F. De Flaviis

Planar microwave integrated phase-shifter design with high purity ferroelectric material

Ferroelectric materials (FEMs) are very attractive because their dielectric constant can be modulated under the effect of an externally applied electric field perpendicular to the direction of propagation of a microwave signal. FEM may be particularly useful for the development of a new family of planar phase shifters which operate up to X-band. The use of FEM in the microwave frequency range has been limited in the past due to the high losses of these materials; tan /spl delta/=0.3 at 3 GHz is typical for commercial BaTiO/sub 3/ (BTO) and due to the high electric field necessary to bias the structure in order to obtain substantial dielectric constant change. In this paper, a significant reduction in material losses is demonstrated. This is achieved by using a new sol-gel technique to produce barium modified strontium titanium oxide [Ba/sub 1-x/Sr/sub x/TiO/sub 3/ (BST)], which has ferroelectric properties at room temperature. Also demonstrated is how the use of thin ceramics reduces the required bias voltage below 250 V, with almost no power consumption required to induce a change in the dielectric constant. A phase shift of 165/spl deg/ was obtained at 2.4 GHz, with an insertion loss below 3 dB by using a bias voltage of 250 V. Due to the planar geometry and light weight of the device, it can be fully integrated in planar microwave structures.

Power Harvester Design for Passive UHF RFID Tag Using a Voltage Boosting Technique

This paper presents a guideline to design and optimize a power harvester circuit for an RF identification transponder. A power harvester has been designed and fabricated in a CMOS 0.18- process that operates at the UHF band of 920 MHz. The circuit employs an impedance transformation circuit to boost the input RF signal that leads to the improvement of the circuit performance. The power harvester has been optimized to achieve maximum sensitivity by characterizing both the impedance transformation network and the rectifier circuit and choosing the optimum values for the circuit parameters. The measurement results show sensitivity of 14.1 dBm for dc output voltage of 1 V and the output current of 2 mum that corresponds to the output power of 2 muW.

Modeling planar arbitrarily shaped microstrip elements in multilayered media

Microstrip elements of arbitrary shape are modeled in multilayered media. The Greens function for the multilayered structure is developed in a form useful for efficient computation for interacting microstrip elements, which may be located at any substrate layer and separated by an arbitrarily large distance. This result is of significant value to a variety of applications in wave propagation besides those discussed in this paper. The mixed-potential integral-equation (MPIE) method is developed in the spatial domain. Examples for regularly/arbitrarily shaped geometries in single and multilayered media are presented. These involve the optimization of an open-end microstrip, a radial-stub microstrip, a five-section overlay-gap-coupled filter, and a circular-patch proximity-coupled microstrip antenna. Very good agreement with measurement and other published data is observed.

Microwave reflection tomographic array for damage detection of civil structures

Microwave tomographic imaging technology using a bifocusing operator has been developed in order to detect the internal voids/objects inside concrete structures. The imaging system consists of several cylindrical or planar array antennas for transmitting and receiving signals, and a numerical focusing operator is applied to the external signals both in transmitting and in receiving fields. An imaging algorithm using a numerical focusing operator was developed, which allows the recovery of a two-dimensional object from its scattered field. Numerical simulation demonstrated that a subsurface image can be successfully reconstructed by using the proposed tomographic imaging technology. For experimental verification, a prototype planar antenna array was fabricated and tested on a concrete specimen.

A UHF Near-Field RFID System With Fully Integrated Transponder

This paper presents an RF identification (RFID) system with a fully integrated transponder. To enable the on-chip integration of the tags antenna, it is suggested to employ near-field coupling at high-frequency ranges, i.e., the UHF band. The RFID system including the reader and key blocks of the transponder is designed and fabricated in a standard CMOS 0.18-mum process. The system operates at 900 MHz with the coverage range of over 0.4 cm. The tags antenna is integrated on-chip without using any special process. The reader employs multiple coils to increase its coverage area. Using a proper output network, the reader can deliver a current of 225 mA (rms) to its coil, which is designed on a printed circuit board.

A mutual coupling study of linear and circular polarized microstrip antennas for diversity wireless systems

In this paper, an analysis of mutual coupling is presented to examine the benefits of orthogonal polarizations and patterns for adjacent microstrip antennas. The mutual coupling between two linear polarized antennas orientated in parallel polarizations (E and H plane) is reduced using low dielectric constant materials. The mutual coupling can be reduced an additional 20-35dB at the same inter-element spacing when adjacent elements are orientated in orthogonal polarizations, O plane. Similarly, the mutual coupling between two circular polarized antennas orientated in the parallel polarization is reduced using low dielectric constant materials. However, the reduction in mutual coupling between two circular polarized antenna elements orientated in the O plane is only an additional 1-6 dB. The mutual coupling between a linear polarized sum beam (1/2/spl lambda/) and difference beam (1/spl lambda/) antenna is reduced 20-35 dB below the case when using identical antennas only in the H- and O-planes. Compact two- and four-element multielement antennas with inter-element spacings less than 0.15/spl lambda/ are fabricated and the S parameters and radiation patterns are measured.

A Two-Point Modulation Technique for CMOS Power Amplifier in Polar Transmitter Architecture

A two-point modulation technique is presented that improves the performance of nonlinear power amplifiers (PAs) in polar transmitters. In this scheme, the output amplitude modulation is performed by controlling the current of the PA. The current control technique enables the PA to provide wideband amplitude modulation, as well as high power control dynamic range. In addition, the supply voltage of the PA is adjusted based on the output power level. The voltage supply adjustment substantially improves the effective power efficiency of the PA. The voltage supply control is performed using a second-order sigma-delta dc-dc converter, which presents an efficiency of over 95% in its operational range. The PA operates at 900 MHz with maximum output power of 27.8 dBm and power efficiency of 34% at maximum output power. The proposed PA achieves 62-dB power control dynamic range with amplitude modulation bandwidth of over 17.1 MHz. The circuits are fabricated in a CMOS 0.18 mum process with a 3.3-V power supply.

A memory-efficient formulation of the finite-difference time-domain method for the solution of Maxwell equations

With the increase in speed and memory storage in modern computer systems, the finite-difference time-domain (FDTD) method for the solution of electromagnetic problems is rapidly becoming an attractive choice due to its programming simplicity and flexibility in the analysis of a wide range of structures. However, this technique has the drawback of high computer memory requirements and computational power, when analyzing large geometries. In this paper, a modified version of the FDTD method with increased memory efficiency is presented and applied to the calculation of the resonant frequencies of a dielectric resonator coupled to a microstrip line. In this novel approach, the divergence relationship, which spatially links the three electric-field and three magnetic-field components, is used to eliminate one component each of E and H. This leads to a more memory-efficient formulation, where only four field components are stored in the whole domain, with a direct memory reduction of 33% in the storage of the fields.

Integrated MEM antenna system for wireless communications

This paper presents the results on the fabrication and testing of a compact broadband antenna integrated with RF MEMS switches for wireless communications. The system consists of two individual broadband CPW fed antennas (having 50% bandwidth) sequentially addressed using low-loss RF-MEMS switches. The antennas, as well as, the RF-MEMS switches are fabricated on a glass substrate using surface micromachining batch processing method. The design and fabrication process of the system are outlined. The results showing the circuit and radiation characteristics are discussed in terms of radiation pattern reconfigurability and spatial and angular diversity capabilities.

Reduced size single and dual band linear polarized microstrip antennas for mobile communications

Two techniques to reduce the physical size of circular and rectangular microstrip patch antennas are presented. By utilizing the symmetry of the resonant current distribution, the size of the antenna can be reduced by one-half of the original patch. Second, slits are inserted within the antenna which are perpendicular to the resonant current. This forces the current to flow around the obstacles which has the effect of reducing the resonant frequency. Using slits, an antenna of one-fourth the area of the original patch can be realized. Lastly, using two patches displaced laterally or vertically an antenna with dual band capabilities can be realized. The two frequency bands, 1.8, 2.4 GHz and 5.2, 5.7 GHz, are chosen to lie within the existing PCS and Bluetooth bands and the two future Bluetooth frequency bands, respectively. The antenna geometry is an aperture coupled microstrip antenna. The reduced size and thin substrate thicknesses give compatibility with portable communications systems.