학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2017. 2. 남상욱.This thesis proposes novel wall-clutter rejection techniques and frequency-modulated continuous-wave (FMCW) radar architectures for through-the-wall radar applications. In the through-the-wall radar applications, wall-cutter rejection is very important because the wall-clutter requires high dynamic range of receiver and analog-to-digital converter (ADC). Wideband FMCW radars are good candidates for a high-resolution wall-penetrating detection radar because they can achieve high dynamic range by using a range-gating filter at intermediate frequency or baseband to fully eliminate wall-clutter. However, homodyne FMCW radars require a very high-order high-pass filter (HPF) to fully eliminate wall-clutter when a target is located behind and in close proximity to the wall.
In this thesis, we first investigate a delay-line technique. The inserted delay-line decreases the time-gap between the received signal and the chirp signal. Therefore, the beat-frequencies of the target and wall are shifted to lower frequencies, and the ratio of pass-band to stop-band frequency is increased. As a result, the low-order HPF meets the filter specification. A design methodology and an example for short-range target is provided in this thesis. And the validity of this technique has been verified with a system simulation.
Even though, the delay-line technique allows the low-order HPF can fully attenuate the wall-clutter, the delay-line in an RF signal path or LO signal path cause several problems: 1) A conventional long delay-line makes considerable signal loss at a high frequency such as X-band or Ka-band2) The line-loss also introduces amplitude modulation due to the loss, depending on frequency. Therefore, amplitude modulation increases as radar bandwidth increases, representing a particular problem for high-resolution radar. This amplitude modulation can lead to large side-lobes near a target-beat-frequency3) The delay-line requires a large area single-layer substrate or a multi-layer substrate with additional process or expensive process, such as surface acoustic wave (SAW) process to produce a long delay4) It is difficult to achieve a controllable delay, from a short delay to a long delay, with a fine time-resolution. It requires abundant delay-lines, control circuits, and loss-compensate circuits, resulting in a bulky system.
Therefore, we propose a novel FMCW radar architecture that employs two phase-locked loops (PLL) and a phase controller. One PLL generates a chirp signal for transmitting (TX chirp) while the other PLL generates a chirp signal for mixing at the mixer (LO chirp). The PLLs share a reference clock, but the transmitter PLL input path includes a digital phase-controller. When a digital phase control function is activated, the controller advances the reference clock as a half-period by generating one more edge and inversing the following edges. Each reference clock is divided by two and compared to the corresponding PLLs feedback clock. When the phase controller invokes a half-period advance, the transmitter PLL starts tracking. After some cycles, the PLL locks onto the advanced clock, producing a corresponding advanced time in the TX chirp. By repeating this process (advance reference, PLL tracking and PLL locking), it is theoretically possible to produce an infinite time-difference. In practice, due to the finite period of the TX chirp and the LO chirp, the maximum time-difference is limited. This method solves all of the above problems: It does not result in any loss in RF or LO signals, nor produce any amplitude modulation including wideband FMCW radarsdoes not require greatly increased volumeand permits infinite time-delay with fine time-resolution. This method allows a low-order HPF to highly attenuate wall-clutter and also decouples the relationship between the walls distance and the HPFs cut-off frequency. The proposed radar was implemented and measured. The wall was located at 1.5 m and the target was located at 3 m at the middle of the room. The measured results show a second-order HPF attenuates by more than 20 dB for the wall-beat-frequency signal while it does not attenuate the target-beat-frequency signal. The proposed radar is highly appropriate for wall-penetrating detection applications.Chapter 1. Introduction 1
Chapter 1.1 Radars of the high-resolution wall-penetrating applications 2
Chapter 1.2 Research strategy 5
Chapter 1.3 Dissertation organization 5
Chapter 2. FMCW radar with a delay-line 12
Chapter 2.1 Introduction 13
Chapter 2.2 Design methodology 22
Chapter 2.3 Conclusion 34
Chapter 3. FMCW radar with two PLLs and a digital controller 35
Chapter 3.1 Introduction 37
Chapter 3.2 Design methodology 45
Chapter 3.3 Measurement results 55
Chapter 3.3 Conclusion 60
Chapter 4. Conclusion 61
Appendix A Wideband DC block design for wideband radar applications 63
Appendix A.A Introduction 64
Appendix A.B Analysis and design 69
Appendix A.C Implementation and measurements 73
Appendix A.D Conclusion 78
Appendix B FMCW radar components in chap. 3 79
Appendix B.A High-pass filter 79
Appendix B.B Chirp source 82
Bibliography 85
Abstract in Korean 88Docto
