• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.19 no.10
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• pp.1028-1033
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• 2009

To detect faults and monitor thinning on a pipe, many people use ultra sonic sensors that are operated in high frequency range. Because there are many modes in high frequency range, it is difficult to find faults and monitor pipe thinning on a structure. If we deal with signals in a low frequency range which include only A0 wave and S0 wave, the information of monitoring and diagnosis can be easily obtained. In this paper, the technique for exciting low frequency range using ultra sonic sensors is proposed. The main idea of the proposed method comes from the beat phenomenon. The beat frequency is equal to the absolute value of the difference in frequency of the two waves. If the beat frequency is tuned by two ultra waves, we can excite A0 mode and S0 mode of structures. To verify the proposed method, we have performed a steel plate and pipe experiments. Experimental results show that two ultra sonic sensors can well excite low frequency range.

• The Korean Journal of Applied Statistics
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• v.31 no.5
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• pp.667-675
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• 2018

When a financial time series consists of daily (closing) returns, traditional volatility models such as autoregressive conditional heteroskedasticity (ARCH) and generalized ARCH (GARCH) are useful to figure out daily volatilities. With high frequency returns in a day, one may adopt various multivariate GARCH techniques (MGARCH) (Tsay, Multivariate Time Series Analysis With R and Financial Application, John Wiley, 2014) to obtain intraday volatilities as long as the high frequency is moderate. When it comes to the ultra high frequency (UHF) case (e.g., one minute prices are available everyday), a new model needs to be developed to suit UHF time series in order to figure out continuous time intraday-volatilities. Aue et al. (Journal of Time Series Analysis, 38, 3-21; 2017) proposed functional GARCH (fGARCH) to analyze functional volatilities based on UHF data. This article introduces fGARCH to the readers and illustrates how to estimate fGARCH equations using UHF data of KOSPI and Hyundai motor company.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.17 no.3
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• pp.340-347
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• 2004

Ultra-small size, high-performance, solenoid-type RF chip inductors utilizing low-loss A1$_2$O$_3$ core materials were investigated. The dimensions of the RF chip inductors fabricated were 1.0mm${\times}$0.5mm${\times}$0.5mm and copper coils were used. The materials (96％ A1$_2$O$_3$) and shape (I-type) of the core, the diameters (40${\mu}{\textrm}{m}$) and position (middle) of the coil, and the lengths (0.35mm) of solenoid were determined by a high-frequency structure simulator (HFSS) to maximize the performance of the inductors. The high-frequency characteristics of the inductance (L) and quality-factor (Q) of the developed inductors were measured using a RF impedance/material analyzer (E4991A with E16197A test fixture). The developed inductors exhibit an inductance of 11 to 11.3nH and a qualify factor of 22.3 to 65.7 over the frequency ranges of 250 MHz to 1.7 GHz, and show results comparable to those measured for the inductors prepared by Coilcraft$^{TM}$. The simulated data described the high-frequency data of the L and Q of the fabricated inductors well.

• Transactions on Electrical and Electronic Materials
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• v.14 no.4
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• pp.211-215
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• 2013

Partial discharge diagnosis techniques using ultra high frequencies do not affect load movement, because there is no interruption of power. Consequently, these techniques are popular among the prevention diagnosis methods. For the first time, this measurement technique has been applied to the GIS, and has been tested by applying an extra high voltage switchboard. This particular technique makes it easy to measure in the live state, and is not affected by the noise generated by analyzing the causes of faults ? thereby making risk analysis possible. It is reported that the analysis data and the evaluation of the risk level are improved, especially for poor location, and that the measurement of Ultra high frequency (UHF) partial discharge of the real live wire in industrial switchgear is spectacular. Partial discharge diagnosis techniques by using the Ultra High Frequency sensor have been recently highlighted, and it is verified by applying them to the GIS. This has become one of the new and various power equipment techniques. Diagnosis using a UHF sensor is easy to measure, and waveform analysis is already standardized, due to numerous past case experiments. This technique is currently active in research and development, and commercialization is becoming a reality. Another aspect of this technique is that it can determine the occurrences and types of partial discharge, by the application diagnosis for live wire of ultra high voltage switchgear. Measured data by using the UHF partial discharge techniques for ultra high voltage switchgear was obtained from 200 places in Gumi, Yeosu, Taiwan and China's semiconductor plants, and also the partial discharge signals at 15 other places were found. It was confirmed that the partial discharge signal was destroyed by improving the work of junction bolt tightening check, and the cable head reinforcement insulation at 8 places with a possibility for preventing the interruption of service. Also, it was confirmed that the UHF partial discharge measurement techniques are also a prevention diagnosis method in actual industrial sites. The measured field data and the usage of the research for risk assessment techniques of the live wire status of power equipment make a valuable database for future improvements.

• Journal of Welding and Joining
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• v.21 no.3
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• pp.85-91
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• 2003

Narrow gap welding has many advantages over conventional V-grooved butt welding such as high productivity, small deformation and improved mechanical property of joints. With narrower groove gap, less arc heat input is expected will all the other advantages of narrow gap welding. The main defects of narrow gap welding include the lack of root fusion, convex bead surface and irregular surface, all of which have negative effects on the next welding pass. This paper suggests an up-and-down torch oscillation for ultra narrow gap welding with gap size of 5mm and investigates the proper welding conditions to fulfill the reliable and high welding quality. First, GMA welding model was suggested for ultra narrow gap welding system with Halmoy's model referenced for wire melting modeling. And the arc length in ultra narrow gap was defined. Secondly, based on the experimental results of up-and-down torch oscillation welding, phase shift of current and wire extension length were simulated for varying oscillation frequency to show that weld the bead shape in ultra narrow gap welding can be predicted. As the result, it was confirmed that reliable weld quality in ultra narrow gap welding can be achieved with up-and-down torch oscillation above 15Hz due to its ability to provide uniform heat input along the sidewall of gap.

• Nuclear Engineering and Technology
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• v.56 no.4
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• pp.1532-1537
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• 2024

Time-of-flight (TOF) PET detectors with fast-rise-time scintillators and fast-single photon time resolution silicon photomultiplier (SiPM) have been developed to improve the coincidence timing resolution (CTR) to sub-100 ps. The CTR can be further improved with an optimal bandwidth and minimized electronic noise in the readout circuit and this helps reduce the distortion of the fast signals generated from the TOF-PET detector. The purpose of this study was to develop an ultra-high frequency and fully-differential (UF-FD) readout circuit that minimizes distortion in the fast signals produced using TOF-PET detectors, and suppresses the impact of the electronic noise generated from the detector and front-end readout circuits. The proposed UF-FD readout circuit is composed of two differential amplifiers (time) and a current feedback operational amplifier (energy). The ultra-high frequency differential (7 GHz) amplifiers can reduce the common ground noise in the fully-differential mode and minimize the distortion in the fast signal. The CTR and energy resolution were measured to evaluate the performance of the UF-FD readout circuit. These results were compared with those obtained from a high-frequency and single ended readout circuit. The experiment results indicated that the UF-FD readout circuit proposed in this study could substantially improve the best achievable CTR of TOF-PET detectors.

• Korean Journal of Optics and Photonics
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• v.14 no.4
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• pp.351-354
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• 2003

We present a 12.5-GHz interleaved bidirectional ultra-dense wavelength-division-multiplexing transmission over a conventional single mode fiber of 80 km achieving spectral efficiency as high as 0.8-bit/s/Hz. The beat-frequency-locking method is used to stabilize the channel frequency within $\pm$200 MHz error. To facilitate the identification of multiple beat frequency signals, we use a radio-frequency spectrum analyzer. The bidirectional transmission penalty is about 0.3 dB compared with the unidirectional transmission over the same fiber.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.08a
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• pp.132-132
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• 2010

In the nano-scale Si processing, patterning processes based on multilevel resist structures becoming more critical due to continuously decreasing resist thickness and feature size. In particular, highly selective etching of the first dielectric layer with resist patterns are great importance. In this work, process window for the infinitely high etch selectivity of silicon oxynitride (SiON) layers and silicon nitride (Si3N4) with EUV resist was investigated during etching of SiON/EUV resist and Si3N4/EUV resist in a CH2F2/N2/Ar dual-frequency superimposed capacitive coupled plasma (DFS-CCP) by varying the process parameters, such as the CH2F2 and N2 flow ratio and low-frequency source power (PLF). It was found that the CH2F2/N2 flow ratio was found to play a critical role in determining the process window for ultra high etch selectivity, due to the differences in change of the degree of polymerization on SiON, Si3N4, and EUV resist. Control of N2 flow ratio gave the possibility of obtaining the ultra high etch selectivity by keeping the steady-state hydrofluorocarbon layer thickness thin on the SiON and Si3N4 surface due to effective formation of HCN etch by-products and, in turn, in continuous SiON and Si3N4 etching, while the hydrofluorocarbon layer is deposited on the EUV resist surface.

• Progress in Superconductivity and Cryogenics
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• v.21 no.1
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• pp.10-14
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• 2019

Radio-frequency (RF) amplifiers based on direct current (DC) superconducting quantum interference device (SQUID) have low-noise performance for precision physics experiments. Gain curves of SQUID RF amplifiers depend on several parameters of the SQUID and operation conditions. We are developing SQUID RF amplifiers for application to measure very weak RF signals from ultra-low-temperature high-magnetic-field microwave cavity in axion search experiments. In this study, we designed, fabricated and characterized SQUID RF amplifiers with different SQUID parameters, such as number of input coil turn, shunt resistance value of the junction and coupling capacitance in the input coil, and compared the results.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2003.05a
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• pp.770-774
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• 2003

We propose the re-design method of modal parameters to depress the 2nd resonance peak of the ultra-slim-height optical pick-up actuator. With the addition of tile counter mode near the 2nd resonance frequency, we can achieve the gain margin which is sufficient to meet the system requirement. It would alleviate the burden of the additional filter for a high-speed drive.