High Tuning Ratio Research Articles

Compact frequency‐reconfigurable circular patch antenna with varactor diode: Design, tuning, and compactness optimization

AbstractIn this study, a frequency‐reconfigurable circular patch antenna with a varactor diode has been presented. The proposed antenna has been designed and investigated in four steps: circular patch antenna (Antenna 1), antenna with double slots (Antenna 2), antenna with quad slots (Antenna 3), and antenna with varactor diode (Antenna 4). The proposed antenna has been made compact with rectangular slots and a hyperabrupt junction tuning varactor diode (SMV1249‐079LF). The proposed antenna has been designed and produced in 32 × 32 × 0.64 mm3 dimensions, approximately 80% minimized compared to a microstrip circular patch antenna operating at 850 MHZ resonance frequency. This frequency‐reconfigurable antenna can perform between the range of 0.85–1.3 GHz frequency with 0–3 V DC voltage tuning of the varactor diode. So, using with the varactor diode, the proposed antenna has obtained a 53% tuning range between 0.85 and 1.3 GHz frequencies. This study offers a different antenna design, compactness, and high tuning ratio compared to previous studies. This frequency reconfigurable antenna can be preferred with its wideband structure in different communications areas and wireless energy transfer systems.

The Design, Simulation, and Parametric Optimization of an RF MEMS Variable Capacitor with an S-Shaped Beam

This study presents the design and simulation of an RF MEMS variable capacitor with a high tuning ratio and high linearity factor of capacitance–voltage response. An electrostatic torsion actuator with planar and non-planar structures is presented to obtain the high tuning ratio by avoiding the occurrence of pull-in point. In the proposed design, the capacitor plate is connected to the electrostatic actuators by using the s-shaped beam. The proposed design shows a 138% tuning ratio with the planar structure of the actuator and 167% tuning ratio by implementing the non-planar structure. A linearity factor of 99% is attained by adjusting the rates at which the capacitor plate rises as the actuation voltage increases and the rate at which the capacitance decreases as the plate rises. Parametric optimization of the design is performed by utilizing the finite element method (FEM) analysis and high-frequency structural simulator (HFSS) analysis to obtain an optimized high-tuning ratio RF MEMS varactor at low actuation voltage. S-parameters of the design are presented on HFSS, with a 50 ohm coplanar waveguide (CPW) serving as the transmission line. The proposed RF MEMS varactor can be utilized in tunable RF devices.

Adaptive under-actuated control for capacitive micro-machined ultrasonic transducer based on an accurate nonlinear modeling

A capacitive micromachined ultrasonic transducer (CMUT) due to many benefits is being considered as an imaging and therapeutic technology recently. The critical challenge is to stabilize the system beyond the pull-in voltage considering imposed perturbations. The CMUT system, on the other hand, has a low range of movement and it is intrinsically unstable, which can result in a pull-in phenomenon. Consequently, in order to use the CMUT systems in a variety of medical applications that require high tuning ratio, a closed-loop control technique has been designed for these systems aims at increasing the maximum capacitance and enhancing tunability as well. In this study, using the closed-loop control, the resistance of micro-plate against the pull-in phenomenon has been examined. Also, in the description of the system a more accurate nonlinear modeling has been considered in the presence of an under-actuated sliding model control strategy with finite convergence time. Besides, adaptive protocols with unknown upper bounds have been designed to compensate the effects of uncertainty, unmodeled dynamics and external disturbances. The performance of controller in terms of improving output pressure, stabilizing the CMUT and its robustness to imposed perturbations have been investigated. Finally, numerical simulations are presented to verify the usefulness and applicability of the proposed control strategy.

Switchable adhesion with a high tuning ratio achieved on polymer surfaces with embedded low-melting-point alloy

The ability to control adhesion with a high tuning ratio and robust performance in a reversible manner is highly desirable for many engineering applications. Here, by embedding low-melting-point alloy (LMPA) blocks in a polymer matrix, we design a composite structure, whose surface adhesion can be actively switched between high and low when the LMPA is cooled to solid state or heated to liquid state. The proposed strategy does not need to modify the surface/interface directly yet can easily achieve an adhesion switching ratio about 100. As revealed by a contact mechanics model and finite element simulation, the adhesion tuning capability is enabled by rational regulation of the interface decohesion mode. More specifically, the interface will separate uniformly and exhibit high adhesion when the LMPA is solid; however, the interface will detach through crack-like propagation and exhibit much weaker adhesion when the LMPA is liquid. Based on this essential mechanism, we proceed to design a composite array structure, demonstrating that programmable inhomogeneous adhesion can be potentially achieved with pixel-by-pixel control capability.

A Novel Technique for Design of Ultra High Tunable Electrostatic Parallel Plate RF MEMS Variable Capacitor

This paper introduces a novel method for designing of low actuation voltage, high tuning ratio electrostatic parallel plate RF MEMS variable capacitors. It is feasible to achieve ultra-high tuning ratios way beyond 1.5:1 barrier, imposed by pull-in effect, by the proposed method. The proposed method is based on spring strengthening of the structure just before the unstable region. Spring strengthening could be realized by embedding some dimples on the spring arms with the precise height. These dimples shorten the spring length when achieved to the substrate. By the proposed method, as high tuning ratios as 7.5:1 is attainable by only considering four dimple sets. The required actuation voltage for this high tuning ratio is 14.33 V which is simply achievable on-chip by charge pump circuits. Brownian noise effect is also discussed and mechanical natural frequency of the structure is calculated.

Flexible liquid‐metal capacitors

Researchers in the US report liquid-metal based tuneable capacitors with linear behaviour and large tuning ratios. Compact and flexible, they hold promise for reconfigurable and wearable RF circuits. The liquid metal research team conducting research at the University of Hawai'i at Manoa. Tuneable capacitors could allow next generation wireless communication systems to adapt to the needs of the user and environment by changing the frequency and bandwidth of RF circuits, optimising their performance in response to the circumstances. Work in tuneable capacitors can be broadly divided into mechanically and electrically controlled approaches. Mechanically controlled capacitors can provide high-power handling, a high Q-factor and often a linear response. However, they tend to be bulkier than electrically controlled capacitors and have a limited tuning range. On the other hand, Electrically-controlled tuneable capacitors tend to have nonlinear behaviour, also limiting their useful range of tuneability. The range of tuneability and the linearity of the change in capacitance are two critical parameters for RF tuneable capacitors. In this issue of Electronics Letters a team at the University of Hawai'i report tuneable capacitors which not only combine linear response with a wide tuning range but add mechanical flexibility, giving them potential in flexible electronics. These benefits derive from their use of liquid metal. “The significance of this work is the integration of rapid prototyping with the dynamic shape-changing characteristics of liquid metal to develop a tuneable element that provides better performance in several categories compared to existing tuneable capacitors,” explains Hawai'i team Member, Prof. Aaron Ohta. “Liquid metal is electrically and thermally conductive, and naturally deformable, allowing it to conform to the shape of its container. Liquid metal can also provide better contact with rough solid surfaces compared to a solid-solid contact. It is also stretchable, enabling its use in flexible electronics. The inherently dynamic behaviour of liquid metal makes it appropriate for reconfigurable electronics.” One of the capacitors they have developed has a tuning ratio of 42:1, and the team say there is potential to increase this. The tuneability of the capacitors is achieved by varying the overlap of parallel conductors, which is an established principle in tuneable capacitors using solid conductors. The key difference in the Hawaii team's approach is that the fluid nature of the liquid-metal conductor allows a wider range of overlap by actuating the liquid metal. Having a large overlap range with solid conductors can easily mean a bulky device; but using liquid metal also allows the team to create reconfigurable conductor shapes that keep the size of the device down. In their Letter, the team present four different geometries, including coils and spirals. The high tuning ratio and linear response of liquid-metal capacitors make them attractive propositions for RF circuits that require a high dynamic range and resolution, such as oscillators, filters, or antennas that need to operate over a wide range of frequency and bandwidth. “In addition, the rapid prototyping of the liquid-metal capacitors described in this Letter is very suitable for laboratory devices,” said Ohta. Left: tuneable liquid metal coil capacitor, formed by a pair of plastic tubes coiled together. One tube is completely filled with liquid metal and the other tube is incrementally filled with liquid metal, providing a tuneable capacitance. Right: The capacitor is attached to a PET substrate, demonstrating its flexibility. The team at the University of Hawai'i are now integrating their tuneable capacitors into various RF circuits including filters, resonators, oscillators and antennas. This includes optimising the frequency response, bandwidth, and loss in each case. They are also working to increase the tuning range of the capacitors and include electrical actuation of the liquid metal for tuning. This is an area in which their group already has significant expertise; having developed several electrical actuation methods to tune networks, phase shifters, switches and antennas. Developing actuation methods for liquid metals that are more easily integrated into common circuit technologies will be critical to seeing this kind of capacitor applied in real-world applications.

High quality factor RF MEMS tunable capacitor

This paper presents the design of a tunable capacitor that integrates two vertical comb-drive tunable capacitors and a parallel plate tunable capacitor. The parallel plate tunable capacitor is in the middle of the comb-drive tunable capacitors. The vertical comb-drive part has sets of fixed and moving fingers while the parallel plate has fixed and moving plates. The capacitor is fabricated using the MetalMUMPs microfabrication process, which has only one thick structural layer i.e. metal (20 μm nickel and 0.5 μm gold). The nickel of the metal layer is subject to residual stress gradients along its thickness. After release of the metal layer, the stress gradients bend the curve-up beams to raise the moving fingers and the moving plate of the capacitor above the substrate and the fixed plate. Hence, one structural layer (i.e. the metal) is used to form the moving and the fixed comb fingers without requiring two structural layers. Vertical comb-driven tunable capacitors offer high capacitance density when a large number of fingers with narrow gap are used. Therefore, high capacitance ratio and high quality factors can be achieved by using vertical comb-drives with in a relatively small device area compared to lateral comb-drives. The parallel plate driving part increases the displacement and capacitance ratio while reducing the actuation voltage required for driving the capacitor. The quality factor and tuning ratio of the fabricated tunable capacitor are 118.5 and 101% at 0.8 GHz; respectively at a driving voltage of 100 V. Based on the experimental results of the fabricated tunable capacitor, an optimized design is presented for both high tuning ratio and high quality factor. The tuning ratio of the optimized capacitor is found to be 143.1% at a “pull-in” displacement of 9.5 µm and a “pull-in” voltage of 90 V. The quality factors of the optimized capacitor are 520 at 0 V and 363.5 at 90 V, respectively at 0.8 GHz. The tunable capacitor achieves a displacement of more than one-half of the gap between the parallel plates without “pull-in” effect.

A novel high tuning ratio MEMS cantilever variable capacitor

This paper presents a novel cantilever-type MEMS variable capacitor with high tuning ratio. In previous works, the cantilever is used as a switch but in our design, it is applied as a variable capacitor. For increasing the maximum achievable capacitance of the cantilever, the suspended capacitive plate should be moved as parallel with fixed plate. The parallel movement can be obtained using the novel structure which utilized two additional lateral beams. Also to overcome the pull-in phenomenon in new structure, the different membrane thickness technique is used. The novelty of our design is adding the lateral beams to make parallel movement of the suspended plate to increase the variable capacitor of the cantilever. The new device is designed on a thick silicon substrate with a thin poly silicon membrane. The results show the tuning range of tunable capacitor with initial capacitance of 560.1 fF can be improved from 6:1 for conventional cantilever to 22.5:1 for the new cantilever. In other words the capacitance tuning range increased three times.

SiC Varactors for Dynamic Load Modulation of High Power Amplifiers

SiC Schottky diode varactors with a high breakdown voltage, a high tuning ratio, and a low series resistance have been designed and fabricated. These characteristics are particularly necessary for the dynamic load modulation of high power amplifiers (PAs), which is an attractive alternative to other efficiency enhancement techniques. For a SiC Schottky diode varactor with a 50- radius fabricated by using a graded doping profile, a breakdown voltage of 40 V, a tuning range of 5.6, and a series resistance of 0.9 were achieved. The results show the great potential of this type of varactors for the use in the dynamic load modulation of high power amplifiers.