Maximum Length Sequences for Radar and Synchronization

Abstract

This dissertation investigates the feasibility of implementing ultra-wideband (UWB) radar sensors using commercial off-the shelf electronics. The sensors are based on the correlation of binary pseudo noise sequences, combining low transmit power requirements with excellent noise and interference suppression. Maximum length sequences (M-sequences) are used as stimulus in this work. Their characteristics are compared to other established radar waveforms. Generators for M-sequences are demonstrated that allow high-speed operation at 2 GHz. In the practical part of this thesis, sensor prototypes are developed for two different fields: positioning as well as ground penetrating radar. A modulated ranging system is introduced that features 1 GHz of baseband bandwidth. The system is operating at a 2 GHz carrier frequency using a sequential IQ downconverter. The sequential IQ downconverter ensures close to perfect IQ balance even for large bandwidths. It is shown that the system is able to track moving objects with a standard deviation of 1.73 mm at 2 m range. When combining multiple ranging systems to construct a complete three-dimensional positioning system, a synchronization backbone is often required. A demonstrator is developed which can synchronize itself to a reference sequence with 1.96 ps RMS jitter. The proposed solution uses an analog correlating control loop (delay lock loop) which allows tracking the reference to 0.38% of one chip duration in a cabled configuration. The final application shown is a ground penetrating radar (GPR). GPR is commonly used to locate buried utility pipes and voids avoiding costly drilling and excavation. A prototype radar system is developed that integrates the complete transmitter and receiver chain, as well as signal processing. A very compact system can be built by using programmable logic as a high-speed binary signal source and sink. In addition to the central FPGA only two additional components are needed: a fast output driver for the transmitter and a commercial analog-to-digital converter for the receiver. Comparative measurements on buried pipes and cables prove that this system has achieved detection capability comparable to commercially available pulsed GPRs.