Hyunil Kang

A new "active" predistortor with high gain using cascode-FET structures

An MMIC-compatible miniaturized active predistorter using cascode FET structures is presented. The predistorter has gain expansion and negative phase deviation characteristics required to compensate for the distortion of typical power amplifiers. Unlike the previously reported predistorters, the predistorter of this work is capable of providing gain as high as 7-17 dB, eliminating the need for additional buffer amplifiers. This makes our approach well suited to MMIC implementation. Analysis of gain and phase deviation was also conducted. From the analysis, the origin of nonlinear distortion of predistorter was found to be G/sub m/ of the upper FET and R/sub ds/ of the lower FET. The predistorter of this work was applied to linearize a 2-GHz MMIC power amplifier for CDMA handset applications. ACPR improvement of 5 dB was achieved.

Superconductivity in the Ba1−xKxBiO3 system

Abstract For the first time, superconductivity in the Ba1−xKxBiO3 system has been investigated in the whole region of cubic phase from x=0.37 up to x=1. Samples were synthesized using the high pressure and high temperature technique. The superconducting transition temperature Tc of Ba1−xKxBiO3 samples determined from dc susceptibility measurements monotonously goes down with increasing x from the well known Ba0.63K0.37BiO3 composition with Tc=30.4 K to Ba0.24K0.76BiO3 with very low diamagnetic signal at nearly 3 K. However, following this, we have found an increase of Tc up to 9 K for x=1 according to KBiO3 nominal composition.

1-26 GHz high power p-i-n diode switch

Wideband high power GaAs p-i-n diode SPDT switch is presented. Insertion loss is less than 0.7 dB and isolation is better than -38 dB for 1-26 GHz frequency range. Maximum power handling capability is 35 dBm at 1.8 GHz where the bias voltage is -15 V. Composite buffer layer of superlattice and low temperature grown GaAs prevent the problems of power limitation related to epitaxial buffer layer of p-i-n diode structure.

Characteristic Analysis of Functional Films according to the Annealing Temperature

Because of the low pollution resistance of the porcelain electrical insulator itself, in this work the anti-pollution performance of insulator was improved by using the functional coating. The ceramic substrates that components were same as the porcelain electrical insulator were used in this experiment. The functional films were coated on the ceramic substrate by using a spray coating method, and then the coated substrate were annealed under different coating condition such as natural curing and annealing temperature of , and . Then, the contact angles of the coated surfaces were measured and the minimum angle () was obtained at . The anti-contamination properties were measured, revealing that as the contact angle decreased, the anti-contamination properties improved. The hardness and adhesion were small at the natural curing condition however the excellent mechanical properties were obtained under higher temperature annealing.

Estimation of Power Collector Wear Considering the Operating Environment of a Maglev

The wear of the third track power collector is one of the essential check factors for safe train operation. Rapid wear of the current collector accelerates the line of the catenary. In addition, the arc generated when the catenary line is turned off causes a malfunction in the minute portion of the catenary line, thereby shortening the life of the catenary line. In this paper, to analyze the mechanical wear of the current collector during driving according to the environmental factor of the Maglev(magnetic levitation train), it was divided into dry season and wet season. the wear of metallized collector, copper alloy collector and carbon collector were measured and compared with each other. The wear rate was measured according to the position of the wire, the position of the power collector and the position per hour. Microscopic photographs of the cross section and surface of the power collector were measured. The electrical currents of the metallized collector, copper alloy collector and carbon collector were measured.

Effects of Annealing Ambient on the Anti-Pollution and Mechanical Properties of Functional Film Coated on the Ceramic Substrate

For the improvement of the anti-pollution properties of porcelain electrical insulators, in this study, we have applied the functional film to the surface of insulator. The functional films were coated on the ceramic substrates which components were like the porcelain electrical insulator. The coating material was applied to ceramic substrate by spray coating method and then the film was cured at around 300C for 10 minutes with different gas ambient, such as O2, N2, and only vacuum. We have measured the contact angle of the coated surface, and obtained the lowest angle (8.9) and a strong hydrophilic property at vacuum condition. The anti-pollution properties were measured, revealing that as the contact angle decreased, the anti-pollution properties improved. The mechanical hardness and adhesion were both excellent regardless of the annealing ambient.

Effects of Plasma Treatment on Carbon Nanowalls Grown by Microwave Plasma Enhanced Chemical Vapor Deposition.

In this study, the effects of post-plasma treatment on synthesized carbon nanowalls (CNWs) grown with a microwave were investigated. CNWs were synthesized by microwave plasma enhanced chemical vapor deposition (PECVD), employing a mixture of CH4 and H2 gases. The plasma treatment was done in different plasma environments (O2 and H2) but under the same condition of synthesized CNWs. Raman spectroscopy, field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), and fourier transform infrared spectroscopy (FT-IR) were used to analyze the effects of the post-plasma treatment on the synthesized CNWs. After the H2 post-plasma treatment, no significant changes in the appearance and characteristics of the CNWs were observed. After the O2 post-plasma treatment, on the other hand, the CNWs were etched at a rate of 18.05 nm/sec. The Raman analysis confirmed, however, that the structural changes in the CNWs caused by the O2 post-plasma treatment were insignificant.