• 대한기계학회논문집B
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• 제31권2호
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• pp.202-207
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• 2007

In order to understand the phenomena of combustion instability, an experimental study was conducted at the moderate pressure and ambient temperature conditions. The flame behavior and the pressure fluctuations were measured in a dump combustor. Various types of combustion modes occurred in accordance with the equivalence ratio and the fuel supplying conditions. The fluctuation of pressure, heat release and equivalence ratio were measured by piezoelectric pressure sensor, high speed Intensified Charge Coupled Device (HICCD) camera and gas chromatography respectively. Two representative modes were self-excited pressure oscillations at the resonance of combustion chamber (200Hz) and instabilities related to the modulated fuel flow rate through the fuel holes (10Hz). It is found that, especially in an unchoked fuel flow condition, the modulation of the fuel flow rate affects the characteristics of flame behavior and pressure fluctuations in a lean premixed flame.

• JSTS:Journal of Semiconductor Technology and Science
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• 제13권2호
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• pp.170-183
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• 2013

This paper gives an overview of fractional-N phase-locked loops (PLLs) with practical design perspectives focusing on a ${\Delta}{\Sigma}$ modulation technique and a finite-impulse response (FIR) filtering method. Spur generation and nonlinearity issues in the ${\Delta}{\Sigma}$ fractional-N PLLs are discussed with simulation and hardware results. High-order ${\Delta}{\Sigma}$ modulation with FIR-embedded filtering is considered for low noise frequency generation. Also, various architectures of finite-modulo fractional-N PLLs are reviewed for alternative low cost design, and the FIR filtering technique is shown to be useful for spur reduction in the finite-modulo fractional-N PLL design.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2003년도 하계학술대회 논문집 B
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• pp.1250-1252
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• 2003

This Paper presents Sigma-Delta Modulation(SDM) schemes to generate switching waveform for a high-power factor boost-type rectifier. The SDM scheme can be implemented by simple digital algorithm unlike conventional PWM schemes with several hardware, and has the characteristics of spectrum-spreading and noise-shaping effects, which are profitable in harmonic reduction of input current in the boost-type rectifier. The comparison results of their spectrum performances shows that the 1st-order SDM has better harmonic suppression effect than conventional PWM scheme and Dithered SDM scheme.

• 융합신호처리학회 학술대회논문집
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• 한국신호처리시스템학회 2000년도 하계종합학술대회논문집
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• pp.69-72
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• 2000

Multilevel modulation schemes are known to be highly bandwidth efficient. By varying modulation level adaptively according to channel conditions (i.e. adaptive QAM schemes or AQAM), high bandwidth efficiency can be achieved. This paper considers the effect of time delay on the adaptive QAM schemes in dispersive fading channels. In order to undertake investigations effectively, a simulator has been developed. The simulation results show that the BER performance obtained for the wideband channel with a delay spread of 0.75 $\mu$sec is better than 10$^{-4}$ with a SNR value of 40 ㏈ and ABPS is found to be approximately 5.5.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2013년도 제45회 하계 정기학술대회 초록집
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• pp.220-220
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• 2013

Surface-normal transmission electro-absorption modulator (EAM) are attractive for high-definition (HD) three-dimensional (3D) imaging application due to its features such as small system volume and simple epitaxial structure [1,2]. However, EAM in order to be used for HD 3D imaging system requires uniform modulation performance over large area. To achieve highly uniform modulation performance of EAM at the operating wavelength of 850 nm, it is extremely important to remove the GaAs substrate over large area since GaAs material has high absorption coefficient below 870 nm which corresponds to band-edge energy of GaAs (1.424 eV). In this study, we propose and experimentally demonstrate a transmission EAM in which highly selective backside etching methods which include lapping, dry etching and wet etching is carried out to remove the GaAs substrate for achieving highly uniform modulation performance. First, lapping process on GaAs substrate was carried out for different lapping speeds (5 rpm, 7 rpm, 10 rpm) and the thickness was measured over different areas of surface. For a lapping speed of 5 rpm, a highly uniform surface over a large area ($2{\times}1\;mm^2$) was obtained. Second, optimization of inductive coupled plasma-reactive ion etching (ICP-RIE) was carried out to achieve anisotropy and high etch rate. The dry etching carried out using a gas mixture of SiCl4 and Ar, each having a flow rate of 10 sccm and 40 sccm, respectively with an RF power of 50 W, ICP power of 400 W and chamber pressure of 2 mTorr was the optimum etching condition. Last, the rest of GaAs substrate was successfully removed by highly selective backside wet etching with pH adjusted solution of citric acid and hydrogen peroxide. Citric acid/hydrogen peroxide etching solution having a volume ratio of 5:1 was the best etching condition which provides not only high selectivity of 235:1 between GaAs and AlAs but also good etching profile [3]. The fabricated transmission EAM array have an amplitude modulation of more than 50% at the bias voltage of -9 V and maintains high uniformity of >90% over large area ($2{\times}1\;mm^2$). These results show that the fabricated transmission EAM with substrate removed is an excellent candidate to be used as an optical shutter for HD 3D imaging application.

• Journal of Power Electronics
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• 제9권1호
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• pp.18-25
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• 2009

Conventional series-resonant pulse frequency modulation controlled DC-DC high power converters with a high-frequency transformer link which is designed for driving the high power microwave generator has the problem of hard switching commutation at turn-on and turn-off of active power switching devices. This problem is due to the influence of the magnetizing current of the high-frequency transformer. This paper presents a novel prototype for a high-frequency transformer using parasitic parameters with a lossless inductive snubber and a series resonant capacitor assisted series-resonant zero current switching pulse frequency modulated DC-DC power converter, which is designed using a high power magnetron for microwave ovens. In order to implement a complete and efficient soft switching commutation, the performance of the new converter topology is practically confirmed and evaluated in the prototype of a power microwave generator.

• 방송공학회논문지
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• 제19권6호
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• pp.967-970
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• 2014

이 논문에서는 M-FSK(frequency shift keying) 변조신호를 자동으로 분류하는데 필요한 효율적인 첨두 검출 방법을 제안하였다. 다른 디지털 변조신호와 FSK 신호는 진폭 스펙트럼의 특성을 이용하여 분류할 수 있다. FSK 신호의 진폭 스펙트럼은 다른 디지털 변조신호와 다르게 변조차수와 동일한 수의 첨두를 나타낸다. 일반적으로 신호의 첨두를 검출하기 위해서는 임계치가 필요한데, 변조인식과 같이 사전에 신호에 대한 정보가 없는 경우 임계치를 정하기 어려운 점이 있다. 이 논문에서는 진폭 스펙트럼의 히스토그램을 이용하여 자동으로 간단하게 임계치를 결정하는 방법을 제시하였다. 모의실험 결과 적은 수의 표본과 잡음이 많은 환경에서도 매우 우수한 분류확률을 나타내었다.

• 한국통신학회논문지
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• 제40권6호
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• pp.981-989
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• 2015

최적 ML(Maximum Likelihood) 기법 및 sphere decoding(SD), QRM-MLD(QR decomposition with M-algorithm Maximum Likelihood Detection) 기반의 준 최적 검출 기법을 적용한 MIMO(Multiple-Input Multiple-Output) 시스템에서의 LLR(Log Likelihood Ratio) 계산은 변조 차수 및 송/수신 안테나의 수가 증가할수록 그 복잡도가 지수적으로 증가하여 구현 및 성능 면에서 큰 문제점을 야기한다. 본 논문에서는 고차 변조 방식 기반의 $N_T{\times}N_R$ MIMO시스템 수신기의 QRM-MLD 기반 MIMO 검출기에서 연판정 시 아주 낮은 복잡도로 1dB 이내의 ML 검출 기법에 대한 오류 성능 접근도를 갖는 LLR 계산 방법을 제시하고, 컴퓨터 시뮬레이션을 통해 여러 M 값에 대한 MIMO 시스템의 BER(Bit Error Rate) 결과를 도출하고 분석하여 제시된 방법의 유효성을 검증한다.

• 대한전자공학회논문지TC
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• 제38권4호
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• pp.11-17
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• 2001

본 논문에서는 High-Order QAM(Quandrature Amplitude Modulation)을 적용하는 모뎀에서 강인하고 넓은 범위의 주파수 포착 범위를 가지는 극성판단(Polarity Decision) 반송파 동기용 PD(Phase Detector) 알고리즘을 제안하고 이에 대한 평균 출력특성(S-curve)과 분산특성을 수학적으로 유도하여 기존의 DD(Decision Directed)방식과 비교 분석한다. 기존의 DD 방식의 선형영역은 256 QAM의 경우 $3.5^{\circ}{\sim}3.5^{\circ}$ 이었으나 제안한 알고리즘의 선형영역은 ${\gamma}-17.9$에서 $36^{\circ}{\sim}36^{\circ}$ 의 넓은 구간을 가진다. 또한 기존의 DD 방식에서는 256 QAM의 주파수 오프셋 포착 성능이 ${\pm}10\;KHz$ 이하였다. 이는 아날로그 front-end 회로에서 주파수 오프셋이 일반적으로 ${\pm}100\;KHz$ 정도까지 줄어들 수 잇는 것을 감안하면 AFC(Automatic Frequency Control) 또는 반송파 복구를 위한 보조적인 위상검출회로가 필요하게 됨을 의미한다. 그러나 제안된 극성판단 반송파 동기 알고리즘을 사용하면 보조적인 회로의 도움없이 SNR = 30 dB에서 최대 ${\pm}300\;KHz$의 주파수 오프셋까지도 포착 가능하다.

• 전자공학회논문지 IE
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• 제45권1호
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• pp.24-30
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• 2008

DMT 변조 방식은 VDSL에 적용된 다중 반송파 변조 방식이다. DMT 변조 방식의 문제는 송수신 반송파 주파수들 사이의 주파수 오프셋(Offset)에 민감하다는 것이다. 이러한 주파수 오프셋은 DMT 심볼에서 반송파간 간섭을 야기한다. 본 논문에서는 칼만(Kalman) 필터링을 이용하여 ICI를 감소하는 방법을 제안한다. 제안한 방식은 다른 방식과의 성능과 대역폭 그리고 계산상 복잡도를 비교한다. 시뮬레이션 결과는 제안한 방식의 성능이 주파수 오프셋이 높은 경우와 높은 차수의 변조 방식에 대해서 다른 방식에 대해서 칼만 필터링을 이용한 방식이 더 좋은 성능을 나타내었다.