Thesis (PhD (Electrical and Electronic Engineering)--University of Stellenbosch, 2007. / 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 the
physical limitations and variable electromagnetic environment of different borehole diameters and
conditions. This dissertation discusses the complete design process of the analog section of a
monostatic 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 current
flowing on a borehole radar antenna when excited by pulse waveforms. The characteristics of this tail
depend strongly on the borehole environment. A measurement technique is developed that accurately
quantifies this exponential decay by digitizing a logarithmic representation of the antenna current
while it is operating in various boreholes. Transmitters are then designed to drive these antennas with
waveforms that prevent the formation of current tails. This is achieved through the use of pole-zero
networks or alternatively by generating certain asymmetric, bipolar waveforms. The transmitters are
simultaneously designed to have an output impedance approximating a short circuit after the transient
is generated. In the series configuration proposed here, the duplexing of the antenna between
transmitter and receiver is then reduced to simply isolating the receiver during transmit-mode. The
switch responsible for this isolation disconnects the receiver and presents a short circuit between
antenna and transmitter during transmit-mode, while connecting the receiver terminals between the
antenna and the short circuited transmitter terminals in receive-mode. The required close-in
performance of the transceiver dictates that the transition between these two states of the isolation
switch occur in a time similar to the duration of the transmitter waveform. The switching artefacts
generated by the switch are consequently similar to the radar data signal. The isolation switch
employs an innovative configuration (using both transistors and diodes) which accepts a single control
signal and causes the switching artefacts to be generated as a common mode signal, while a
differential path is created for the radar data signal which is being switched. This leads to effective
suppression of the switching signal in the signal passed to the receiver. Dissipative filtering is
advocated as a fundamental design principle for high fidelity receivers and it is shown how it can be
applied by using constant impedance equalizers and diplexers as basic building blocks. This principle
is used as the basis for the design of this transceiver's receivers, which incorporate both standard
gain 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 are
presented and suggest that the monostatic system compares well with current state of the art bi-static
systems. / AFRIKAANSE OPSOMMING: Die behoefte is geïdentifiseer vir 'n monostatiese basisbandradar wat oor die vermoë beskik om
nabygeleë teikens op 'n afstand soortgelyk aan die resolusie van die stelsel waar te neem, sonder om
die maksimum bereik van die stelsel in te kort. Die toepassing daarvan in 'n boorgatradarstelsel lei tot
verdere vereistes vanweë die fisiese beperkings en veranderende elektromagnetiese omgewing van
boorgate met verskillende deursnitte en toestande. Hierdie proefskrif is gemoeid met die volledige
ontwerpsprosedure van die analoog gedeelte van 'n monostatiese radar wat al hierdie kwessies
aanspreek.
'n Serie verbinding van antenne, sender, ontvanger en isolasieskakelaar word ingespan vir hierdie
ontwerp. Eksponensieel wegsterwende stertjies word waargeneem in die antennestroom van 'n
boorgatradarantenne wanneer dit aangedryf word deur puls golfvorms. 'n Meettegniek word ontwikkel
wat hierdie eksponensiële verslapping noukeurig kan monitor deur 'n logaritmiese voorstelling van die
antennastroom te versyfer terwyl dit ontplooi word in verskillende boorgate. Senders word dan
ontwikkel om hierdie antennes aan te dryf met golfvorms wat juis die vorming van hierdie stertjies
voorkom. Dit word bewerkstellig deur die gebruik van pool-zero netwerke of andersins deur die opwek
van sekere asimmetriese, bipolêre golfvorms. Die senders se uittree-impedansies moet egter
terselfdertyd ontwerp word om 'n kortsluiting te benader sodra die oorgang klaar opgewek is. Met die
serie verbinding wat hier gebruik word, raak die vereiste tyddeling van die antenna tussen die sender
en ontvanger dan bloot 'n geval van ontvanger-isolasie gedurende uitsaai-modus. Die skakelaar wat
verantwoordelik is vir hierdie isolasie ontkoppel die ontvanger en vertoon soos 'n kortsluiting tussen
sender en antenne tydens uitsaai-modus, maar verbind weer die terminale van die ontvanger tussen
die antenne en kortgeslote senderterminale tydens ontvang-modus. Die vereiste kortafstand vermoë
van die stelsel veroorsaak dat die tysduur van die oorgang tussen hierdie twee modusse soortgelyk is
aan dié van die sender golfvorm en enige skakelverskynsels wat opgewek word deur die skakelaar is
gevolglik soortgelyk aan die radardatasein self. Die isolasieskakelaar gebruik egter 'n innoverende
konfigurasie (met transistors sowel as diodes) wat funksioneer met 'n enkele beheersein en die
skakelverskynsels as gemene modus seine opwek, terwyl 'n differensiële seinpad geskep word vir die
radardatasein wat geskakel word. Die skakelseine word gevolglik effektief onderdruk in die sein wat
oorgedra word aan die ontvanger. Die gebruik van verkwistende filters word voorgestel as 'n
fundamentele ontwerpsbeginsel vir hoëtrou ontvangers en daar word getoon hoe dit toegepas kan
word met konstante impedansie vereffeningsbane en dipleksers. Hierdie beginsel is dan ook gebruik
as basis vir die ontwerp van hierdie stelsel se ontvangers, wat gebruik maak van beide standard
aanwinsblokke sowel as operasionel versterkers.
'n Volledige boorgatradarstelsel, gebaseer op die stelsel wat hier ontwikkel is, is gebou en getoets.
Die gevolg is die eerste bekende, praktiese monostatiese boorgatradarstelsel. Data wat hiermee
verwerf is word aangebied en dui daarop dat die monostatiese stelsel baie goed opweeg teen huidige
bi-statiese stelsels.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/328 |
| Date | 12 1900 |
| Creators | Van der Merwe, P.J. (Paulus Jacobus) |
| Contributors | Cloete, J. H., Van der Walt, P. W., University of Stellenbosch. Faculty of Engineering. Dept. of Electrical and Electronic Engineering. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 146 p. : ill. ; 30 cm. |
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