• Proceedings of the Acoustical Society of Korea Conference
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• 1998.06e
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• pp.27-30
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• 1998

본 연구에서는 청각계의 시간 및 주파수 특성을 고려한 과도음의 시간-주파수 신호해석 기법인 VFT-STFT (STFT with Variable Frequency Resollution)을 제안하고자 한다. VFT-STFT은 downsampling와 FFT를 반복적으로 수행하여 주파수 대역에 따라 주파수 및 시간 분해능이 청각계의 특성과 유사한 기존의 VFR-FFT에 그 뿌리를 두고 있다. 그러나, 본 연구에서는 기존의 VFT-FFT 알고리즘에 overlap인자를 도입하여 시간-주파수 해석 결과를 구하고, 2/3-rate resampling에 의해 추가로 구성된 시간-주파수 해석 결과의 일부를 기존의 시간-주파수 해석 결과에 이식시킴으로서 기존의 VFT-FFT가 갖는 overlap과 spectral loss 등의 문제점을 최소화하고자 한다.

• Journal of the Korea Institute of Military Science and Technology
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• v.24 no.4
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• pp.368-373
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• 2021

This paper presents a method of independent pulse repetition frequency(PRF) generation and control for frequency phase calibration on mono-pulse radar at a remote location. In order to generate an independent PRF signal of 320[Hz], pulse width modulation(PWM) of 16-bit timer/counter was applied. For a precision control of PRF signal, 16-bit timer/counter interrupt was changed for each period. Therefore, average frequency of PRF could be controlled by 0.0001[Hz]. To calibrate a frequency phase of mono-pulse radar at a remote location, the proposed PRF generator with a precision control of frequency was used regardless of receiving PRF signal from a radar. For the verification of the proposed PRF generator, theoretical analysis and experimental results are included.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.32 no.2
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• pp.117-124
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• 2019

Parallel sparse solvers are essential for solving large-scale finite element models. This paper introduces the combination of iterative and direct solver that can be applied efficiently to problems that require continuous solution for a subtly changing sequence of systems of equations. The iterative-direct sparse solver combination technique, proposed and implemented in the parallel sparse solver package, PARDISO, means that iterative sparse solver is applied for the newly updated linear system, but it uses the direct sparse solver's factorization of previous system matrix as a preconditioner. If the solution does not converge until the preset iterations, the solution will be sought by the direct sparse solver, and the last factorization results will be used as a preconditioner for subsequent updated system of equations. In this study, an improved method that sets the maximum number of iterations dynamically at the first Krylov iteration step is proposed and verified thereby enhancing calculation efficiency by the frequency domain analysis.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.27 no.6
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• pp.512-520
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• 2016

In this paper, a miniaturized frequency selective surface(FSS) for bandstop operation that provides stability for an angle of incidence and polarization is presented. The proposed miniaturized FSS has the unit cell of hexagonal structure with triangular loops and size of the unit cell is $0.081{\lambda}{\times}0.081{\lambda}$ at 2.5 GHz operating frequency, which is very small compared to operating wavelength. In addition, unlike the conventional design, which requires complicated design parameters, the proposed FSS is easily expanded to the desired operating frequency for 2~8 GHz, by controlling the specific design parameters. To validate the simulation results, the FSS structures having different operating frequencies, 2.5 GHz, 5 GHz and 8.2 GHz were designed, fabricated and measured. The comparisons between the simulation and the measured results show good agreement. The proposed miniaturized FSS can provide better frequency stability for different incidence angles and polarizations.

• Proceedings of the Optical Society of Korea Conference
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• 2002.07a
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• pp.224-225
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• 2002

펨토초 모드록 레이저는 공진기의 왕복시간 $\tau$마다 펨토초 폭을 갖는 펄스를 발생시키고, Fourier 변환에 의해 주파수 공간에서는 일정한 주파수 간격 $\Delta$ = 1/$\tau$을 가지는 불연속 스펙트럼, 즉 광빗 스펙트럼을 갖게 된다(1). 광빗의 간격, 즉 펄스의 반복률, 은 $\Delta$ ： v$_{g}$l$_{c}$의 관계식에 의해 그룹속도(v$_{g}$)와 공진기 길이(l$_{c}$)에 의해 결정된다. 광빗의 간격을 고정시키기 위한 가장 일반적인 방법은 공진기 길이를 일정하게 유지하는 것이다. (중략)

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2009.04a
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• pp.131-134
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• 2009

Krylov 부공간 모델차수축소법은 초기 유한요소모델과 축소모델의 전달함수의 계수인 모멘트를 일치시키는 방법을 이용하는 축소기법으로 이미 대형 유한요소모델의 주파수응답 해석의 효율적인 계산에 많이 사용되고 있는 방법 중의 하나이다. 본 논문에서는 Krylov 부공간 축소기법을 이용한 관심 주파수영역에 대한 주파수응답 해석 및 이를 통하여 계산된 주파수응답의 여러 가지 설계변수에 대한 설계민감도 해석방법을 제안하였다. 일반적으로 구조물의 주파수응답을 고려한 최적설계를 위해서는 설계변수에 대한 관심 주파수영역에서의 주파수응답 및 그의 민감도 정보가 요구되므로, 고려하는 유한요소모델이 대형일 경우에 관심 주파수영역에서의 반복적인 해석으로 인한 계산비용의 문제가 대두된다. 본 논문에서는 축소모델을 이용하여 주파수응답과 주파수응답의 설계민감도 해석을 수행하여 계산의 효율성을 극대화하였다. 민감도 계산에는 시간측면과 구현의 용이성 측면에서 장점이 있는 준해석적 방법을 이용하였다. 수치 예제를 통하여 축소기법을 이용한 주파수응답의 설계민감도 해석 결과를 유한차분법에 근거한 민감도 결과와 비교하였다. 본 논문에서 제안된 방법을 이용하는 경우, 주파수응답을 고려한 최적설계를 계산비용 측면에서 매우 효율적으로 수행할 수 있을 것이다.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.32 no.5A
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• pp.484-489
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• 2007

The main disadvantage of orthogonal frequency division multiplexing (OFDM) systems is its sensitivity to carrier frequency offset and timing offset. This paper proposes a simple way of improving the performance of the integer frequency offset (IFO) estimator in OFDM-based digital video broadcasting (DVB) system. By modifying the conventional maximum likelihood (ML) estimator to have multi-stage estimation strategy, IFO estimator is derived. Simulations indicate that the proposed IFO estimator works robustly with reduced computational burden when compared to ML estimator.

• The Journal of the Korea institute of electronic communication sciences
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• v.13 no.4
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• pp.729-736
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• 2018

Due to the frequency offset, interchannel interference (ICI) is occurred in the received symbols of the orthogonal frequency division multiplexing (OFDM) systems. The ICI self-cancellation (ICI-SC) technique appropriately adjusts the subchannel signal assignment of the OFDM symbols, thereby canceling the interference caused by other subchannels. The conventional adjacent symbol repetition (ASR) method can reduce the interference caused by remote subchannels. However, it may not mitigate or even increases the ICI produced by some nearest subchannels. To solve the problem, a new ASR based ICI-SC technique is proposed and its performance is analyzed in this paper. Here, a t-parameter obtained by the interference coefficients of 3 successive subchannels is applied. As a result, the proposed method has the same capability to reduce the influence of remote subchannels. However, it can reduce the ICI caused by the nearest subchannels significantly.

• The Journal of the Korea Contents Association
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• v.7 no.2
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• pp.125-131
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• 2007

An iterative Green's function method (IGFM) is introduced in order to analyze complex electromagnetic waveguide stub structures in view of a university student. The IGFM utilizes a Green's function approach and an regional iteration scheme. A physical iteration mechanism with simple mathematical equations facilitates clear formulations of the IGFM. Scattering characteristics of a standard E-plane T-junction stub in a parallel-plate waveguide are theoretically investigated in terms of the IGFM. Numerical computations illustrate the characteristics of reflection and transmission powers versus frequency.

• The Journal of Engineering Geology
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• v.16 no.3 s.49
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• pp.235-244
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• 2006

Granite cores were sampled within Korea Atomic Energy Research Institute and cyclic loadings up to 1550 cycles were applied. Microcrack development in samples due to cyclic loading was estimated using Acoustic Emission(AE) method. AE showed two different types depending on numbers of cycle. Type 1 appeared at low cycles and had low energy and diverse frequencies, while type 2 appeared at high cycles and had high energy and uniform frequency. AE property of type 1 indicates voids and pre-existing microcracks in samples may close or propagate up to certain length. Microcracks may be sheared or closed during loading and are recovered from shear or opened during unloading when AE of type 2 were measured. P wave velocities and Felicity ratios were measured at 50, 150, 350, 750, 1550 cycles. P wave velocities were almost the same regardless of number of cycles applied. However, Felicity ratios were much lower than 0.9, indicating that microcracks were developed within samples. This result indicates that Felicity ratio is a better tool than P wave velocity to estimate the damage of rock.