Design and Development of High-Power Pulse Transmitters for Underground Environmental Perception

Abstract

The objective of this thesis is associated with designing a high-power Gaussian pulse transmitter for high-depth imaging applications. Two types of pulse transmitters are presented in this work. The former configuration is based on Step Recovery Diodes (SRDs), while the latter is based on Nonlinear Transmission Lines (NLTLs). Both configurations are thoroughly studied and tested theoretically and experimentally. The first system consists of three main parts: an avalanche transistor-based pulse shaping network, an improved SRD-based configuration, and a pulse-shifting circuit with a broadband combiner to create a second derivative Gaussian pulse for high-power applications. A Gaussian pulse transmitter comprises a Gaussian pulse shaping network that converts a sine/square signal to a first derivative Gaussian pulse, the monopulse. The monopulse is used to excite the antenna to radiate through the medium. The pulse width and amplitude of a monopulse determine the image resolution and detection range, respectively. The transistor-based circuit plays a role as an input pulse generator for the SRD pulse shaping circuit that produces ultra-short pulses. The SRD-based part of the circuit is developed to have a high amplitude output pulse. A balun is accommodated at the output of the avalanche circuit to convert the output pulse into two opposite polarity pulses. Two parallel SRD-based pulse shaping networks are placed after the balun to sharpen the output pulses of the avalanche circuit. The outputs of these two branches are two oppositely polarized narrow Gaussian pulses. Note that a delay line is attached for one of the branches to provide the delay between the two opposite pulses. A broadband combiner is used after the two SRD-based circuits to add the opposite pulses. The final output of the combiner is a monopulse. The last part of the transmitter can be any broadband antenna such as a Vivaldi antenna or a monopole antenna placed after the combiner to radiate the monopulse. In order to check the validity of the transmitter for imaging purposes, several experiments for buried objects in the sand are conducted. All reconstructed 3D images clearly represent the target shape and dimension, confirming the practicality (functionality) of the designed transmitter for sensing and imaging applications. The second configuration is the Gaussian pulse generator based on the NLTL. The NLTL consists of the twenty-one cascaded unit cells of transmission lines with reversed biased varactors connected in parallel, which sharpen the rising edge. An analysis of the sharpening process of the rising and falling time of a Gaussian pulse is presented in this thesis. Several different simulations and circuit tests were established to conduct the analysis, including a lumped-element model and a distributed-element model.