Note: Descriptions are shown in the official language in which they were submitted.
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"Method and device for transceiver isolation"
Cross-Reference to Related Applications
The present application claims priority from Australian Provisional Patent
Application No 2004904543 filed on 10 August 2004.
Field of the Invention
The present invention relates to a transmit/receive switch (T/R switch). More
particularly, the invention relates to a T/R switch for a transceiver
comprising a transmitter
and a receiver, for providing high isolation between the transmitter and
receiver,
particularly in broadband applications where the delay between a transmitted
signal and
detection of a target echo is comparable to the duration of the transmission
process.
Background to the Invention
A typical environment in which a transceiver may be used is borehole radar,
and in
particular a monostatic borehole radar using a single-feed point antenna. A
borehole radar
comprises a relatively powerful transmitter (T,), a wide-band antenna, and a
sensitive
receiver (Rx). The transmitter (Tx) is positioned in a borehole and excites
the antenna to
radiate electromagnetic pulses which propagate into the surrounding rock or
earth.
Usually, the transmitted electromagnetic pulses are characterized by short
rise/fall times to
obtain sufficient 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 reflect off
geological features such as interfaces between rock media having differing
electromagnetic properties. Should such geological features be proximal to the
transceiver,
reflected signals may have a short two way propagation time. A receiver (Rx)
is used to
detect such reflected signals. The receiver is usually located in the same
borehole as the
transmitter, and must be sufficiently sensitive to detect signals which have
suffered
attenuation and reflection losses. The receiver detects and amplifies the
received signal,
and records the arrival time relative to the time that the transmitted pulse
was radiated.
The sensitive receiver may saturate or even be damaged if exposed to excessive
input signals, for example if a substantially unattenuated signal is received
directly
from the transmitter. Any degradation of the signal to noise ratio in the
receiver
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reduces the range of the system. The bandwidth of the receiver should at least
equal
the bandwidth of the radiated pulse to preserve resolution and integrity of
the data.
Borehole radar systems often employ a bi-static configuration, with the
transmitter and receiver deployed as two completely separate units (probes) in
the same
borehole. 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 perform well but 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.
T/R-switches are typically 3-port (6-terminal) devices used in conjunction
with
a parallel connection of the transmitter, receiver and antenna, with the
transmitter,
receiver and antenna connected to separate ports, 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.
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 transmit/receive
(T/R) switch comprising:
a first switch terminal for connection to a first terminal of an antenna;
a second switch terminal for connection to a first terminal of a transmitter;
and
switch receiver terminals for connection to a 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 switch receiver terminals;
wherein the T/R switch is operable to implement a receive state by causing
short
circuiting of a transmitter connected to the second switch terminal such that
signals
from a second terminal of an antenna may be received at the second switch
terminal via
the short circuited 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
According to a second aspect the present invention provides a transceiver
comprising:
an antenna for transmitting and receiving signals, having a first antenna
terminal
and a second antenna terminal;
a transmitter for producing transmit signals in a transmit state and operable
to be
short circuited in a receive state, the transmitter having a first transmitter
terminal and a
second transmitter terminal;
a receiver for receiving signals from the antenna; and
a transmit/receive (T/R) switch having a first switch terminal, a second
switch
terminal, and switch receiver terminals,
wherein the first switch terminal is connected to the first antenna terminal,
the
second switch terminal is connected to the first transmitter terminal, the
second
transmitter terminal is connected to the second antenna terminal, and the
switch
receiver terminals are connected to the receiver;
wherein the T/R switch is operable in the transmit state to connect transmit
signals from the second switch terminal to the first switch terminal, and to
isolate the
receiver terminals;
wherein the T/R switch is operable in the receive state to receive signals
from
the second antenna terminal at the second switch terminal via the short
circuited
transmitter; and
wherein the T/R switch is operable in the receive state to pass signals
received at
the first switch terminal and the second switch terminal to the receiver
terminals.
According to a third aspect the present invention provides a method of
operating
a T/R switch, comprising:
in a transmit state connecting transmit signals received at a second switch
terminal to a first switch terminal, and isolating switch receiver terminals;
and
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in a receive state causing short circuiting of a transmitter connected to the
second switch terminal, and passing received signals from the first switch
terminal and
the second switch terminal to the receiver terminals.
According to a fourth aspect the present invention provides a method of
operating a transceiver, comprising:
in a transmit state, connecting transmit signals of a transmitter from a
second
switch terminal to a first switch terminal, and isolating switch receiver
terminals; and
in a receive state, short circuiting the transmitter, passing a received
signal from
the antenna to the first switch terminal and via the short circuited
transmitter to the
second switch terminal, and passing the received signal from the first and
second
switch terminals to the receiver terminals.
According to a fifth aspect the present invention provides a computer program
for operating a T/R switch; comprising:
code for connecting transmit signals received at a second switch terminal to a
first switch terminal in a transmit state;
code for isolating switch receiver terminals in the transmit state;
code for causing short circuiting of a transmitter connected to the second
switch
terminal in a receive state; and
code for passing received signals from the first switch terminal and the
second
switch terminal to the receiver terminals in the receive state.
According to a sixth aspect the present invention provides a computer program
for operating a transceiver, comprising:
code for connecting transmit signals of a transmitter from a second switch
terminal to a first switch terminal in a transmit state;
code for isolating switch receiver terminals in the transmit state;
code for short circuiting the transmitter in a receive state, such that a
received
signal from the antenna is passed to the first switch terminal and via the
short circuited
transmitter to the second switch terminal; and
code for connecting the first and second switch terminals to the receiver
terminals such that the received signal is passed from the first and second
switch
terminals to the receiver terminals.
In preferred embodiments of the invention, 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
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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 immediately after settling of major switching
transients,
and thus enabling short two way propagation time signals, such as reflections
from
5 close targets, to be sensed.
In further preferred embodiments of the invention, a bypass circuit is
incorporated between the first and second switch terminals on the one hand and
the first
and second receiver terminals on the other hand. Preferably, the bypass
circuit is
configured to minimise transmission of undesired low-frequency/long time
constant
signals caused by residual antenna transients to the receiver terminals, while
allowing
transmission of desired high-frequency signals such as target echoes to the
receiver
terminals. In preferred embodiments the bypass circuit minimises transmission
of
differential transient signals by damping residual currents caused by antenna
relaxation.
The bypass circuit may comprise a diplexer, and may be tuneable. Additionally
or
alternatively, the bypass circuit may provide for imposing a delay before the
first and
second switch terminals are connected to the receiver terminals, the delay
being
sufficient to allow post-transmission differential transients to decay below a
receiver
saturation level.
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
substantially removed by use of a transformer, or by use of a differential
amplifier.
The present invention recognises that, in designing a transceiver operable to
both transmit and receive signals, it is desirable to use a T/R switch in
order to enable a
transmitter of the transceiver and a receiver of the transceiver to share a
single antenna,
thus realizing a monostatic system. Use of a single antenna ensures identical
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transmitting and receiving beam shape, and further avoids the cost and
increased physical
size associated with providing two antennas. In applications such as borehole
radar, size
restrictions on the maximum dimensions of the radar device exist, and in such
applications
use of a single antenna can assist in meeting such restrictions. The mining
environment also
promotes the use of the shortest possible system. Thus, in preferred
embodiments, a T/R
switch in accordance with the present invention is used to facilitate
implementation of a
mono static system, where the receiver and the transmitter use the same
antenna.
In these systems, the time associated with switching from the transmit state
to the
receive state is typically significantly longer than the duration of the
transmitted signal. This
allows for frequency domain filtering to separate the switching signal,
created by the T/R-
switch, from the RF-signal to be detected.
Embodiments of the present invention may be used in implementing borehole
radar,
and in particular monostatic borehole radar using a single feedpoint antenna.
The antenna
configuration may comprise a damped antenna, an undamped antenna, a symmetric
antenna,
an asymmetric antenna, a single feedpoint antenna and/or a double feedpoint
antenna. A
mono static antenna as disclosed in International Application No
PCT/AU02/001382 may be
used.
In these and other embodiments, an antenna in conjunction with which the
transmit/receive switch is used is preferably matched to an expected medium of
propagation. For example, where it is anticipated that the borehole radar is
to be used in an
air-filled borehole, the antenna is preferably matched to air. The expected
medium may
additionally or alternatively comprise one or more of water, drilling fluid,
or oil. By
matching the antenna to the expected medium, reflections from the antenna and
thus
residual transients in the transmit switch may be minimised. Further, where
expected use
will occur in more than one medium, such as in a borehole partially filled
with water, the
antenna is preferably more closely matched to the medium of greater
electrostatic
susceptibility then to the medium of lesser electrostatic susceptibility, in
order to reduce the
worst case transients generated by the antenna's mismatch to each medium.
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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A borehole radar embodying the present invention may be deployed in accordance
with the teachings of Australian Provisional Application No. 2004906114.
A borehole radar embodying the present invention may be deployed in accordance
with the teachings of Australian Provisional Application No. 2005900071.
Brief Description of the Drawings
Examples of the invention will now be described with reference to the
accompanying drawings in which:
Figures la and lb are schematics of a borehole radar transceiver in accordance
with
a first embodiment of the invention;
Figure 2 is a schematic of a borehole radar transceiver in accordance with a
second
embodiment of the invention;
Figure 3 is a circuit diagram of the borehole radar transceiver of Figure 2;
Figures 4a and 4b are schematics of a borehole radar transceiver in accordance
with
a third embodiment of the invention;
Figure 5 is a schematic of a borehole radar transceiver in accordance with a
fourth
embodiment of the invention;
Figure 6 is a circuit diagrams of the borehole radar transceiver of Figure 5;
and
Figure 7 is a circuit diagrain of a borehole radar transceiver in accordance
with a
fifth embodiment of the invention.
Detailed Description of the Preferred Embodiments
Figures la and lb are schematics of a mono static borehole radar transceiver
100 in
accordance with a first 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 la 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
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would typically comprise high gain 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 lb illustrates transceiver 100 in a receive state, wherein
electromagnetic
signals detected by antenna 105 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. Thus, in the receive state, transceiver 100
passes
received signals to receiver 130.
Figure 2 is a schematic of a monostatic 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-switch 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 P1 and P2 and a pair of
identical
series switches Si and S2. P1 is connected between TR1 and ground, while P2 is
connected between TR2 and ground. S1 is connected between TR1 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 Al and TX2 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 series switches Si and S2 are on, in a low impedance state. The
transmitter 220
is in a low RF 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 PI, P2, S1 and S2 between low and high impedance
states introduces switching transients into the signal path. Such switching
transients
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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. Consequently the
signal
received on the antenna 210 cannot be separated from the switching transient
by
frequency domain filtering.
Accordingly, 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
thus made to be a substantially common mode event and consequently the
receiver 240
which detects the differential mode signal between RX1 and RX2, is
substantially
protected from saturation.
Figure 3 is a circuit diagram of the monostatic borehole radar transceiver of
Figure 2. The transmitter 220 comprises an N-channel metal oxide semiconductor
(NMOS) transistor QTX connected to a DC power supply VCC1 through resistor
RTX.
A control voltage TX-CTRL is applied between the gate and source of QTX. 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 TX-CTRL 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
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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 P1 and P2 of Figure 2 are realized by two identical
NMOS transistors Q1 and Q2, with their drains connected to nodes TR1A and
TR2A,
5 respectively, and to a second, common DC power supply VCC2 through two
identical
resistors R1 and R2, respectively. The sources of Q1 and Q2 are connected to
the
ground of the T/R-switch 230. The single control voltage TR-CTRL of the T/R-
switch
230 is applied to the gates of both Q1 and Q2.
Two identical Schottky diodes D1 and D2 and two identical resistors R3 and R4
10 are used to implement the series switches Si and S2 of Figure 2. The
anodes of D1 and
D2 are connected to nodes TR1A and TR2A, respectively. The cathodes of D1 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 the node 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 D1 and D2 to substantially
simultaneously
become reverse biased, and thus present a high impedance. This combination of
the
low impedance between TR1A and TR2A, the high impedance between TR1A and
TR1B and the high impedance between TR2A and TR2B 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
transient generated by the transmitter 220 between TX1 and TX2 will therefore
be
applied to and radiated by 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
voltages on TR1A and TR2A to recover concurrently to a value slightly higher
than
VCC3. At this point D1 and D2 become conducting substantially simultaneously
and
enter a low impedance state. This causes the quiescent voltage at TR1A to rise
further
to a bias point determined by VCC2, VCC3, R1, R3 and the forward voltage of
Dl.
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The quiescent voltage at TR2A rises to substantially the same bias point,
determined by
VCC2, VCC3, R2, R4 and the forward voltage of D2. The resultant high impedance
between TR1A and TR2A, the low impedance between TR1A and TR1B 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 Al, A2 is therefore passed through
to the
receiver terminals RX1, RX2 with minimum loss.
The synchronous switching action of Q1 and Q2 causes substantially identical
switching transient waveforms on TR1A and TR2A relative to ground. D1 is
consequently activated in a substantially identical manner as D2 and similar
waveforms
are generated between TR1A and TR1B and between TR2A and TR2B. The waveform
observed on TR1B relative to ground is therefore substantially identical to
that
observed on TR2B relative to ground, and thus switching transients are
controlled to be
a common mode signal. As transformer TF2 can only couple a differential signal
from
its primary winding to its secondary winding, the common mode switching
transients
are rejected whereas the differential signal applied between TR1C and TR2C
passes
through TF2 to terminals RX1 and RX2.
The parasitic reactance of the devices used in this embodiment limits the
bandwidth and phase response of the differential path of the T/R switch 230 in
receive
mode. The main contributors of parasitic reactance are the drain-source
capacitance of
the NMOS transistors Q1 and Q2 and the leakage inductance of the transformers
TF1
and TF2. The inclusion of Li, Cl and C2 compensates for the parasitic
reactance of
Ql, Q2 and TF2 in a second order, flat phase, band pass filter, to ensure a
controlled
frequency response. The T/R-switch 230 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 230, quick recovery
time of the
respective drain voltages of Q1 and Q2 as well as sufficient bias current for
D1 and D2.
While the embodiment of the invention set out in Figure 3 and described in the
preceding operates in a desired manner, particularly when the antenna Al, A2
is in an
air environment, it has been recognised that immersion of antenna Al, A2 in a
high-
susceptibility electrically polarizable medium, such as water, drilling fluid
or oil, can
cause performance degradation. It has further been recognised that this
degradation
arises for the following reasons. Prior to closing of switch QTX, the antenna
Al, A2 is
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charged to a high energy state in preparation for electromagnetic (EM) pulse
radiation.
When QTX closes, the desired EM pulse is promptly radiated by the antenna Al,
A2.
Thereafter the antenna Al, A2 discharges or relaxes electroquasistatically by
means of
a residual current flowing through the primary winding of TF1, between arms Al
and
A2. This residual current induces a secondary current in the low resistance
loop
created by the secondary winding of transformer TF1 and the two switches Q1
and Q2
in their low impedance state. No signal is produced at the output of the TR
switch,
while switches D1 and D2 are open with the TR switch in transmit mode or
isolate
mode.
When switches Q1 and Q2 start opening, the stored energy in the circulating
secondary current excites the high Q parallel LC resonant circuit formed by
the
parasitic capacitances of Q1 and Q2 and the inductor Li. When the switches D1
and
D2 close about 5Ons later a damped differential transient is passed to the
receiver
cascade. It has further been recognised that the damped differential transient
will be
small enough not to saturate the receiver chain, provided the amplitude of the
residual
secondary current has decayed sufficiently before the TR switch 230 leaves the
transmit state to enter the receive state.
Importantly, the differential transient is generated by the antenna. It is
fundamentally dependent on antenna capacitance. Notably, antenna capacitance,
and
thus potential energy stored on the antenna, is significantly increased when
the antenna
is immersed in a high-susceptibility electrically polarizable medium, and the
associated
significant increase in electroquasistatic (EQS) charge relaxation time.
However, it has
been realised that the amplitude of the differential transient may be
substantially
reduced by incorporating a low-frequency bypass circuit in the T/R switch, as
exemplified by the embodiments of Figures 4 to 7.
Figures 4a and 4b are schematics of a mono static borehole radar transceiver
400
in accordance with a first embodiment of the invention. Transceiver 400
comprises an
antenna 405, transmitter 410, transmit/receive (T/R) switch 420, and a
receiver 430. A
first antenna terminal 406 is connected to a first switch terminal 421, and a
second
antenna terminal 407 is connected to a second transmitter terminal 411. A
first
transmitter terminal 412 is connected to a second switch terminal 422. First
switch
receiver terminal 423 and second switch receiver terminal 424 are connected to
receiver 430.
Figure 4a illustrates transceiver 400 in a transmit state, in which second
switch
terminal 422 is connected to first switch terminal 421 via bypass circuit 428
due to
switching means 425 being closed. Switches 426 and 427 are open thus isolating
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switch receiver terminals 423 and 424, and thus isolating receiver 430 in the
transmit state. As
receiver 430 would typically comprise high gain amplifiers, isolation is
important during the
production of high power signals by transmitter 410. The closing of switching
means 425
during the transmit state allows signals produced by transmitter 410 to be
transmitted by
antenna 405. Following such transmission, bypass circuit 428 acts to damp
residual currents
caused by relaxation of antenna 405.
Figure 4b illustrates transceiver 400 in a receive state, wherein
electromagnetic signals
detected by antenna 405 are passed to receiver 430. Transmitter 410 has been
short circuited
such that signals from second antenna terminal 407 are passed to second switch
terminal 422.
Opening of switching means 425, and closing of switching means 426 and
switching means
427, permits received signals to pass from first switch terminal 421 to first
receiver terminal
423, and from second switch terminal 422 to second receiver terminal 424, via
bypass circuit
428. Thus, in the receive state, transceiver 400 passes received signals to
receiver 430.
Figure 5 is a schematic of a monostatic borehole radar transceiver 500 in
accordance
with a second embodiment of the invention. The antenna 510 has a first
terminal A51 and a
second terminal A52, transmitter 520 has a first terminal TX51 and a second
terminal TX52
and T/R-switch has two input terminals TR51 and TR52. A51 is connected to
TR51, TR52 is
connected to TX52, and TX51 is connected to A52. The T/R-switch is also
connected to the
input terminals RX51 and RX52 of the receiver 540. The T/R-switch has a pair
of identical
shunt switches P51 and P52 and a pair of identical series switches S51 and
S52. P51 is
connected between TR51 and ground via bypass circuit 550, while P52 is
connected between
TR52 and ground via bypass circuit 550. S51 is connected between TR51 and RX51
via
bypass circuit 550, while S52 is connected between TR52 and RX52 via bypass
circuit 550.
When the transceiver 500 is in a transmit state, the shunt switches P51 and
P52 are on
and thus in a low impedance state, and the series switches S51 and S52 are
off, in a high
impedance state. This effectively connects TX52 to A51 via bypass circuit 550
and
disconnects RX51 and RX52 from A51 and TX52 respectively. The transmitter 520
consequently drives the antenna 510 directly and the receiver 540 is isolated
in two stages.
Following transmission, bypass circuit 550 acts to damp residual currents
caused by
relaxation of antenna 510, prior to switching the circuit 500 to a receive
state. The bypass circuit
550 imposes resistive damping to dissipate the residual transient, while
having a minimal effect
on insertion loss to the receiver.
In the receive state, shunt switches P51 and P52 are off, in a high impedance
state, and
the series switches S51 and S52 are on, in a low impedance state. The
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transmitter 520 is in a low RF impedance state so that A52 appears to be
connected to
TR52. Thus, RX51 is effectively connected to A51 and RX52 is effectively
connected
to A52. The antenna 510 consequently drives the receiver 540 directly with
minimum
loss in the transmitter 540 and T/R-switch.
Switching the switches P51, P52, S51 and S52 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 540 if applied directly to the receiver's input.
Further, it is desirable for the T/R-switch to be able to switch the
transceiver 500
from the transmit state to the receive state in a time comparable to the
duration of the
signal transmitted and received on the antenna 510, in order for the receiver
540 to
discern close-in targets. This causes the frequency spectrum of the switching
transients
to overlap that of the signal received on the antenna 510. Consequently the
signal
received on the antenna 510 cannot be separated from the switching transient
by
frequency domain filtering.
Accordingly, in the embodiment of Figure 5, shunt switch P51 is operated so
that the transient it creates on TR51 relative to ground is substantially
identical to the
transient created by shunt switch P52 on TR52 relative to ground. Similarly,
series
switch S51 is operated so that the transient it creates between TR51 and RX51
is
substantially identical to the transient created by series switch S52 between
TR52 and
RX52. Such switching causes the transient observed on RX51 relative to ground
to be
substantially identical to that observed on RX52 relative to ground. The large
switching transient is thus made to be a substantially common mode event and
consequently the receiver 540 which detects the differential mode signal
between RX51
and RX52, is substantially protected from saturation.
Figure 6 is a circuit diagram of the monostatic borehole radar transceiver of
Figure 5. The transmitter 620 comprises an N-channel metal oxide semiconductor
(NMOS) transistor QTX6 connected to a DC power supply VCC61 through resistor
RTX6 A control voltage TX-CTRL6 is applied between the gate and source of
QTX6.
The drain of QTX6 is at a voltage level close to that of VCC61 in the steady
state
condition, where the transmitter's control voltage TX-CTRL6 is zero.
The gate voltage of QTX6 rises rapidly relative to the source voltage during
transmit mode. This causes QTX6 to create a sharp falling voltage transient
between
terminals TX61 and TX62 (QTX6 drain-source voltage). The equivalent drain-
source
impedance of QTX6 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
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620 is allowed to recover to its steady state condition upon completion of the
receive
state.
The second terminal TR62 of T/R switch 630 is connected to TX62 of the
transmitter 620, and the first input terminal TR61 of T/R switch 630 is
connected to an
5 antenna terminal A61. Transformer TF61 allows the T/R-switch 630 and
transmitter
620 to have a common ground, even though the signal monitored by the T/R-
switch
630 is superimposed on the drain voltage of transistor QTX6.
The two shunt switches S51, S52 of Figure 5 are realized by two identical
NMOS transistors Q61 and Q62, with their drains connected to nodes TR61A and
10 TR62A, respectively, and to a second, common DC power supply VCC62 through
two
identical resistors R61 and R62, respectively. The sources of Q61 and Q62 are
connected to the ground of the T/R-switch 630. The single control voltage TR-
CTRL6
of the T/R-switch 630 is applied to the gates of both Q61 and Q62.
Two identical PIN diodes D61 and D62 and two identical resistors R63 and R64
15 are used to implement the series switches S61 and S62 of Figure 5.
The anodes of D61
and D62 are connected to nodes TR61A and TR62A, respectively. The cathodes of
D61 and D62 are in turn connected to a third, common DC power supply VCC63
through R63 and R64, respectively. The node at the cathode of D61 is TR61B and
the
node at the cathode of D62 is TR62B. The voltage level of VCC63 is slightly
higher
than the anticipated voltage levels associated with leakage from the
transmitted pulse
through to TR61A and TR61B, and is significantly lower than the voltage of
VCC62.
A positive voltage higher than the gate-source threshold voltage of the NMOS
transistors Q61 and Q62 is applied to the control voltage input TR-CTRL6 of
the T/R-
switch 630 during the transmit state. This causes Q61 and Q62 to enter a low
drain-
source impedance state and the voltages on nodes TR61A and TR62A (being the
drain
voltages of Q61 and Q62) will drop substantially simultaneously to a value
close to
zero, below VCC63. These voltage drops cause D61 and D62 to substantially
simultaneously become reverse biased, and thus present a high impedance. This
combination of the low impedance between TR61A and TR62A, the high impedance
between TR61A and TR61B and the high impedance between TR62A and TR62B
isolates the receiver 640 from the transmitted pulse. The impedance seen
between
TR61 and TR62 is close to zero so that TX62 is in effect directly connected to
A61.
TX61 is already hard wired to A62. The transient generated by the transmitter
620
between TX61 and TX62 will therefore be applied to and radiated by the antenna
610.
The control voltage TR-CTRL6 of the T/R-switch 630 is reduced to zero soon
after the transmitter 620 radiates the voltage transient on the antenna 610,
to initiate the
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receive state. This causes the drain-source impedance of Q61 and Q62 to rise
and the
voltages on TR61A and TR62A to recover concurrently to a value slightly higher
than
VCC63. At this point D61 and D62 become conducting substantially
simultaneously
and enter a low impedance state. This causes the quiescent voltage at TR61A to
rise
further to a bias point determined by VCC62, VCC63, R61, R63 and the forward
voltage of D61. The quiescent voltage at TR62A rises to substantially the same
bias
point, determined by VCC62, VCC63, R62, R64 and the forward voltage of D62.
The
resultant high impedance between TR61A and TR62A, the low impedance between
TR61A and TR61B and the low impedance between TR62A and TR62B allows a
differential signal to pass through with minimum loss from the input TR61,
TR62 to the
output RX61, RX62 of the T/R-switch 630.
The low output impedance of the NMOS transistor QTX6 of the transmitter 620
during the receive state effectively connects TR62 to A62. TR61 is already
hard wired
to A61. The signal received on the antenna A61, A62 is therefore passed
through to the
receiver terminals RX61 and RX62 with minimum loss.
The synchronous switching action of Q61 and Q62 causes substantially identical
switching transient waveforms on TR61A and TR62A relative to ground. D61 is
consequently activated in a substantially identical manner as D62 and similar
waveforms are generated between TR61A and TR61B and between TR62A and
TR62B. The waveform observed on TR61B relative to ground is therefore
substantially identical to that observed on TR62B relative to ground, and thus
switching
transients are controlled to be a common mode signal. As transformer TF62 can
only
couple a differential signal from its primary winding to its secondary
winding, the
common mode switching transients are rejected whereas the differential signal
applied
between TR61C and TR62C passes through TF62 to terminals RX61 and RX62.
The parasitic reactance of the devices used in this embodiment limits the
bandwidth and phase response of the differential path of the T/R switch 630 in
receive
mode. The main contributors of parasitic reactance are the drain-source
capacitance of
the NMOS transistors Q61 and Q62 and the leakage inductance of the
transformers
TF61 and TF62. The inclusion of L61, C61 and C62 compensates for the parasitic
reactance of Q61, Q62 and TF62 in a second order, flat phase, band pass
filter, to
ensure a controlled frequency response. The T/R-switch 630 then combines with
the
receiver to create a path with fixed characteristic impedance, in the pass
band.
The value of R61 and R62 and the value of R63 and R64 are compromises
between minimum loss in the differential path of the T/R-switch 630, quick
recovery
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time of the respective drain voltages of Q61 and Q62 as well as sufficient
bias current
for D61 and D62.
T/R switch 630 further includes a bypass circuit comprising resistors RD62,
RD61, and capacitors CD61, CD62, CD63. Following transmission by antenna A61,
A62, the bypass circuit damps residual currents caused by relaxation of
antenna A61,
A62.
Figure 7 is a circuit diagram of a further embodiment of the present
invention.
The transmitter 720 comprises an N-channel metal oxide semiconductor (NMOS)
transistor QTX7 connected to a DC power supply VCC71 through resistor RTX7 A
control voltage TX-CTRL7 is applied between the gate and source of QTX7. The
drain
of QTX7 is at a voltage level close to that of VCC71 in the steady state
condition,
where the transmitter's control voltage TX-CTRL7 is zero.
The gate voltage of QTX7 rises rapidly relative to the source voltage during
transmit mode. This causes QTX7 to create a sharp falling voltage transient
between
terminals TX71 and TX72 (QTX7 drain-source voltage). The equivalent drain-
source
impedance of QTX7 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
720 is allowed to recover to its steady state condition upon completion of the
receive
state.
The second terminal TR72 of T/R switch 730 is connected to TX72 of the
transmitter 720, and the first input terminal TR71 of T/R switch 730 is
connected to an
antenna terminal A71. Transformer TF71 allows the T/R-switch 730 and
transmitter
720 to have a common ground, even though the signal monitored by the T/R-
switch
730 is superimposed on the drain voltage of transistor QTX7.
Two shunt switches are realized by two identical NMOS transistors Q71 and
Q72, with their drains connected to nodes TR71A and TR72A, respectively, and
to a
second, common DC power supply VCC72 through two identical resistors R75 and
R76, respectively. The sources of Q71 and Q72 are connected to the ground of
the
T/R-switch 730. The single control voltage TR-CTRL7 of the T/R-switch 730 is
applied to the gates of both Q71 and Q72.
Two identical Schottky or PIN diodes D73 and D74 and two identical inductors
L73 and L74 are used to implement series switches. The anodes of D73 and D74
are
connected to nodes TR71A and TR72A, respectively. The cathodes of D73 and D74
are in turn connected to a third, common DC power supply VCC74 through L73 and
L74, respectively. The node at the cathode of D73 is TR71B and the node at the
cathode of D74 is TR72B. The voltage level of VCC74 is slightly higher than
the
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anticipated voltage levels associated with leakage from the transmitted pulse
through to
TR71A and TR71B, and is significantly lower than the voltage of VCC72.
A positive voltage higher than the gate-source threshold voltage of the NMOS
transistors Q71 and Q72 is applied to the control voltage input TR-CTRL7 of
the T/R-
switch 730 during the transmit state. This causes Q71 and Q72 to enter a low
drain-
source impedance state and the voltages on nodes TR71A and TR72A (being the
drain
voltages of Q71 and Q72) will drop substantially simultaneously to a value
close to
zero, below VCC74. These voltage drops cause D73 and D74 to substantially
simultaneously become reverse biased, and thus present a high impedance. This
combination of the low impedance between TR71A and TR72A, the high impedance
between TR71A and TR71B and the high impedance between TR72A and TR72B
isolates the receiver 740 from the transmitted pulse. The impedance seen
between
TR71 and TR72 is close to zero so that TX72 is in effect directly connected to
A71.
TX71 is already hard wired to A72. The transient generated by the transmitter
720
between TX71 and TX72 will therefore be applied to and radiated by the antenna
710.
The control voltage TR-CTRL7 of the T/R-switch 730 is reduced to zero soon
after the transmitter 720 radiates the voltage transient on the antenna 710,
to initiate the
receive state. This causes the drain-source impedance of Q71 and Q72 to rise
and the
voltages on TR71A and TR72A to recover concurrently to a value slightly higher
than
VCC74. At this point D73 and D74 become conducting substantially
simultaneously
and enter a low impedance state. This causes the quiescent voltage at TR71A to
rise
further to a bias point determined by VCC72, VCC74, R75, L73 and the forward
voltage of D73. The quiescent voltage at TR72A rises to substantially the same
bias
point, determined by VCC72, VCC74, R76, L74 and the forward voltage of D74.
The
resultant high impedance between TR71A and TR72A, the low impedance between
TR71A and TR71B and the low impedance between TR72A and TR72B allows a
differential signal to pass through with minimum loss from the input TR71,
TR72 to the
output RX71, RX72 of the T/R-switch 730.
The low output impedance of the NMOS transistor QTX7 of the transmitter 720
during the receive state effectively connects TR72 to A72. TR71 is already
hard wired
to A71. The signal received on the antenna A71, A72 is therefore passed
through to the
receiver terminals RX71 and RX72 with minimum loss.
The synchronous switching action of Q71 and Q72 causes substantially identical
switching transient waveforms on TR71A and TR72A relative to ground. D73 is
consequently activated in a substantially identical manner as D74 and similar
waveforms are generated between TR71A and TR71B and between TR72A and
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TR72B. The waveform observed on TR71B relative to ground is therefore
substantially identical to that observed on TR72B relative to ground, and thus
switching
transients are controlled to be a common mode signal. As transformer TF72 can
only
couple a differential signal between its primary winding and its secondary
winding, the
common mode switching transients are rejected whereas the differential signal
applied
between TR71C and TR72C passes through TF72 to terminals RX71 and RX72.
The parasitic reactance of the devices used in this embodiment limits the
bandwidth and phase response of the differential path of the T/R switch 730 in
receive
mode. The main contributors of parasitic reactance are the drain-source
capacitance of
the NMOS transistors Q71 and Q72 and the leakage inductance of the
transformers
TF71 and TF72.
In contrast to the embodiment of Figure 6, in switch 700 inductor L75 is
positioned on a downstream side of capacitors C73, C74. This location of L75
is
precautionary should there be energy transmitted through switch 700. Further,
inductors L73 and L74 are used in place of resistors. The inclusion of L73,
L74, L75,
C73 and C74 implements a fourth order filter, as opposed to the second order
filter
provided by L61, C61 and C62 in the embodiment of Figure 6, and compensates
for the
parasitic reactance of Q71, Q72 and TF72 to ensure a controlled frequency
response.
The fourth order filter further reduces the amplitude of low frequency
differential
transients. The T/R-switch 730 then combines with the receiver to create a
path with
fixed characteristic impedance, in the pass band.
The value of R75 and R75 and the value of L73 and L74 are compromises
between minimum loss in the differential path of the T/R-switch 730, quick
recovery
time of the respective drain voltages of Q71 and Q72 as well as sufficient
bias current
for D73 and D73.
T/R switch 730 further includes a bypass circuit comprising resistors RD72,
RD71, and capacitors CD71, CD72, CD73. Following transmission by antenna A71,
A72, the bypass circuit damps residual currents caused by relaxation of
antenna A71,
A72.
Thus, the exemplary embodiments relate to a duplexer or transmit/receive
switch (T/R-switch) that enables a transmitter and a receiver to use the same
antenna.
The T/R-switch of these embodiments 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 in accordance
with such
embodiments of the invention provides adequate isolation between the
transmitted
pulse and the receiver during transmit-mode to prevent the receiver from
saturating for
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prolonged periods. Further, 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.
In the present embodiments, the T/R-switch possesses the ability to switch
from
5 maximum isolation in the transmit state to minimum attenuation in the
receive state in a
time comparable to the transmission process, in order to enable the receiver
to record
reflections from close-in targets.
Notably, in the present embodiments 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
10 responsible for isolating the receiver from the transmitted signal such
that the energy
from the transmitted pulse is substantially radiated by the antenna. In the
receive state,
the transmitter itself presents a radio frequency (RF) short circuit, thus
substantially
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
15 are delivered to the receiver with minimal dissipation.
Thus, the T/R switch of the preferred embodiments 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.
20 The described embodiments of the invention specifically relate 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 transmission process. -
The bypass circuit enables the T/R switch to be relatively unaffected by
changes
in the antenna impedance due to changing conditions in the surrounding medium.
The receiver used in conjunction with the transmit receive switch of any of
the
previously described embodiments may comprise an analogue to digital converter
(ADC) to allow digital sampling of a received signal. Preferably, a
sensitivity time
control (STC) and/or automatic gain control (AGC) is provided upstream of the
STC
and/or AGC to match the received analogue signal to the dynamic range of the
ADC.
Use of a soft limiter in the receiver is also preferable due to the
possibility of large
transient signals. A first order high pass filter may be implemented by use of
an
inductor across the input of the STC or AGC, in order to speed recovery of the
receiver
electronics during decay of differential transients caused by antenna
relaxation. Such
differential transients can have substantial low frequency components outside
a
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frequency band of interest, and thus a high pass filter at the input to the
STC or AGC
assists in removing such low frequency components.
While the present embodiments have been described with reference to a ground
penetrating monostatic radar for use in a borehole environment, it is to be
appreciated
that devices embodying the invention may have application elsewhere. For
example, in
borehole radar, perimeter and area surveillance such as radar arrays mounted
on fence
posts or poles using directional antennas such as Hertz parabolic cylinders,
surface
ground penetrating radar, airborne ground penetrating radar, impulse response
measurements, and other pulsed imaging systems, such as MM, time domain
reflectometry, and non-destructive testing.
453580U
