Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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"Method and apparatus for monostatie borehole radar"
Cross-Reference to Related Applications
The present application claims priority from Australian Provisional Patent
Application No AU2004904543 filed on 10 August 2004.
Field of the Invention
The present invention relates to an improved ground penetrating radar. In
particular, it relates to a compact radar in which the transmitter and the
receiver share
all or part of the same antenna.
Background to the Invention
For many years it has been desired to implement a radar system capable of
imaging subterranean features and buried objects. Many systems have been
developed
for a broad range of applications in the mining, geoteehnical, environmental
and safety
IS areas. For example, applications include the detection of underground pipes
and
cables, detection of buried landmines and bombs, the delineation of ore
bodies, the
detection of aquifers, road evaluation, and hazardous waste detection. Two
types of
ground penetrating radar exist. For deep applications, borehole radars are
used. For
shallow applications, a surface ground penetrating radar is generally more
suitable.
In rocks that favour radar wave propagation, borehole radars are able to
contribute to the safety of drillers and miners by allowing defects such as
high-pressure
gas-bearing dykes and paleo-stressed faults in the rock volume around the hole
to be
placed under surveillance, mapped, and monitored in order to alert miners to
their
proximity and allow the dangerous ground either to be steered around and
avoided, or
to be broken into at a chosen instant.
A borehole radar typically comprises a relatively powerful transmitter
positioned in a borehole for generating electromagnetic pulses which excite an
antenna
to radiate energy into the surrounding rock or earth. Usually, the transmitted
electromagnetic pulses are characterized by short rise/fall times to obtain
sufficient
frequency spread and resolution in the eventual radar data, and by
sufficiently high
energy levels to overcome attenuation and spreading losses in the surrounding
rock
medium. Transmitted electromagnetic pulses propagate through the rock and/or
reflect
off geological features such as interfaces between rock media having differing
electromagnetic properties.
REPLACEMENT PAGE
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- The
receiver must be sufficiently sensitive to detect signals which have suffered
attenuation and/or reflection losses. Borehole radar systems Usually employ a
bi-static
configuration, with the transmitter and receiver deployed as two completely
separate
units (probes). The physical distance between the two probes is increased
until
adequate isolation between the receiver and the transmitted pulse is achieved.
Signal
synchronization is often achieved by use of an optic fibre between the two
probes. The
closest discernible target is determined by the duration of' saturation (if
present) in the
receiver, and'whether the resultant oblique signal path is within the
radiation pattern of
the transmitter and receiver antennas. Bi-static systems are awkward to deploy
in "
constrained spaces, for example mining stope faces. The optic fibre link is
also
susceptible to damage in mining and other industrial environments. Further,
the hi
static deployment necessitates use of a first antenna for transmitting and a
second
= antenna for receiving, introducing the likelihood of mismatched
characteristics between
the first and second antennae, which may result in performance degradation.
Any discussion of documents, acts, materials, devices, articles or the like
which
has been included in the present specification is solely for the purpose of
providing a
context for the present invention. It is not to be taken as an admission that
any or all of
these matters form part of the prior art base or were common general knowledge
in the
field relevant to the present invention as it existed before the priority date
of each claim
of this application.
Throughout this specification the word "comprise", or variations such as
"comprises" or "comprising", will be understood to imply the inclusion of ,a
stated
element, integer or step, or group of elements, integers or steps, but not the
exclusion of
= any other element, integer or step, or group of elements, integers or
steps.
= Summary of the Invention
According to a first aspect, the present invention provides a ground
penetrating
radar comprising:
" a transmitter for generating electromagnetic transmissions for ground
penetration;
a receiver for receiving reflected electromagnetic signals; and
an antenna;
wherein at least a portion of the antenna is used by the transmitter for
transmitting and by the receiver for receiving.
According to a second aspect the present invention provides a method of
constructing a ground penetrating radar comprising:
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providing a transmitter for generating electromagnetic transmissions for
ground
penetration;
providing a receiver for receiving reflected electromagnetic signals; and
providing an antenna;
wherein at least a portion of the antenna is used by the= transmitter for
transmitting and by the receiver for receiving.
According to a third aspect the present invention provides a method of ground
imaging using ground penetrating radar, comprising:
=
transmitting an electromagnetic ground penetrating signal using at least one
transmit antenna portion; and
receiving reflected electromagnetic signals using at least one receive antenna
portion;
wherein at least one antenna portion is both a transmit antenna portion and a
receive antenna portion.
Preferred embodiments of the invention may provide a ground penetrating radar
in accordance with the first aspect of the invention and wherein the antenna
is damped
to minimise resonance between an antenna portion used exclusively by the
transmitter
and an antenna portion used exclusively by the receiver via the portion of the
antenna
used by the transmitter and by the receiver. Such damping is advantageous in
maintaining the broad bandwidth both of the signal launched by the transmitter
and of
the radar echoes sensed by the receiver, and in providing some level of
shielding of the
receiver from powerful transmitted signals or pulses_
Further embodiments of the invention may provide a ground penetrating radar in
accordance with the first aspect of the invention, wherein the antenna is a
dipole
antenna having two transmitting elements. In further such embodiments, at
least one of
the transmitting elements may be used by both the transmitter and the receiver
Further embodiments of the invention may provide a ground penetrating radar in
accordance with the first aspect of the invention, wherein a counter electrode
of the
antenna is used by both the transmitter and the receiver.
The transmitter and receiver may share the whole antenna. In such
embodiments, the ground penetrating radar preferably further comprises a
transmit/receive switch for isolating the receiver from the antenna during
transmissions
by the transmitter, and for passing post-transmission received signals to the
receiver. In
preferred such embodiments, the radar may comprise a symmetrical dipole
antenna
wholly shared by the transmitter and the receiver, and wherein the dipole
antenna
functions as a housing for both the transmitter and receiver. In such
embodiments, first
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and second elements of the dipole antenna may be connected by a low pass
filter
allowing DC power flow between the first and second elements and allowing
radar
frequency AC voltages to develop between the first and second elements. Such
embodiments allow a DC power supply of the radar to be provided via the radar
housing. The low pass filter may comprise a RF choke.
In alternate embodiments of the invention, the transmitter and receiver may
share only a portion of the antenna. For example, the transmitter and receiver
may
sharc a common counter electrode. In such embodiments, the transmitter may
employ
a transmit antenna element, and the receiver may employ a receive antenna
element.
The transmit antenna element may be positioned remote from the receive antenna
element in the radar, to provide some isolation of the receiver from
transmitted signals.
For example, in a longitudinal borehole radar, the transmit antenna element
may extend
from the radar in a first direction along a longitudinal axis of the radar,
while the
receive antenna element may extend from the radar in a second direction
opposite to
the first direction along the longitudinal axis of the radar. Alternatively,
at least one of
the transmit antenna element and the receive antenna element may be
resistively loaded
to provide broadband antenna capability and to provide resistive isolation of
the
receiver, and in such embodiments the transmit antenna element may be
positioned
proximal to the receive antenna element. For example in a longitudinal
borehole radar,
the transmit antenna element and the receive antenna element may each extend
from
the radar in a common direction parallel to the longitudinal axis of the
radar.
In particularly preferred embodiments of the invention, at least one portion
of
the antenna is interchangeable. For example, the transmit element and receive
element
may be interchangeable to allow site specific optimisation of antenna
frequency and/or
resistive loading.
By providing a ground penetrating radar in which at least part of an antenna
is
used by both the transmitter and the receiver, embodiments of the present
invention
may provide a ground penetrating radar constructed as a single device_ Such
embodiments provide for easy deployment of a single device to a desired
position,
without requiring separate deployment of a transmitter and receiver to desired
positions. Such simplified deployment may further lead to cost savings.
Preferably,
the single device ground penetrating radar of such embodiments is provided in
a
housing suitable for insertion into mining drill holes, thus providing a
borehole ground
penetrating radar. For instance, the housing of such a ground penetrating
radar may
have a diameter or largest cross-sectional dimension of no greater than
substantially
32mm to enable deployment of such a borehole radar in 47rrim drill holes.
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Further, in embodiments of the present invention providing a single device
ground penetrating radar, the need for an optical fibre for synchronisation of
a receiver
relative to a transmitter may be obviated. Additionally, signal processing
requirements
and associated costs may be minimised in embodiments of the present invention
in
5 which the physical offset or spacing between the transmitter and receiver is
small. If
this offset is small compared to both the radiated principal wavelength and
the distance
to a typical target, the radar is known as a short offset system; if the
distance becomes
substantially zero, the radar is said to be "monostatie". The term
'monostatic' is used
herein to refer to a radar in which transmission and receiving points are
separated by
less than or substantially equal to a wavelength of a transmission centre
frequency of
= the radar. For example the transmission and receiving portions of the
ground
penetrating radar may be separated by a half wavelength measured at
substantially the
centre of the frequency band transmitted by the radar. By sharing at least a
portion of
the antenna for use by both the transmitter and the receiver, embodiments of
the present
invention may provide for such a monostatic borehole radar.
By providing a ground penetrating radar in which at least a portion of the
antenna is used by the transmitter for transmitting and by the receiver for
receiving,
= embodiments of the present invention may provide for a monostatic
borehole radar
having a length significantly less than the length of a bi-static borehole
radar. In
preferred embodiments, the length of such monostatic borehole radars is
sufficiently
small to enable synthetic aperture radar measurements to be obtained within a
borehole
drilled as a blast hole- Such blast holes are typically so shallow as to
prevent synthetic
aperture radar measurements by use of a relatively lengthy bi-static borehole
radar.
Still further, it has been realised that a large part of the total lifetime
cost of a
ground penetrating radar is determined by the antenna(e) and by the mechanical
= skeleton or housing. Accordingly, embodiments of the present invention
may enable
= reduced construction costs and improved durability in the field, by
enabling the
transmitter and receiver to share all or even a part of the same physical
housing and
some or all of an antenna.
In embodiments of the invention in which the radar is a borehole radar with a
= single housing accommodating both the transmitter and receiver, a part of
the housing
of the borehole radar may be conductive and may function as a counter
electrode for
both the transmitter and the receiver modules. Where the borehole radar is
deployed
inside a conductive drill string within a borehole, the borehole radar housing
may
further be electrically connected to the conductive drill string, such that
the drill string
itself is employed as a counter electrode for both the transmitter and the
receiver. Tn
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such embodiments, the borehole radar should be configured to have a second
antenna
element operable to radiate electromagnetic energy into and/or receive energy
from the
medium surrounding the drill string. For example, the second antenna element
may
protrude from the drill string, for example the second antenna element may
protrude
from a distal end of the drill string. Alternatively the second antenna
element may be
positioned within a portion of the drill string which is substantially
electromagnetically
transparent. The substantially electromagnetically transparent drill string
portion may
comprise an open slot in the drill string allowing EN propagation to and from
the
second antenna element, or may comprise a non-metallic substantially
electromagnetically transparent drill string portion enclosing the second
antenna
element.
It will be appreciated that in accordance with embodiments of the present
= invention, the ground penetrating radar may be of the surface or borehole
type. In
= preferred embodiments of the invention, transmission and reception do not
occur
simultaneously.
In preferred embodiments of the invention, a transmit/receive (T/R) switch is
provided in order to further facilitate the transmitter and the receiver
wholly or partially
sharing a single antenna. Embodiments in which the transmitter and the
receiver
wholly share a single antenna may be advantageous in ensuring near-identical
transmitting and receiving beam shape, and may further avoid the cost and
increased
physical size associated with providing a partially shared antenna or two
wholly
separate antennas for transmission and reception. The T/R-switch may be used
in
conjunction with a parallel connection of the transmitter, receiver and
antenna, such
that the T/R-switch may in the transmit state disconnect the terminals of the
receiver, or
in the receive state may disconnect the terminals of the transmitter from the
parallel
connection.
Alternatively, in preferred embodiments of the invention, the T/R switch may
operate to shunt transmitter current past the receiver into at least one
element of the
antenna during transmission, and may operate to shunt a received signal past
the
transmitter into the receiver during the reception interval that follows
transmission.
Such embodiments of the invention may thus further comprise a transmit/receive
(T/R)
switch comprising:
a first switch terminal for connection to a first terminal of the at least one
antenna;
a second switch terminal for connection to a first terminal of the
transmitter; and
switch receiver terminals for connection to the receiver;
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wherein the T/R switch is operable to implement a transmit state by connecting
transmit signals from the second switch terminal to the first switch terminal,
and by
isolating the s. witch receiver terminals;
wherein the T/R switch is operable to implement a receive state by causing
short
circuiting of
transmitter connected to the second switch terminal such that signals
from a second terminal of the antenna may be received at the second switch
terminal
via the short.eircuited transmitter; and
wherein in the receive state the T/R switch is operable to pass signals
received at
the first switch terminal and the second switch terminal to the receiver
terminals.
'En fuuther preferred embodiments, the switch receiver terminals comprise a
first
switch receiver terminal and a second switch receiver terminal. In such
embodiments
= of the invention, switching between the transmit state and the receive
state is preferably
balanced switching, such that switching transients appearing at the first
switch receiver
terminal are substantially equal to switching transients appearing at the
second switch
receiver terminal. Such embodiments enable common mode rejection of such
switching transients in the receiver, thus providing for sensing of received
signals to be
achieved prior to settling of switching transients, and thus enabling short
two way
propagation time signals to be sensed.
In preferred embodiments of the invention, a controllable connection between
the first switch terminal and the second switch terminal is provided by a
first switch
= element and a second switch element in series and having a ground
connection between
= the first switch element and the second switch element. Where the first
switch terminal
and second switch terminal are matched, such embodiments provide for balanced
switching of the controllable connection between the first switch terminal and
the
second switch terminal.
To further provide balanced switching, a controllable connection between the
first switch terminal and the first switch receiver terminal is preferably
provided by a
third switch element, and a controllable connection between the second switch
terminal
and the second switch receiver terminal is preferably provided by a fourth
switch
element, wherein the third switch element and the fourth switch element are
matched.
In preferred embodiments, common mode switching transients may be removed
by use of a transformer, or by use of a differential amplifier.
Preferably, the transmitter comprises an N-channel metal oxide semiconductor
(NMOS) transistor, operable to produce transmit signals in the transmit state,
and
presenting a low impedance when the drain-source voltage is small, in the
receive state.
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It has been realised that, contrary to conventional wisdom, platinum reefs
such
as the UO2 platinum deposit in South Africa are transparent or translucent to
ground
penetrating radar.
Accordingly, in a fourth aspect of the present invention, there is provided a
method o r radar imaging in the vicinity of a platinum deposit, comprising:
applying a ground penetrating electromagnetic signal; and
receiving returned electromagnetic signals.
Embodiments of the fourth aspect or the invention may be particularly
advantageous in determining a thickness or volume of a platinum deposit for
assisting
in determining the economic or practical viability of mining that deposit.
Embodiments
of the fourth aspect of the invention may further be advantageous in imaging
the rock
= volume surrounding the platinum reef during mining, and particularly the
rock volume
on a distal side of the deposit from the radar imaging apparatus.
For example, the UG2 platinum deposit is overlaid by chromatite layers known
as stringers having low tensile strength. Where those stringers are less than
around 3m
above the platinum deposit, there is a high risk of the stringers collapsing
into the mine
void during or following removal of the platinum, with attendant implications
for
employees and mining equipment in the mine. Even when the presence of
stringers is
determined and roof bolts are placed in order to prevent collapse, the length
of such
bolts must be sufficient that the bolts pass through the stringer layers and
are secured
into high strength rock above the stringer layers. Determining a suitable
length for the
roof bolts can be difficult and is time sensitive. Accordingly, embodiments of
the
fourth aspect of the present invention may be particularly advantageous in
imaging the
stringer layers in order to determine the proximity of the stringer layers to
the roof of
the mine void, and in determining the thickness of the stringer layers and
thus a suitable
length for roof bolts.
In preferred embodiments of the invention, the method of the fourth aspect of
the invention is applied by use of a ground penetrating radar in accordance
with the
first aspect of the invention. In such embodiments, the single device borehole
radar is
preferably adapted for insertion into blast holes drilled for breaking up the
platinum
reef, prior to the insertion of explosives into the blast holes. In
particularly preferred
such embodiments, where a drill array is used for drilling an array of blast
holes, a
plurality of single device borehole radars are preferably mounted for
simultaneous
insertion into at least a subset of the array of blast holes.
Brief Description of the Drawings
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Examples of the invention will now be described with reference to the
accompanying drawings in which:
Figure 1 illustrates a symmetric monostatic ground penetrating radar; =
Figure 2 illustrates an asymmetric monostatic ground penetrating radar:
Figure 3 illustrates an asymmetric monostatic ground penetrating radar housed
in a wire-lining core barrel;
Figure 4 illustrates a monostatic ground penetrating radar with parallel
mounted
resistively loaded transmit and receive antennas;
Figures 5a and 5b are schematics of a borehole radar transceiver in accordance
with an embodiment of the invention;
Figure 6 is a schematic of a borehole radar transceiver in accordance with a
second embodiment of the invention; and
Figure 7 is a circuit diagram ofthe borehole radar transceiver of Figure 6.
Detailed Description of the Preferred Embodiments
Most surface ground penetrating radars (GPR.$) illuminate only shallow
surticial
layers. Near-surface attenuation usually screens deeper targets; there is room
enough
on the surface to make separate transMitting and receiving antennas for
standard GPR.
Bistatic surface GPRs are therefore the norm. Borehole radars (BHRs), in
contrast,
rarely work in the attenuating near-surface. Most BHR are run in relatively
low loss
hard rock, and targets in that rock can be far enough away for a broadband
transmit-
receive switch to have had time to settle before echoes of interest arrive.
FIG I shows an electrically symmetric form of a ground penetrating borehole
radar 10 in accordance with a, first embodiment of the invention. BHR 10
comprises
antenna elements 11 and 12 of a single dipole antenna used for both
transmitting and
receiving. The electrically symmetric form of radar 10 provides a radiation
pattern
which is symmetrical about a plane that lies normal to the axis of the
borehole.
A battery array 18 is mounted within a first half of housing 13, while
transceiver
electronics 14 are mounted in a second half of housing 13, with the two
mechanically
similar halves of the housing 13 being separated by a ferrite bead RF choke
15,
illustrated in greater detail in the exploded portion of Figure 1.
The RF choke 15 allows DC power to flow from the battery array 18 to the
transceiver electronics 14 , while resisting the flow of AC power between the
battery
and transceiver compartments of the housing 13. AC voltage develops across the
choke, thus enabling the antenna elements 11, 12 to be electrically excited to
transmit
and receive radar signals. BHR 10 further comprises a transmit/receive switch
16, for
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example of the type set out in Figures 5 to 7, to isolate the receiver
electronics from
high power transmission signals during a transmit stage, and to pass signals
received by
antenna elements 11, 12 during a receive stage to the receiver electronics.
FIG 2 shows an asymmetric borehole radar 20 in accordance with a second
5 embodiment of the invention. BHR 20 comprises an antenna element 21 for
transmitting, and an antenna element 22 for receiving. BHR 20 further
comprises an
electrically Conductive housing 23 which connects the antenna elements 21, 22
and
serves as a counter electrode to each antenna element 21, 22.
Transmitter electronics 24 are positioned proximal to transmit antenna element
10 21, while transmit/receive switch 26 and accompanying receiver
electronics are located
at an opposite end of conducting housing 23, proximal to receive antenna
element 22_
Housing 23 further houses batteries 28.
By providing a BHR 20 in which only the counter electrode is used ler both
transmitting And receiving and having a transmit antenna element 21 distinct
from
receive antenna element 22, and further by physically separating the
transmitter
electronics 24 and transmit/receive switch and receiver electronics 26, the
BHR 20
reduces direct coupling between transmitter and receiver. Where the transmit
electronics are spaced apart from the switch/receiver electronics by around
1.5m,
coupling may be reduced by perhaps 10dB, depending on the physical and
electrical
configuration of the electrical path between the transmitter electronics 24
and
switch/receiver electronics 26 . Such a reduction in coupling may be
particularly
valuable in easing the isolation burden placed on a transmit/receive switch,
and/or may
enable the use of transmissions of a correspondingly greater magnitude (eg
10dB
greater magnitude) to improve the signal-to-noise (SNR) ratio and/or range of
the
borehole radar 20. The transmit/receive switch, suitably timed, further
assists in
isolating the: receiver during transmissions by transmitter 24. Asymmetric
antenna 20
may provide a radiation pattern having radiation lobes oriented in desired
directions.
FIG 3 shows an asymmetric borehole radar 30 in accordance with a third
embodiment of the present invention, suitable for housing in a standard wire-
lining core
barrel. In BHR 30, the transmitter, receiver and associated controller
electronics are
mounted within a conductive housing 33 co-located proximal to the end of. a
conducting tube 39, such as a drill string. Again, BHR 30 includes a battery
array 38,
and antenna element 31, being resistively loaded for broadband performance and
housed in a protective casing 32 which is substantially electromagnetically
transparent.
Housing 33 and tube 39 together function as a counter electrode relative to
antenna element 31. Radar signals are developed between the counter electrode,
and
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loaded, damped antenna 31 which protrudes from the end of the tube 39. Antenna
element is used for both transmitting and receiving, with a transmit/receive
switch The
configuration of BHR 30 is advantageous in that the tube 39 provides physical
protection for all of the BHR 30 apart from the antenna 31. Antenna 31 may be
relatively inexpensive and adapted for regular replacement, and/or housed in a
sturdy
and substantially electromagnetically transparent protective casing. -
Figure 4 illustrates a BHR 40 in accordance with a fourth embodiment of the
present invention. The receiver antenna element 41 and transmitter antenna
element 42
lie side by side in close proximity, and share a counter electrode 43 which
also serves
as a housing for the radar electronics. Each antenna element 41, 42 has
distributed
resistors along the length of the antenna element for damped broadband
performance.
Advantageously, such resistors also form a potential divider, which limits the
power
that can be delivered directly from the transmitter into the receiver. This
eases the
burden carried by the receiver, or by a transmit-receive switch at the
receiver front end.
In the present embodiment, the resistors distributed in series along each
antenna
element 41, 42 arc selected to have increasing resistance with distance from
the
transmitter/receiver electronics, with the sum of all the resistors of each
antenna
coming to substantially 5k.Q. In considering the amount of isolation of the
receiver
from transmitted pulses afforded by this configuration, we consider the
circumstance in
which a lkV transmitted pulse is generated, and arcs from the transmit antenna
element
42 to the receiver element 41 at the first resistor of each antenna element.
Assuming
the first resistor of each antenna element has a value of 5on, and that the
receiver has a
50S-1 input, then only about 330V is generated at the receiver, thus providing
around
10dB isolation between transmitter and receiver in this circumstance.
Symmetric radars of the type shown in Figure 1 are simpler to analyse than
asymmetric radars of the type shown in Figure 2, with which they may share the
advantage df interchangeable antennas to match site-specific ranges and
attenuation
conditions, by screwing these different antennas onto the ¨1 meter long
central section.
Asymmetric BHR such as the BHR of Figure 2 reduce stress on the T/R switch,
and
enable higher powered transmissions, with consequent rise in SNR.
Monostatic radars are shorter than the conventional bistatic equivalents. The
short offset in a monostatic radar makes it possible to create viable
synthetic apertures
in blast holes, which might be only three or four radar wavelengths long.
Robust
monostatic radars are easier to build than their bistatic equivalents.
The present invention may incorporate a transmit/receive switch (T/R switch)
of
the type set out in Figures 5 to 7, for providing high isolation between the
transmitter
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and receiver, particularly in broadband applications where the delay between a
transmitted signal and detection of a received signal is comparable to the
duration of
the transmitted signal.
Figures 5a and 5b are schematics of a borehole radar transceiver 100 in
accordance with an embodiment of the invention. Transceiver 100 comprises an
antenna 105, transmitter 110, transmit/receive (T/R) switch 120, and a
receiver 130. A
first antenna terminal 106 is connected to a first switch terminal 121, and a
second
antenna terminal 107 is connected to a second transmitter terminal 111. A
first
transmitter terminal 112 is connected to a second switch terminal 122. First
switch
receiver terminal 123 and second switch receiver terminal 124 are connected to
receiver 130.
Figure 5a illustrates transceiver 100 in a transmit state. in which second
switch
terminal 122 is connected to first switch terminal 121 =due to switching means
125
being closed. Switches 126 and 127 are open thus isolating switch receiver
terminals
123 and 124, and thus isolating receiver 130 in the transmit state. As
receiver 130
would typically comprise high gain pre-amplifiers, isolation is important
during the
production of high power signals by transmitter 110. The closing of switching
means
125 during the transmit state allows signals produced by transmitter 110 to be
transmitted by antenna 105.
Figure 5b illustrates transceiver 100 in a receive state, wherein
electromagnetic
signals detected by antenna 305 are passed to receiver 130. Transmitter 110
has been
short circuited such that signals from second antenna terminal 107 are passed
to second
switch terminal 122. Opening of switching means 125 and closing of switching
means
126 and switching means 127 permits received signals to pass from first switch
terminal 121 to first receiver terminal 123, and from second switch terminal
122 to
second receiver terminal 124. This, in the receive state, transceiver 100
passes received
signals to receiver 130.
Figure 6 is a schematic of a borehole radar transceiver 200 in accordance with
a
second embodiment of the invention. The antenna 210 has a first terminal Al
and a
second terminal A2, transmitter 220 has a first terminal TX1 and a second
terminal
TX2 and T/R-svvitch has two input terminals TR1 and TR2. Al is connected to
TR1,
TR2 is connected to TX2, and TX1 is connected to A2. The T/R-switch is also
connected to the input terminals RX1 and RX2 of the receiver 240. The T/R-
switch
has a pair of identical shunt switches PI and P2 and a pair of identical
series switches
Si and S2. P1 is connected between TR1 and ground, while P2 is connected
between
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TR2 and ground. SI is connected between TR.1 and RX1, while S2 is connected
between TR2 and RX2.
When, the transceiver 200 is in a transmit state, the shunt switches P1 and P2
are
on and thus in a low impedance state, and the series switches Si and S2 are
off, in a
high impedance state_ This effectively connects TX2 to Al and disconnects RX1
and
RX2 from TX2 and Al respectively. The transmitter 220 consequently drives the
antenna 210 directly and the receiver 240 is isolated in two stages.'
In the receive state, shunt switches P1 and P2 are off, in a high impedance
state,
and the seriei switches S1 and S2 are on, in a low impedance state. The
transmitter 220
is in a low ItF impedance state so that A2 appears to be connected to TR2.
Thus, RX1
is effectively connected to Al and RX2 is effectively connected to A2. The
antenna
210 consequently drives the receiver 240 directly with minimum loss in the
transmitter
240 and T/R-switch.
Switching the switches P1, P2, Si and S2 between low and high impedance
states introduces switching transients into the signal path. Such switching
transients
have sufficient amplitude to cause prolonged saturation of the input
amplifiers of
receiver 240.,if applied directly to the receiver's input.
Further, it is desirable for the T/R-switch to be able to switch the
transceiver 200
from the transmit state to the receive state in a time comparable to the
duration of the
signal transmitted and received on the antenna 210, in order for the receiver
240 to
discern close-in targets. This causes the frequency spectrum of the switching
transients
to overlap that of the signal received on the antenna 210. The signal received
on the
antenna can consequently not be separated from the switching transient by
frequency
domain filtering.
Accardingly, in the present exemplary embodiment, shunt switch P1 is operated
so that the transient it creates on TR1 relative to ground is substantially
identical to the
transient created by shunt switch P2 on TR2 relative to ground. Similarly,
series switch
Si is operated so that the transient it creates between TR1 and RX1 is
substantially
identical to the transient created by series switch S2 between TR2 and RX2.
Such
switching causes the transient observed on RX1 relative to ground to be
substantially
identical to that observed on RX2 relative to ground. The large switching
transient is a
common mode event and consequently can not saturate the receiver 240 which
detects
the differential mode signal between RX1 and RX2.
Figure 7 is a circuit diagram of the borehole radar transceiver of Figure 2.
The
transmitter 220 comprises an N-channel metal oxide semiconductor (NM(DS)
transistor
QTX connected to a DC power supply VCC1 through resistor RTX. A control
voltage
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14
is applied between the gate and source of qrx. The drain of QTX is at a
voltage level
close to that of VCC1 in the steady state condition, where the transmitter's
control
voltage is zero.
The gate voltage of QTX rises rapidly relative to the source voltage during
transmit mode. This causes QTX to create a sharp falling voltage transient
between
terminals TX1 and TX2 (QTX drain-source voltage). The equivalent drain-source
impedance of QTX is then very low and remains in this state during the receive
mode,
which commences after the sharp falling transient has been generated. The
transmitter
220 is allowed to recover to its steady state condition upon completion of the
receive
state.
The second terminal TR2 of T/R switch 230 is connected to TX2 of the
transmitter, and the first input terminal TR1 of T/R switch 230 is connected
to an
antenna terminal Al. Transformer TF1 allows the T/R-switch 230 and transmitter
220
to have a common ground, even though the signal monitored by the T/R.-switch
230 is
superimposed on the drain voltage of transistor QTX.
The two shunt switches of Figure 6 are realized by two identical NMOS
transistors Q1 and Q2, with their drains connected to a second, common DC
power,
supply VCC2 through two identical resistors RI and R2, respectively. The
sources of
Q1 and Q2 are connected to the ground of the T/R-switch 230 and their drains
are
connected to nodes TR1A and TR2A, respectively. The single control voltage TR-
CTRL of the T/R-switch 230 is applied to the gates of both Ql and Q2.
Two identical Schottky diodes D1 and D2 and two identical resistors R3 and R4
are used to implement the series switches Si and S2 of Figure 2. The anodes of
DI and
D2 are connected to nodes TR1A and TR2A, respectively. The cathodes of DI and
D2
are in turn connected to a third, common DC power supply VCC3 through R3 and
R4,
respectively. The node at the cathode of D1 is TR1B and at the cathode of D2
is
TR2B. The voltage level of VCC3 is slightly higher than the anticipated
voltage levels
associated with leakage from the transmitted pulse through to TR1A and TR1B,
and is
significantly lower than the voltage of VCC2.
A positive voltage higher than the gate-source threshold voltage of the NMOS
transistors Q1 and Q2 is applied to the control voltage input TR-CTRL of the
T/R-
switch 230 during the transmit state. This causes Q1 and Q2 to enter a low
drain-
source impedance state and the voltages on nodes TR1A and TR2A (being the
drain
voltages of Q1 and Q2) will drop substantially simultaneously to a value close
to zero,
below VCC3. These voltage drops cause Dl and D2 to substantially
simultaneously
become reverse biased, and thus present a high impedance. This combination of
the
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low impedance between TR1A and TR2A, the high impedance between TR 1 A and
TRIB and the high impedance between TR2A and TR28 isolates the receiver 240
from
the transmitted pulse. The impedance seen between TR1 and TR2 is close to zero
so
that TX2 is in effect directly connected to Al. TX1 is already hard wired to
A2. The
5 transient generated by the transmitter 220 between TX1 and TX2 will
therefore be
radiated on the antenna 210.
The control voltage TR-CTRL of the T/R-switch 230 is reduced to zero soon
after the transmitter 220 radiates the voltage transient on the antenna 210,
to initiate the
receive state. This causes the drain-source impedance of Q1 and Q2 to rise and
the
10 voltages on TR1A and TR2A to recover concurrently to a value slightly
higher than
VCC3, D1 'and D2 simultaneously become conducting at this point and enter a
low
impedance state. This causes TR1A to rise further to a bias point determined
by VCC2,
VCC3, R1, R3 and the forward voltage of Dl. TR2A rises to substantially the
same
bias point, set by VCC2, VCC3, R2, R4 and the forward voltage of D2. The
resultant
15 high impedance between TR1A and TR2A, the low impedance between TR1A and
TR113 and the low impedance between TR2A and TR2B allows a differential signal
to
pass through with minimum loss from the input TR1, TR2 to the output RX1. RX2
of
the T/R-switch 230.
The low output impedance of the NMOS transistor QTX of the transmitter 220
during the receive state effectively connects TR2 to A2. TR1 is already hard
wired to
the Al. The signal received on the antenna is therefore passed through to the
receiver
with minimum loss.
TR1A and TR1B and between TR2A and TR213. The waveform observed on TRIB
relative to the ground is therefore substantially identical to that observed
on TR213
relative to the ground, and thus switching transients are controlled to be a
common
mode signal- As TF2 can only couple a differential signal from its primary
winding to
its secondary winding, the comrnon mode switching transients arc rejected
whereas he
differential signal applied between TR1 and TR2 passes through TF2.
The parasitic reactance of the devices used in this embodiment limits the
bandwidth and phase response of the T/R-switch's differential path in receive
mode.
The main contributors are the drain-source capacitance of the NMOS transistors
Ql
and Q2 and the leakage inductance of the transformers TF I and TF2. The
inclusion of
Li, Cl and C2 absorb the parasitic reactance of Q I, Q2 and TF2 in a second
order, flat
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phase, band pass filter, to ensure a controlled frequency response. The T/R-
switch then
combines with the receiver to create a path with fixed characteristic
impedance, in the
pass band.
The value of R1 and R2 and the value of R3 and R4 are compromises between
minimum loss in the differential path of the T/R-switch, quick recovery time
of Q1 and
Q2's drain voltages as well as sufficient bias current for DI and D2.
Thus, the exemplary embodiments include a duplexer or transmit/receive switch
(UR-switch) that enables a transmitter and a receiver to use the same antenna.
The
T/R-switch has sufficient instantaneous bandwidth and linearity in its phase
response to
function in a pulse system without compromising the pulse shape, range or
resolution
of the system. The T/R-switch implemented in such embodiments the invention
provides adequate isolation between the transmitted pulse and the receiver
during
transmit-mode to prevent the receiver from saturating for prolonged periods
and the
loss introduced by the T/R-switch between the antenna and the receiver in
receive-
mode, is small enough to avoid substantial reduction in the signal to noise
ratio, which
would otherwise reduce the range of the system.
Such a T/R-switch provides for a ground penetrating radar having the ability
to
switch from maximum isolation in the transmit state to minimum attenuation in
the
receive state in a time similar to the period of the transmitted and received
signals, in
order to enable the receiver to record reflections from close-in targets.
Notably, in the present embodiment the T/R-switch and transmitter are inserted
in front of the receiver, in the receiver path. In a transmit state, the T/R-
switch is
responsible for isolating the receiver from the transmitted signal and all the
energy
from the transmitted pulse is sent from the transmitter to the antenna. In the
receive
state, the transmitter itself presents a RF (radio frequency) short circuit,
thus preventing
echoed or reflected signals collected by the antenna from dissipating in the
transmitter
circuitry. Simultaneously the T/R-switch ensures that the received signals are
delivered
to the receiver with minimal dissipation,
Thus, the T/R switch used in the preferred embodiment relates to transceivers
in
which a transmit/receive switch connects an antenna to the transmitter while
transmitting a signal and connects the same antenna to the receiver while
receiving the
reflected signal. The embodiment in accordance with the invention specifically
relates
to a T/R-switch with wide instantaneous bandwidth (such as a pulsed system),
where
the delay between the radiation of the transmitted signal and the detecting of
the
reflected signal is comparable to the duration of the signal itself, the
performance of
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17
which is relatively unaffected by changes in the antenna impedance due to
changing
conditions in the surrounding medium.
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