• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2005.11a
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• pp.107-111
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• 2005

It has been reviewed whether it would be suitable that the application of the time-frequency signal analysis techniques to estimate the location of the impact source in plate structure. The STFT(Short Time Fourier Transform), WVD(Wigner-Ville distribution) and CWT(Continuous Wavelet Transform) methods are introduced and the advantages and disadvantages of those methods are described by using a simulated signal component. The essential of the above proposed techniques is to separate the traveling waves in both time and frequency domains using the dispersion characteristics of the structural waves. These time-frequency methods are expected to be more useful than the conventional time domain analyses fer the impact localization problem on a plate type structure. Also it has been concluded that the smoothed WVD can give more reliable means than the other methodologies for the location estimation in a noisy environment.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.14 no.10
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• pp.1079-1088
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• 2003

In this paper, we propose a high-resolution time-frequency domain reflectometry technique as a methodology of detection and estimation of faults on a wire. This method adopts the time-frequency cross correlation characteristics of the observed signal in both time and frequency domains simultaneously. The accuracy of the proposed method is verified with experiments using a RG type coaxial cable and comparing it with traditional time domain as well as frequency domain reflectometry methods. It is clearly shown here that the proposed algorithm produces excellent results compared to the conventional methods for single as well as multiple fault cables.

• Proceedings of the KIEE Conference
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• 2000.07b
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• pp.1168-1170
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• 2000

A half bridge time sharing type high frequency resonant inverter to give VVVF function in the inverter used as power source of induction heating at high frequency is presented in this paper, this paper also realize the output control of independence irrespective of the switching frequency using Phase-Shift. The analysis of the proposed circuit is generally described by using the normalized parameters. Also, the principle of basic operation and the its characteristics are estimated by the parameters, such as switching frequency, the variation of phase angle(${\varphi}$) of Phase-Shift. It is certain that the proposed circuit will be used and expanded in the high frequency power supplies like induction heating systems.

• Journal of electromagnetic engineering and science
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• v.18 no.4
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• pp.254-260
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• 2018

In this study, we suggest an emulator for generating multiple heterogeneous interference signals in the Korean radio frequency identification/ubiquitous sensor network (RFID/USN) frequency band. The proposed emulator uses only one universal software radio peripheral to generate multiple heterogeneous interference signals more economically. Moreover, the physical and media access control parameters can be adjusted in real time using the LabVIEW program, thereby making it possible to create various time-varying interference environments easily. As an example showing the capability of the proposed emulator, multiple interference signals consisting of a frequency-hopping RFID signal and two LoRa signals with different spreading factors were generated. The generated signals were confirmed in both frequency and time domains. From the experimental results, we verified that our emulator could successfully generate multiple heterogeneous interference signals with different frequency and time domain characteristics.

• Proceedings of the IEEK Conference
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• 1999.06a
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• pp.223-226
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• 1999

We study the propagation property of the transient signals along the microstrip using the wavelet transform. Wavelet transform can offer the time-frequency windows. It makes the resolution of time high in high frequency range and the resolution of frequency high in low frequency range. So It is useful to analyze the signals which have both low and high frequency components.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.12 no.4
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• pp.365-372
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• 1987

This paper describes the basic theory and construction of the frequency stability measurement system by beat frequency method, one kind of the accurate measurement technologies of frequency stability in time domain. The characteristics of the real system is also investigated.

• Journal of Ocean Engineering and Technology
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• v.25 no.2
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• pp.120-126
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• 2011

The fatigue damages in structural details of offshore plants can be accumulated due to various environmental loadings such as swell, wave, wind and current. It is known that load histories acting on mooring and riser systems show stationary and ergodic bimodal wide-banded process. This paper provides refined approach to obtain time signals representing stress range histories from wide-banded bimodal spectrum which consists of ideally narrow-banded and fully separated two spectrums. Variations of the probabilistic characteristics for time signals according to frequency and sampling time increments are compared with the reference data to be the probabilistic characteristics such as zero-crossing period, peak period, and irregularity factor obtained from an assumed ideal spectrum. It is proved that the sampling time increment more affects on the probabilistic characteristics than frequency increment. The fatigue damages according to the frequency and sampling time increments are also compared with the ones with minimum increment condition which are thought to be exact fatigue damage. It is concluded that the maximum sampling time increment to obtain reliable time signals should be determined that ratio of applied maximum sampling time increment and minimum period is less than approximately 0.08.

• Nuclear Engineering and Technology
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• v.55 no.3
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• pp.968-980
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• 2023

The dynamic structural responses are sensitive to the time-frequency content of seismic waves, and seismic input motions in time-history analysis are usually required to be compatible with design response spectra according to nuclear codes. In order to generate spectra-compatible input motions while maintaining the intrinsic non-stationarity of seismic waves, an improved time-domain approach is proposed in this paper. To maintain the nonstationary characteristics of the given seismic waves, a new time-frequency envelope function is constructed using the Hilbert amplitude spectrum. Based on the intrinsic mode functions (IMFs) obtained from given seismic waves through variational mode decomposition, a new corrective time history is constructed to locally modify the given seismic waves. The proposed corrective time history and time-frequency envelope function are unique for each earthquake records as they are extracted from the given seismic waves. In addition, a dimension reduction iterative technique is presented herein to simultaneously superimpose corrective time histories of all the damping ratios at a specific frequency in the time domain according to optimal weights, which are found by the genetic algorithm (GA). Examples are presented to show the capability of the proposed approach in generating spectra-compatible time histories, especially in maintaining the nonstationary characteristics of seismic records. And numerical results reveal that the modified time histories generated by the proposed method can obtain similar dynamic behaviors of AP1000 nuclear power plant with the natural seismic records. Thus, the proposed method can be efficiently used in the design practices.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.33 no.12
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• pp.1442-1448
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• 2009

Transient characteristics of a signal can be effectively exhibited in time-frequency domain. Hilbert-Huang Transform (HHT) is one of the time-frequency domain analysis methods. HHT is known for its several advantages over other signal analysis methods. The capability of analyzing non-stationary or nonlinear characteristics of a signal is the primary advantage of HHT. Moreover, it is known that HHT can provide fine resolution in high frequency region and handle large size data efficiently. In this study, the effectiveness of Hilbert-Huang transform is illustrated by employing structural systems undergoing impact. A simple discrete system and an axially oscillating cantilever beam undertaking periodic impulsive force are chosen to show the effectiveness of HHT.

• Journal of Biosystems Engineering
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• v.23 no.1
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• pp.57-66
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• 1998

This study was conducted to investigate the relationships between parameters affecting internal quality of watermelon and its acoustic characteristics. The measuring and analyzing system was established to study the acoustic characteristics of watermelon. Algorithms for analyzing sound signals were developed. Sound signals which was detected with the microphone were filtered, and their spectrum was computed by means of the Fast Fourier Transform. As watermelon changed from the unripe stage to ripe, acoustic waves in time domain became complicated, and several components appeared in frequency domain. The correlationship was investigated between some parameters affecting internal quality of the watermelon and several peak frequencies. Results indicated that weight, density and sugar content had high correlations with several frequencies(the first peak frequency, the second peak frequency, and the third peak frequency). And the sugar content and the volume of the watermelon were highly correlated with the third peak frequency.