A close range baseband radar transceiver for application in borehole radar systems

[摘要] ENGLISH ABSTRACT: A monostatic baseband radar is required with the capability of detecting close range targets that appear at distances comparable to the system's resolution, without compromising the radar's maximum range. The application in borehole radar imposes further constraints associated with thephysical limitations and variable electromagnetic environment of different borehole diameters andconditions. This dissertation discusses the complete design process of the analog section of amonostatic radar that successfully addresses these issues.The proposed transceiver employs a series duplexing arrangement consisting of an antenna,transmitter, receiver and an isolation switch. An exponentially decaying tail is observed in the currentflowing on a borehole radar antenna when excited by pulse waveforms. The characteristics of this taildepend strongly on the borehole environment. A measurement technique is developed that accuratelyquantifies this exponential decay by digitizing a logarithmic representation of the antenna currentwhile it is operating in various boreholes. Transmitters are then designed to drive these antennas withwaveforms that prevent the formation of current tails. This is achieved through the use of pole-zeronetworks or alternatively by generating certain asymmetric, bipolar waveforms. The transmitters aresimultaneously designed to have an output impedance approximating a short circuit after the transientis generated. In the series configuration proposed here, the duplexing of the antenna betweentransmitter and receiver is then reduced to simply isolating the receiver during transmit-mode. Theswitch responsible for this isolation disconnects the receiver and presents a short circuit betweenantenna and transmitter during transmit-mode, while connecting the receiver terminals between theantenna and the short circuited transmitter terminals in receive-mode. The required close-inperformance of the transceiver dictates that the transition between these two states of the isolationswitch occur in a time similar to the duration of the transmitter waveform. The switching artefactsgenerated by the switch are consequently similar to the radar data signal. The isolation switchemploys an innovative configuration (using both transistors and diodes) which accepts a single controlsignal and causes the switching artefacts to be generated as a common mode signal, while adifferential path is created for the radar data signal which is being switched. This leads to effectivesuppression of the switching signal in the signal passed to the receiver. Dissipative filtering isadvocated as a fundamental design principle for high fidelity receivers and it is shown how it can beapplied by using constant impedance equalizers and diplexers as basic building blocks. This principleis used as the basis for the design of this transceiver's receivers, which incorporate both standardgain blocks and operational amplifiers.A complete borehole radar system, based on the transceiver developed here, was built and tested;resulting in the first known practical monostatic borehole radar system. Data obtained in field trials arepresented and suggest that the monostatic system compares well with current state of the art bi-staticsystems.

[发布机构] Stellenbosch University