PCM/FM 변조시스템에서 가변 컷오프 주파수 특성의 Pre-modulation 필터

다국어 초록 (Multilingual Abstract)

PCM/FM modulation is a commonly deployed technique for a wide variety of telemetry in launch vehicle links. In this system, the effect of the pre-modulation filter reduces the energy contained in the sidebands of the modulated carrier. Most existing filters are based on a constant cutoff frequency specification with a simple design and structure. But onboard units of the launch vehicle should not be revised by any hardware modifications during the development from a reliability point of view. If PCM bit rate is changed, the cutoff frequency of pre-modulation filter should be also changed according to 0.7 times the bit rate. In a constant bandwidth filter, it is needed to modify the hardware. This is surely prohibited in the case of a successful checkout unit up to the final flight test.
In this paper a variable cutoff frequency pre-modulation filter has been developed in order to determine the effective response to this very unusual circumstance which is the change of PCM bit rate during the long-term development. A software programmable function, a magnitude frequency response characteristics with Bessel filter and linear phase frequency response characteristics should be satisfied to the requirements. This filter was comprised of a digital FIR filter and a reconstruction system with a DAC block and a 2nd order low pass filter. It was designed and analyzed for each stage to satisfy the magnitude frequency response of a 7th order low pass Bessel filter. Also group delay simulations have been carried out in order to confirm linear phase characteristics of 2nd order low pass filter with real hardware devices.
Based on the design procedure and circuit scheme, hardware was implemented in video compression unit. Especially the reconstruction system was calibrated to compensate the inescapable tolerances of the real devices using the tone generator. The video compression unit with the proposed pre-modulation filter has been performed by various functional and environmental tests. Video system including the video compression unit has also been equipped in upper stage of KSLV-I launch vehicle. Finally we could verify that the onboard video system has succeeded in acquiring the event pictures and flight dynamic video data during the first flight test.

목차 (Table of Contents)

• Ⅰ. 서 론 1
• 1.1 연구의 배경 1
• 1.2 연구의 내용 3
• 1.3 논문의 구성 5
• Ⅱ. 아날로그 Pre-modulation 필터 6
• 2.1 PCM 신호의 저역통과 필터링 방법 6
• 2.2 고정 대역폭 Pre-modulation 필터 설계 8
• 2.3 저역통과 베셀필터의 감쇄 주파수 응답해석 13
• Ⅲ. 가변 컷오프 Pre-modulation 필터 19
• 3.1 제안한 구조 및 설계 기법 19
• 3.2 필터 구성요소의 감쇄 주파수 응답해석 23
• 3.2.1 Reconstruction 시스템의 감쇄 주파수 응답해석 24
• 3.2.2 디지털 FIR 블록의 감쇄 주파수 응답해석 40
• 3.3 PCM 신호의 시간도메인 필터링 파형 특성 51
• 3.4 다양한 PCM 신호 속도에서의 선형위상 특성 56
• Ⅳ. 구현 및 실험 64
• 4.1 필터 구성요소의 하드웨어 회로설계 64
• 4.1.1 디지털 FIR 블록의 설계 및 시뮬레이션 64
• 4.1.2 DAC 블록의 설계 및 전달함수 72
• 4.1.3 가변 2차 LPF 블록의 설계 및 시뮬레이션 77
• 4.2 Reconstruction 시스템의 성능 평가 방법 85
• 4.2.1 검증방법 85
• 4.2.2 교정방법 89
• 4.3 실험 및 고찰 91
• Ⅴ. 결 론 101
• 참고문헌 104
• 부록 108
• Abstract 123