Bomson Lee

Design of defected ground structures for harmonic control of active microstrip antenna

Photonic bandgap (PBG) structures are usually periodic structures in which propagation of a certain band of frequencies is prohibited. PBG structures for microwave frequencies are applied in planar circuits such as microstrip line and CPW (coplanar waveguide). In this case, they are more frequently termed defected ground structures (DGS). Most of the research performed on DGS has been based on the equivalent circuit consisting of lumped elements, L and C, extracted from EM simulations (see D. Ahn et al., IEEE MTT, vol.49, 2001). In addition, we also consider radiation effects by including resistance, R, in the equivalent circuit. The general 1D periodic structures with N unit cells are analyzed using an ABCD matrix formulation. The effects of the RLC elements of the unit cell, the spacing between the unit cells, and the cell number, N, are investigated in detail. For a design example, a simple 1D DGS with N=2 is designed for harmonic control through a modeling using transmission line theory. This 1D DGS with N=2 is much simpler than the one proposed by Y. Horii and M. Tsutsmi (see IEEE MGWL, vol.9, no.1. p.1895-8, 1999). The proposed approach enables us to design the required DGS quite easily and quickly.

Development of Long-Range UHF-band RFID Tag chip Using Schottky Diodes in Standard CMOS Technology

We present the design of three key building blocks for UHF-band passive RFID tag chip, i.e., voltage multiplier, ASK demodulator, and internal clock generator. An analysis on a simple equivalent circuit of RFID tag chip for long reading range is presented taking into account the finite turn-on voltage of tag chip. The Schottky diodes used in the passive RFID tag chip were fabricated using titanium (Ti/Al/Ta/Al)-silicon (n-type) junction in 0.35 mum CMOS process, and the effect of size of Schottky diode on the turn-on voltage and the input impedance of the voltage multiplier was investigated. For 300 mV RF input voltage, the fabricated voltage multiplier using Schottky diodes generated output voltages of 1.5 V and corresponding voltage conversion efficiency of 45%. In addition, we propose an example circuit for internal oscillator of tag chip with digital calibration, which can generate precise copy of RFID reader timing signals.

Bandgap and slow/fast-wave characteristics of defected ground structures (DGSs) including left-handed features

This paper presents a T-shaped defected ground structure (DGS), which has the bandgap and slow/fast-wave characteristics including left-handed (LH) features. The closed-form expressions for the bandgap center frequency, its fractional bandwidth (BW), and radiation rate are obtained using the R,L, and C values extracted from electromagnetic (EM) simulation results for the unit-cell structure. The equivalent R,L, and C values to realize a DGS with a specific bandgap center frequency, its BW, and allowable radiation rate are also provided for use as design equations for a required unit cell with a design example. Besides, we provide the expressions for the dispersion effects of the DGS based on the equivalent circuit with adequate physical interpretations. At frequencies below and above the bandgap, it is shown to behave as an element for slow and fast waves, respectively. Some LH features when a shunt inductor is added between the two DGSs are also investigated. The measured data for the fabricated 1/spl times/4 and 1/spl times/5 one-dimensional periodic DGSs show a good agreement with those using the provided closed-form expressions and EM simulations.

Dual-Band Circularly Polarized Microstrip RFID Reader Antenna Using Metamaterial Branch-Line Coupler

A dual-band circularly polarized aperture coupled microstrip RFID reader antenna using a metamaterial (MTM) branch-line coupler has been designed, fabricated, and measured. The proposed antenna is fabricated on a FR-4 substrate with relative permittivity of 4.6 and thickness of 1.6 mm. The MTM coupler is designed employing the provided explicit closed-form formulas. The dual-band (UHF and ISM) circularly-polarized RFID reader antenna with separate Tx and Rx ports is connected to the designed metamaterial (MTM) branch-line coupler. The maximum measured LHCP antenna gain is 6.6 dBic at 920 MHz (UHF) and RHCP gain is 7.9 dBic at 2.45 GHz (ISM). The cross-polar CP gains near broadside of the RFID reader antenna are approximately less than - 20 dB compared with the mentioned co-polar CP gains in both bands. The isolations between the two ports are about 25 dB and 38 dB, at 920 MHz and 2.45 GHz, respectively. The measured axial ratios are less than 0.7 dB in the UHF band (917-923 MHz) and 1.5 dB in the ISM band (2.4-2.48 GHz).

A Long-Range UHF-Band Passive RFID Tag IC Based on High-

We present a fully integrated long-range UHF-band passive radio-frequency-identification tag chip fabricated in 0.35-mum CMOS using titanium (Ti/Al/Ta/Al)-silicon Schottky diodes. The diodes showed low turn-on voltages of 95 and 140 mV for diode currents of 1 and 5 muA, respectively. In addition, the Schottky diodes exhibited low-resistive loss, and a high-Q -factor design approach was exploited to achieve a long read range for the tag integrated circuit (IC). An optimized voltage multiplier resulted in an excellent sensitivity of -14.8 dBm and corresponding power-conversion efficiency of 36.2% for generating an output voltage of 1.5 V at 900 MHz. The range analysis of the measured multiplier performance indicated an operating range of more than 9 m at 4-W Effective Isotropically Radiated Power reader power. The subthreshold-mode operation of an ASK demodulator allowed ultralow power operation. Under power consumption as low as 27 nW, the demodulator supported a data rate of 150 kb/s and a modulation depth of 40%. A new architecture for generating a stable system clock (2.2 MHz) for the tag IC was employed to deal with supply voltage and temperature variations. Measurements showed that the clock generator had an error of 0.91% from the center frequency owing to an 8-b digital calibration scheme.

Q

With the recent explosive increase of the use of mobile communication handsets, there has been a growing concern about possible hazard to a human body, especially head part, exposed to the EM fields radiated from handsets. The specific absorption rate (SAR) is normally used for one of the important criteria to evaluate the degree of the hazard. The SAR is influenced by various kinds of parameters such as kind and size of antenna, position of antenna, radiated power, handset structure, distance and angle from human body, area of absorbing material if used, and so on. Most of the studies on this area have been performed to evaluate SAR values based on these parameters. In a previous paper (Minscok Jung and Bomson Lee, IEEE APS, vol. 1, pp. 444-448, 2002), after defining SRF (SAR reduction factor) for a quantitative discussion of effective SAR reduction methods, we analyzed the effects of the ferrite sheet attached to the handset based on power conservation relation. In this paper, we analyze the SAR data for various shapes of the ferrite structure. After investigating the reflection and transmission characteristics of the absorbing material layer, we also present a method to estimate the complex permittivity and permeability having a maximum absorption. We then evaluate SAR and SRF data for handsets coated with those absorbing layers.

Design Approach

We investigated the design trade-off in the development of UHF-band RFID tag for increased reading range. Using the quality factors of the tag antenna and tag chip as design parameters, the effects of the quality factors on the turn-on voltage of the tag chip and the backscattered power from the tag were examined. The design equations from the analysis indicate two regions of good impedances, one for providing high turn-on voltage for the tag chip, and the other resulting in increased radar cross section (RCS) for the antenna. An optimum quality factor which provides the best compromise of the two designs is provided. Based on the analysis, an example of the tag chip and tag antenna was designed, and the reading range test in anechoic chamber resulted in a detection distance of 5.2 m for -75 dBm reader sensitivity

SAR reduction for mobile phones based on analysis of EM absorbing material characteristics

This letter presents two types of metamaterial-inspired absorbers adopting resistive trumpet structures at the X band. The unit cell of the first type is composed of a trumpet-shaped resonator loading a chip resistor, a metallic back plane, and a FR4 (er = 4.4-j 0.02) substrate between them (single-layer). The absorption rate is 99.5% at 13.3 GHz. The full width at half maximum (FWHM) is 95 % at 11.2 GHz (from 5.9 to 16.5 GHz). The size of unit cell is 5.6×5.6×2.4 mm 3 . The second type has been optimized with a 7 Ω/square uniform resistive coating, removing the chip resistors but leading to results comparable to the first type. The proposed absorbers are almost insensitive to polarizations of incident waves due to symmetric geometry.

Design Consideration of UHF RFID Tag for Increased Reading Range

This paper examines various aspects of the electromagnetic responses of the ring resonator located in the transverse electromagnetic cell. In addition, an equivalent circuit for the ring resonator is proposed and analyzed based on the electromagnetic phenomenon of the resonator. The equivalent circuit was simply modeled based on the concept of magnetization. A method for achieving a wider operating bandwidth of the negative permeability is provided. The ring resonator with its resonant frequency of 13.56 MHz was designed and its characteristics were examined in terms of S-parameters, effective permeability, loss rate, bandwidth, etc. The circuit and electromagnetic simulation results show an excellent agreement as well as that of theory.

Design of Metamaterial-Inspired Wideband Absorber at X-Band Adopting Trumpet Structures

For the cell design of mobile communication systems, the path loss prediction models are very important. These models can be classified into theoretical and experimental models. The main experimental models are the Okumura-Hata, Cost231-Hata, and ITU-R model. The problem of these models is that these prediction expressions are based on the qualitative propagation environments such as urban, suburban, and open areas. The Cost231-Walfisch-Ikegami model (Cost231-WI) is a result of the effort to use a quantitative description of the propagation environments. In addition to the height of T/sub x/ and R/sub x/ antennas, the quasi-uniform building height and width of street are considered in this model. A mistake in formulating the Cost23 I-WI from the original Ikegami (see IEEE Trans Antennas Propagat., vol.32, p.822-29) as found by Har, Watson and Chadney (see IEEE Trans. Veh. Technol., p.1451-52, Sep. 1999). The mistake happened due to the misunderstanding between reflection coefficient and reflection loss. We comment on this. For a more quantitative understanding of the propagation environment when Oukumura measured path losses near Tokyo, we compared the path losses of Cost231-Hata with those of Cost231-WI based on the same frequency (f=2000 MHz, and 1500 MHz), T/sub x/ and R/sub x/ antenna heights (h/sub b/=40 m and h/sub m/=1.5m), but changing building height (10-25 m) and street widths (20-25 m). We also compared Okumura-Hata model with the ITU-R model using a similar approach. Through such a parametric study, a quantitative propagation environment based on which Okumura curves were extracted is estimated.