the ranges of 8.12 to 12.63 GHz and 7.81 to 12.10 GHz below −20 dB, respectively.

Transversely Compact Single-Ended and Balanced Bandpass Filters with Source–Load-Coupled Spurlines

Multi-function wireless systems demand multi-channel transmit/receive (TR) modules, particularly as multiple functions are required to operate simultaneously. In each channel, passive components, including bandpass filters, must be compact, or at least transversely compact; thus, the entire circuitry of the channel will be slender, and consequently multiple channels can be parallel-arranged conveniently. In this work, single-ended and balanced bandpass filters for multi-channel applications are presented. As a unique resonator, the U-shaped stepped impedance resonator (USIR) can achieve size miniaturization compared with its corresponding uniform impedance resonator (UIR) counterpart. Hence, with the utilization of USIRs, the proposed bandpass filters are able to acquire compact transverse sizes. Moreover, by using the source–load coupling scheme, two transmission zeros (TZs) are respectively generated at the lower and upper sides of the passbands, which is useful for improvement of the selectivity performance. In addition, spurlines are introduced at the input and output ports to produce another TZ to further enhance the stopband performance, which cannot be acquired by the UIR or stepped impedance resonator (SIR). To verify the aforementioned idea, one single-ended and one balanced bandpass filter are implemented, with experimental results in good agreement with the corresponding simulations. Meanwhile, as compared with some similar works, the proposed balanced filter achieves compact transverse size, sharp selectivity skirt, and wide stopbands up to the fourth-order harmonic with suppression over 20 dB, which illustrates its suitability for differential signal transmission application in microwave circuits and systems.

Structure and Microwave Dielectric Properties of Low Temperature Sinterable NaR5(MoO4)8 (R = La, Pr, Nd, Sm) Ceramics

Phase pure NaR5(MoO4)8 (R = La, Pr, Nd, Sm) ceramics were prepared by a conventional solid-state ceramic route at low sintering temperatures. Powder x-ray diffraction studies were performed to confirm the phase purity of the samples. The presence of MoO 4 2- tetrahedra in the crystal structure was confirmed by Raman spectroscopy of the samples. Surface morphology investigations using scanning electron microscopy show that the sintered samples have compact microstructure and phase homogeneity. The microwave dielectric characterization of the sintered ceramics show that the samples have excellent quality factor, low temperature coefficient of resonant frequency and hence are ideal candidate materials for microwave circuit applications. The NaR5(MoO4)8 (R = La, Pr, Nd, Sm) ceramics possess er from 10.6 to 11.5, quality factor Qu × f from 56200 GHz to 78600 GHz, temperature coefficient of resonant frequency τf from − 42 ppm/oC to − 72 ppm/oC and linear thermal expansion coefficient CTE ranging from + 8.75 ppm/°C to + 11.54 ppm/°C. The chemical compatibility of the ceramics with silver electrodes was determined by x-ray diffraction and energy dispersive x-ray spectroscopic analyses of the silver co-fired ceramic samples.

Metal–Insulator–Graphene Diode Mixer Based on CVD Graphene-on-Glass

In this letter, we present for the first time a mixer circuit based on Metal–Insulator–Graphene (MIG) diodes fabricated with large-scale monolayer graphene grown by chemical vapor deposition. A small-signal model extracted from the diode physical structure is used together with a large-signal model extracted from the dc characteristics of the MIG diode to build a down-conversion mixer. The measured conversion loss at a local oscillator power ( ${P}_{\text {LO}}$ ) of 5 dBm is lower than 15 dB, while RF-to-IF isolation is 36 dB with an input return loss and RF-to-LO isolation better than 10 dB over the frequency band from 1.7–6 GHz. Promising mixer results in combination with the CVD-based process promote the MIG diode-based mixer to be used in low-power, low-cost, microwave, and millimeter-wave circuit applications.

Magneto Tuning of a Ferrite Dielectric Resonator Antenna Based on LiFe5O8 Matrix

LiFe5O8 (LFO) spinel-like material has been studied for use in ferrite resonator antennas (FRAs). Antenna parameters such as gain and return loss were greatly affected when an external magnetic field was applied to the FRA. The temperature coefficient of the resonant frequency (τ f ) for the FRA presented a value of − 482.16 ppm/°C. The magnetic hysteresis results showed that the LFO was a soft ferrite, considering the values of the remanent magnetization (Mr = 5.95 emu g−1), coercive field (0.76 mT), and saturation magnetization (32.15 emu g−1). The magnetodielectric resonator presented a tuning effect in the resonant frequency as a function of the external magnetic field. The antenna bandwidth was also affected by the presence of the magnetic field. LFO is a soft ferrite with applications in microwave circuits, antennas, and devices for operation at microwave frequencies due to its magnetization and demagnetization properties. Impedance study revealed increasing conductivity from room to higher temperature with low activation energy (0.36 eV).

RF Characterization of NiO and TiO2 Based Metal-Insulator-Metal (MIM) Diodes on Flexible Substrates

This paper presents the fabrication and characterization of metal-insulator-metal (MIM) diodes on flexible substrates for RF and microwave circuit applications. Diodes using two types of insulators, titanium dioxide (TiO2) and nickel oxide (NiO), are investigated. These insulators are obtained using different oxidation techniques, i.e., in-situ oxidation for TiO2 and plasma oxidation for NiO. Asymmetric metal contacts (Ti-TiO2-Pd and Ni-NiO-Mo) are utilized to achieve nonlinear $I - V$ characteristics. The fabricated diodes show strong non-linearity, high current densities, and low turn-ON voltage. The diodes show RF to dc rectification with near-ideal behavior and rectification sensitivity of 22 V/W (18 GHz) and 46 V/W (18 GHz) for TiO2 and NiO, respectively. NiO-based diodes barrier shows higher current density and higher cutoff frequency in comparison with TiO2 as expected diodes due to thinner oxide and lower dielectric constant. The diodes also work well as frequency doublers over a wide frequency range of 1–4 GHz for TiO2 and 2–10 GHz for NiO-based diodes. Good dc and RF performance of diodes indicate that good quality oxide can be achieved on plastic substrates and MIM devices can provide a perfect solution for RF and microwave circuits on a flexible substrate.

Influence of SiO2 Addition on Properties of PTFE/TiO2 Microwave Composites

Composite substrates for microwave circuit applications have been fabricated by filling polytetrafluoroethylene (PTFE) polymer matrix with ceramic powder consisting of rutile TiO2 (D 50 ≈ 5 μm) partially substituted with fused amorphous SiO2 (D 50 ≈ 8 μm) with composition x vol.% SiO2 + (50 − x) vol.% TiO2 (x = 0, 3, 6, 9, 12), and the effects of SiO2 addition on characteristics such as the density, moisture absorption, microwave dielectric properties, and thermal properties systematically investigated. The results show that the filler was well distributed throughout the matrix. High dielectric constant (e r > 7.19) and extremely low moisture absorption (<0.02%) were obtained, resulting from the relatively high density of the composites. The ceramic particles served as barriers and improved the thermal stability of the PTFE polymer, retarding its decomposition. The temperature coefficient of dielectric constant (τ e ) of the composites shifted toward the positive direction (from − 309 ppm/°C to − 179 ppm/°C) as the SiO2 content was increased, while the coefficient of thermal expansion remained almost unchanged (∼ 35 ppm/°C).