• Journal of Biomedical Engineering Research
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• v.16 no.2
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• pp.183-190
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• 1995

The use of ultrasound pulsed Doppler systems has become increasingly popular due to the advantages of easy measurements of blood velocity, volume blood blow, and irregularities of the circulatory system. However, the 2-D Doppler systems have several problems, such as range ambiguity, low signal to noise ratio, and slow frame rate. The mean frequency aliasing problem originating from the pulse repetition frequency is one of major limitations in pulsed Doppler systems. A conventional approach to resolve this problem is tracking the mean frequency close to and beyond the Nyquist frequency along the temporal axis. In this paper, a new concept of tracking the mean frequency along the spatial axis is proposed. The proposed technique is fault tolerant by nature and more suitable for multi gate and 2-D Doppler system than conventional methods.

• JSTS:Journal of Semiconductor Technology and Science
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• v.12 no.3
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• pp.270-277
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• 2012

This paper presents a new DAC design strategy to achieve a wideband dynamic linearity by increasing the bandwidth of the output impedance. In order to reduce the dominant parasitic capacitance of the conventional matrix structure, all the cells associated with a unit current source and its control are stacked in a single column very closely (stacked unit cell structure). To further reduce the parasitic capacitance, the size of the unit current source is considerably reduced at the sacrifice of matching yield. The degraded matching of the current sources is compensated for by a self-calibration. A prototype 6-bit 3.3-GS/s current-steering full binary DAC was fabricated in a 1P9M 90 nm CMOS process. The DAC shows an SFDR of 36.4 dB at 3.3 GS/s Nyquist input signal. The active area of the DAC occupies only $0.0546mm^2$ (0.21 mm ${\times}$ 0.26 mm).

• Proceedings of the Korean Society of Broadcast Engineers Conference
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• 2014.06a
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• pp.230-231
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• 2014

Compressive sensing is a new sampling technique, which allows to sample a signal under the Nyquist-Shannon sampling rate. For block-based compressive sensing, a hybrid sensing matrix which contains low-frequency patterns in addition to the random Gaussian numbers is good for exploiting typical property of natural images. By noting that MH-BCS-SPL is well known for its good recovery performance, this paper investigates effect of the hybrid sensing matrix on MH-BCS-SPL in the sense of how large portion of low-frequency patterns can provide performance improvement.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.59 no.9
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• pp.1605-1610
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• 2010

In this paper, the relative stability of magnetic bearing system for the flywheel energy storage is evaluated using both simulation and experimental analysis. We make the simulation model for the magnetic bearing flywheel system using the rigid body shaft model. According to international standard ISO 14839-3, We experimentally analyzed the relative stability of magnetic bearing system. Additionally using both the simulation model and experimental tests, Phase margin and Gain margin is acquired through Nyquist plot.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1994.10a
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• pp.40-45
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• 1994

기계시스템의 비선형특성 해석을 위하여 여러가지 방법이 활용되고 있는데, 이들은 Nyquist 선도의 찌그러짐(distortion), Hilbert 변환, 복원력면(restoring force surface), NARMAX, 고차 주파수응답함수(higher order frequency response function), DPE(direct parameter estimation)를 이용한 방법등이다. 이들중 고차 FRF(frequency response function)는 그 개념이 선형시스템의 FRF와 유사하여 비선형시스템의 해석방법으로서 주목을 받고 있으나 아직은 고차 FRF의 특성에 대한 이론적 연구 단계이고, 고차 FRF로부터 비선형특성을 정량적으로 해석하는 연구는 거의 이루어지지 않고 있다. 다항식으로 표시되는 비선형성을 갖는 시스템이 정현파가진을 받을 때 그 응답의 가진주파수 성분은 가진력진폭과 고차 FRF의 무한급수로 나타낼 수 있다. 가진력의 진폭을 변화시켜가며 응답을 측정하고, 고차항을 무시하면 고차 FRF의 값을 근사적으로 구할 수 있다. 고차 FRF는 비선형 시스템의 매개변수의 식으로 나타낼 수 있으므로 이로부터 비선형 매개변수를 추정할 수 있다. 본 논문에서는 비선형강성과 비선형감쇠를 갖는 1자유도 시뮬레이션 시스템에 이 매개변수 추정법을 각각 적용함으로써 이 방법의 가능성을 고찰하였다.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1998.04a
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• pp.478-485
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• 1998

The interior of open cavities exposed to a grazing flow is known to experience, strong periodic pressure, oscillations sustained for a wide range of flow velocities. In this study, an original approach was followed to develop a describing function model for the flow-excitation mechanism, governed by the shedding of discrete vortices within the shear layer over the orifice. A feedback loop analysis was performed to predict the frequency and the amplitude of the interior pressure fluctuations. Furthermore, a limit cycle stability analysis based on the extended Nyquist Stability criterion allowed the predictions of the onset and termination velocities for various modes. The analytical model was verified experimentally.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.60 no.9
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• pp.1741-1747
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• 2011

Though various techniques have been studied as a way of adjusting parameters of PID controllers, no perfect method of determining parameters is available to date. This paper proposes a new method for enhancing performance of PID controllers by using the characteristics of dual-input describing function (DIDF). In other words, if nonlinear elements with two inputs (DIDF) are connected in series to the plant, the critical point (-1+j0) for Nyquist stability theory can be moved to a position arbitrarily selected on the complex plane by determining necessary coefficients of the DIDF appropriately. This makes the application of the existing conventional PID parameter tuning methods a lot easier, and stability and robustness of the system are improved simultaneously due to the DIDF inserted.

• Proceedings of the Korean Society of Broadcast Engineers Conference
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• 2013.06a
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• pp.212-213
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• 2013

압축 센싱 기술은 성긴 (sparse)신호의 경우 Nyquist 표본화율보다 적은 수의 표본으로도 원신호를 완벽하게 복원할 수 있는 이론을 제시하고 있다. 전통적인 영상 처리분야에 압축 센싱 기술을 적용하는 연구를 시작함에 따라 계산 복잡도 및 메모리 문제로 블록 영상 기반 압축 센싱 방법을 많이 고려하고 있다. 또한, 이러한 압축 센싱 방법에서 복원 과정은 일정 허용 오차 범위 기준을 복원 신호가 만족시키는 경우에 종료되므로, 허용 오차 범위에 따른 복원 신호 품질과 계산 복잡도에 변화가 발생하게 된다. 본 논문에서는 블록 기반 압축 센싱 방법을 이용하여 영상을 복원함에 있어, 허용 오차 값에 따른 복원 영상의 화질 변화와 시간 절감 정도를 비교, 분석하였다.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.40 no.6
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• pp.1024-1031
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• 2015

Compressive sensing is a new data acquisition method enabling the reconstruction of sparse or compressible signals from a smaller number of measurements than Nyquist rate, as long as the signal is sparse and the measurement is incoherent. In this paper, we consider a simple hypothesis testing in target detection and estimation problems using compressive sensing, where the performance depends on the sparsity level of the signals being detected. We provide theoretical analysis results along with some experiment results.

• International Journal of Control, Automation, and Systems
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• v.5 no.1
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• pp.1-7
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• 2007

In this paper, the necessary and sufficient conditions for strict positive realness of the rational transfer functions directly from basic definitions in the frequency domain are studied. A new frequency domain approach is used to check if a rational transfer function is a strictly positive real or not. This approach is based on the Taylor expansion and the Maximum Modulus Principle which are the fundamental tools in the complex functions analysis. Four related common statements in the strict positive realness literature which is appeared in the control theory are discussed. The drawback of these common statements is analyzed through some counter examples. Moreover a new necessary condition for strict positive realness is obtained from high frequency behavior of the Nyquist diagram of the transfer function. Finally a more simplified and completed conditions for strict positive realness of single-input single-output linear time-invariant systems are presented based on the complex functions analysis approach.