Hyukjin J. Kwon

Linear frequency modulation of voltage-controlled oscillator using delay-line feedback

This letter proposes a structure for a voltage-controlled oscillator (VCO) circuit which operates in a frequency range of 4.0-4.3 GHz and can achieve a highly linear frequency sweep without any additional compensation circuit. The VCO consists of an amplifier and a feedback circuit only. The feedback circuit compensates for the phase delay of a transmission line with a varactor and sets the closed-loop phase change close to 360/spl deg/. The measured maximum deviation from a linear frequency sweep is /spl sim/1.2 MHz when the varactor capacitance C/sub V/ of the VCO is related to the control voltage V/sub C/ by C/sub V//spl prop/V/sub C//sup -1.06/. The spectral distribution of the beat frequency between the VCO output and delayed VCO output shows that the proposed VCO has excellent linearity in frequency modulation.

Nonlinearity compensation circuit for voltage-controlled oscillator operating in linear frequency sweep mode

A circuit that can compensate for nonlinearity in frequency modulation of a voltage-controlled oscillator (VCO) is proposed. The circuit uses a fixed-length delay line and an analog phase-locked loop (PLL). It sweeps the frequency of VCO output linearly in time and easily adjusts the rate of frequency sweep. For a VCO operating in a frequency range of 5.3-5.4 GHz, the nonlinearity in frequency modulation was measured by observing the beat frequency f/sub B/(t) between the VCO and delayed VCO outputs. The deviations of f/sub B/ (t) of /spl sim/28 % from its average values (2, 5, and 10 kHz) were reduced to <2% after compensation, indicating that the linearity in frequency modulation was improved significantly independent of the rate of frequency sweep.

Design of 6–18 GHz Wideband Phase Shifters Using Radial Stubs

A 6-18 GHz wideband phase shifter is proposed. This phase shifter consists of one Lange coupler, four radial stubs, and four switching diodes and bias circuits. Wideband phase shifting was achieved using two different pairs of radial stubs as the reflective loads of the Lange coupler. Four phase shifters of 11.25, 22.5, 45, and 90deg were fabricated and tested. Over the frequency band of 6-18 GHz, the measured maximum phase errors were plusmn2.2, 2.2, 3.6, and 5.5deg for the 11.25, 22.5, 45, and 90deg phase shifters, respectively

System Modeling of a MEMS Vibratory Gyroscope and Integration to Circuit Simulation

Recently, consumer applications have dramatically created the demand for low-cost and compact gyroscopes. Therefore, on the basis of microelectromechanical systems (MEMS) technology, many gyroscopes have been developed and successfully commercialized. A MEMS gyroscope consists of a MEMS device and an electrical circuit for self-oscillation and angular-rate detection. Since the MEMS device and circuit are interactively related, the entire system should be analyzed together to design or test the gyroscope. In this study, a MEMS vibratory gyroscope is analyzed based on the system dynamic modeling; thus, it can be mathematically expressed and integrated into a circuit simulator. A behavioral simulation of the entire system was conducted to prove the self-oscillation and angular-rate detection and to determine the circuit parameters to be optimized. From the simulation, the operating characteristic according to the vacuum pressure and scale factor was obtained, which indicated similar trends compared with those of the experimental results. The simulation method presented in this paper can be generalized to a wide range of MEMS devices.

Fabrication of entangled single-wall carbon nanotube films as nanoporous junctions for ion concentration polarization

Ion concentration polarization (ICP) is a distinctive electrochemical phenomenon that occurs near an ion-exchange membrane with an applied DC electric field, generating a significant concentration gradient in back and forth on the membrane. To date, however, there have been only a few attempts to introduce unconventional materials for ion transport in micro?nanofluidic systems. Here, we describe the development of a novel ICP system using an entangled single-wall carbon nanotube (SWNT) film as an ion-selective membrane instead of a Nafion membrane, for investigating the detailed relationship between electrical properties, i.e., ionic conductance through nanojunctions, and nonlinear electrokinetic behavior.

Wearable transparent thermal sensors and heaters based on metal-plated fibers and nanowires

Electrospun metal-plated nanofibers and supersonically sprayed nanowires were used to fabricate hybrid films exhibiting a superior low sheet resistance of 0.18 Ω sq-1, a transparency of 91.1%, and a figure-of-merit of 2.315 Ω-1. The films are suitable to serve as thermal sensors and heaters. Such hybrid transparent conducting films are highly flexible and thus wearable. They can be used as body-temperature monitors and heaters. The employed hybrid approach improved the sheet resistance diminishing it to a minimum, while maintaining transparency. In addition, the low sheet resistance of the films facilitates their powering with a low-voltage battery and thus, portability. The thermal sensing and heating capabilities were demonstrated for such films with various sheet resistances and degrees of transparency. The temperature sensing was achieved by the resistance change of the film; the resistance value was converted back to temperature. The sensing performance increased with the improvement in the sheet resistance. The temperature coefficient of resistivity was TCR = 0.0783 K-1. The uniform distribution of the metal-plated nanofibers and nanowires resulted in a uniform Joule heating contributing to an efficient convection heat transfer from the heaters to the surrounding, demonstrated by an improved convective heat transfer coefficient.

A multiscale-pore ion exchange membrane for better energy efficiency

Ion exchange membranes (IEMs) have been adopted in various environmental, chemical, and energy applications. However, the formation of ion-depletion regions, caused by concentration polarization near IEMs, often leads to significant energy and efficiency loss. While much research has been devoted to solving this challenge, complete removal of ion-depletion regions is still difficult, especially when the membrane systems are operating under near- or over-limiting conditions. This paper proposes a novel multiscale-pore (MP) IEM to reduce the effect of the ion-depletion region, by allowing a fluid flow through the MP-IEM, thereby limiting the size (and the resulting resistance) of the ion-depletion region. The electrical resistance and energy consumption in MP and conventional IEM-embedded electrochemical systems were investigated, and their performance during water desalination processes were compared. The current–voltage response suggests a secondary ohmic regime attributed to an internal flow rate through the MP-IEM. Moreover, the electrochemical desalination of seawater with MP-IEMs demonstrated up to 75% reduction of energy consumption, compared with conventional IEMs under comparable operating conditions.

Controlled production of monodisperse polycaprolactone microspheres using flow-focusing microfluidic device

These days, biodegradable microsphere polymers have been attracting increasing interest as a cosmetic injectable filler. Particularly, polycaprolactone (PCL) microspheres are well known for their safety and long degradation time. However, these microspheres are usually produced by the conventional stirring method, which has an inherent drawback related to the size control of the microspheres; accurate size control is critical for the use of these microspheres as a filler. Here, we demonstrate the fabrication of monodisperse PCL microspheres in the size range of 26.6-161 μm using a flow-focusing microfluidic device. The acquired coefficient of variation of the solidified microspheres is approximately 4.5%; thus, these microspheres meet the requirement of being monodisperse. The study results show the feasibility of manufacturing PCL microspheres using a microfluidic device, and these microspheres have better morphological characteristics, thereby reducing pain and infection after their injection into the skin. Furthermore, the specific target size of the solidified microspheres is met. Therefore, undesired outcomes after microsphere injection through the dermis, such as phagocytosis and inflammatory reactions, are less likely to occur.

Evaluation of Electrospun TiO2/PVP/LiCl Nanofiber Array for Humidity Sensing

Abstract Recently, tremendous application utilizing electrospun nanofibers have been actively reported due to its several advantages, such ashigh surface to volume ratio, simple fabrication and high-throughput manufacturing. In this paper, we developed highly sensitive andconsistent nanofiber humidity sensor by electrospinning. The humidity sensor was fabricated by rapid electrospinning (~2 sec) TiO 2 /PVP/LiCl mixed solution on the micro-interdigitated electrode. In order to evaluate the humidity sensing performances, we measuredcurrent response using DC bias voltage under various relative humidity levels. The results show fast response / recovery time and mar-ginal hysteresis as well as long-term stability. In addition, with the aid of micro-interdigitated electrode, we can reduce a total resistanceof the sensor and increase the total reaction area of nanofibers across the electrodes resulting in high sensitivity and enhanced currentlevel. Therefore, we expect that the electrospun nanofiber array for humidity sensor can be feasible and promising for diverse humiditysensing application.