RF FRONT-END OPTIMIZATION FOR HARMONIC RADAR ARCHITECTURES

The concept of harmonic radars has its foundations in the early development of radio frequency (RF) technologies. Initially conceived in the mid-20th century, the idea of using harmonic frequencies for remote sensing by radar detection arose when researchers sought to overcome the limitations inherent in fundamental frequency radar systems. Practical applications of harmonic radar technology have been seen in defense and entomology, where scientists employed these radars to detect nonlinear devices, track insect movement, and study the behavior of insects or small animals. Over the decades, the technology evolved and found new applications in search and rescue operations (i.e., RECCO®), wildlife monitoring, and automotive security systems. Despite all the previously described applications of harmonic radars, it is noticed that only 2nd-order harmonic radar has been exploited in most cases, it being one of the most popular nonlinear radars. Since the 2nd-order harmonic radar operates in a dual-band, one carrier frequency for transmission (i.e., the fundamental tone) and the other for reception (i.e., the 2nd-order harmonic tone) are expected to exhibit linear behavior. Therefore, these harmonic radars tend to have more complex designs, which rely on high-quality filters (e.g., diplexers) or techniques to suppress the 2nd-order harmonic tone from the transmitter and the fundamental tone from leaking into the mixer (before baseband), thus avoiding any false detection. For this reason, this dissertation investigates the feasibility of understanding and improving the harmonic radar architecture by studying their filtering stages. This is one of the main goal objectives of this research. Furthermore, another important objective is to perform a comparison with a high-order har- monic radar to explore new alternatives besides the popular 2nd–order harmonic radar. To achieve this, simulation and experimental evaluations of three architectures were examined: (1) 7.9 GHz fundamental-mode radar; (2) 7.9/15.8 GHz 2nd-order harmonic radar; and (3) 7.9/23.7 GHz 3rd-order harmonic radar for human sensing and vital signs. This work analyzes the harmonic radar architectures for high-sensitivity motion sensing scenarios (i.e., vital signs detection). This dissertation begins with a comprehensive review of the existing literature on harmonic radar and its motivation. Following this, the concepts of this research and an analysis of the harmonic radar range equation for the harmonic tag and simulations in MATLAB® are presented. Additionally, a summary of the radar harmonic radar applications is described. Then, the methodology section presents the architecture modeling for the 2nd- and 3rd-order harmonic radar architectures in AWR 2.5-D Visual System Simulator (VSS). It also contains a description of the harmonic tags, linear clutter, and experimental setups. The simulation results indicate that the most crucial filtering stage is located between the x2 frequency multiplier and the mixer for the case of active harmonic tag. The experimental results show that it is almost impossible for conventional radar to distinguish the active tag in a cluttered environment. However, the 2nd-order harmonic radar can distinguish active tags from clutter, with or without filtering stages. In conclusion, this dissertation highlights the crucial need for filtering stages in 2nd-order harmonic radar. Furthermore, unwanted tone rejection stages are the most important identified for short-range and high-sensitivity applications, such as vital signs detection in a waiting room (i.e., a few meters from the sensors). In this research, I developed and implemented the 3rd-order harmonic radar, with which it is possible to detect very small motion signals simulating vital signs in a short distance range. Furthermore, it is observed that 3rd-order harmonic radars do not need filtering stages, at least for short-range applications. Another advantage of 3rd-order radars is that a multiple tag scenario is the best at rejecting tags of a different order than the 3rd-order, compared to a conventional radar or a 2nd-order radar.