Background of the Invention
1. Field of the Invention
[0001] The present invention generally relates to electronic switches, and more particularly
to electronic control of switchable antenna units utilizing a common signal bus.
2. Description of the Prior Art
[0002] Antenna arrays are used in a variety of applications, such as underground cable location,
radio navigation, and electromagnetic field detection. It is often necessary to route
the signals from multiple antennas to a signal processing network in order to prepare
the signals for data analysis. Where the application permits, it is desirable to utilize
a single signal processing network for the entire antenna array rather than using
multiple networks, i.e., one for each antenna. Because of this, systems have been
devised which sequentially switch the network input from one antenna to another.
[0003] The simplest mechanism for achieving sequential inputs is a manual switch, such as
a single pole-single throw contact. A plurality of such devices may be used in an
antenna array (one for each antenna), or a multicontact switch may be used. A more
convenient alternative is a conventional relay, such as a solenoid coil utilizing
an armature contact. An example of a relay used in conjunction with two antennas is
disclosed in U.S. Patent No. 4,387,340 issued to E. Peterman (and in the divisional
case, U.S. Patent No. 4,520,317).
[0004] For most applications, however, the switching time of such relays is highly unsatisfactory.
An improved, quicker design is illustrated in U.S. Patent No. 4,542,344 issued to
Darilek et al. A simplified version of the electrical configuration of that device
is shown in Figure 1. The prior art structure includes a plurality of loop antennas
1, each connected to one of a plurality of pre-amplifiers
2. The outputs of preamps
2 are connected to a multiplexer
3 which is regulated by a controller
4. Controller
4 selects the antenna or channel to be analyzed at any given time. The selected signal
is then directed from multiplexer
3 to the signal processor circuit
5, which conveys data in a comprehensible form to an output device (display)
6 via controller
4.
[0005] The foregoing devices still possess a significant disadvantage, to wit, the necessity
of providing a pre-amplifier for each antenna. This drawback, which is uneconomical
and redundant, is accentuated by the fact that other electrical components may have
to be provided for each antenna. For example, Figure 3 of U.S. Patent No. 4,295,095
issued to Thomas et al. shows use of a plurality of filters, one for each channel,
in addition to the preamplifiers. It would, therefore, be desirable and advantageous
to devise means for selectively switching the input of a signal processor from among
a plurality of antennas without requiring the redundant use of electrical components
for each antenna channel.
[0006] Accordingly, the primary object of the present invention is to provide means for
selecting a signal from one of a plurality of antennas.
[0007] Another object of the invention is to provide such means having a rapid switching
time.
[0008] Still another object of the invention is to provide switchable antenna units whose
output may be directed to a common signal bus.
[0009] Yet another object of the invention is to provide a switchable antenna unit that
does not require preamplifiers or other signal conditioning components.
[0010] A further object of the invention is to provide such a switchable antenna unit having
sufficient attenuation of the signal when the unit is not selected.
Summary of the Invention
[0011] The foregoing objects are achieved in a switchable antenna unit having a loop antenna
and switching means for selectively enabling and disabling the antenna. The antenna
includes a first winding, one end of which is connected to a common signal bus. The
second end of the first winding is effectively open in the disabled or unselected
state, resulting in signal attenuation. The switching means is responsive to a controller
and, when enabled, causes the second end of the first winding to become connected
to ground, thereby allowing signal transmission to the common bus.
[0012] A second winding is provided as part of the switching means, the second winding being
inductively coupled to the first winding. When the switching means is disabled, the
second winding is effectively shorted and provides improved attenuation of the signal
from the first winding. In the preferred embodiment, the switching means comprises
a NPN transistor connected to the first winding, and a PNP transistor connected to
the second winding, the base of each transistor being connected to the controller.
Brief Description of the Drawings
[0013] The novel features of the invention are set forth in the appended claims. The invention
itself, however, will best be understood by reference to the accompanying drawings,
wherein:
Figure 1 is a block electrical diagram of a typical prior art antenna array utilizing
a plurality of pre-amplifiers.
Figure 2 is a block electrical diagram of the present invention showing the switchable
antenna units connected to a common signal bus.
Figure 3 is a detailed electrical schematic of the preferred embodiment of the switchable
antenna units of the present invention.
Figure 4 is a detailed electrical schematic of an alternative embodiment utilizing
MOSFET's.
Figure 5 is a detailed electrical schematic of an alternative embodiment providing
a differential input to a paired signal bus.
Figure 6 is a detailed electrical schematic of an alternative embodiment in which
the antenna and a capacitor form a bridge circuit.
Description of the Preferred Embodiment
[0014] With reference now to the figures, and in particular with reference to Figure 2,
there is depicted a network
10 for processing signals obtained from multiple antennas. Network
10 includes a plurality of switchable antenna units
12, a preamplifier
14, a controller
16, a signal processor
18, an input device
20, and an output device
22. Details of switchable antenna units
12 are discussed later in conjunction with Figures 3-6. The remaining components of
network
10 are generally described below but, inasmuch as network
10 has several different applications, it will be appreciated that the exact configuration
of these components greatly depends on the particular purpose for which network
10 is being used. Various configurations will readily become apparent to those skilled
in the art.
[0015] Controller
16 may be a microprocessor (including RAM, ROM, etc.), or simply a clock pulse generator,
as shown in U.S. Patent No. 4,295,095 issued to Thomas et al. If controller
16 is a clock pulse generator or other non-programmable component, then input device
20 is unnecessary. Therefore, its inclusion in network
10 is optional. Even if controller
16 is a microprocessor, input device
20 may still be unnecessary, e.g., where the microprocessor is pre-programmed and no
user inputs are required. An example of such an application is shown in U.S. Patent
No. 4,542,344 issued to Darilek et al. If, however, network
10 is designed to require or allow user inputs (for either initial setup parameters
or interactive participation) then input device
20 may conveniently take the form of a keypad. Controller
16 provides control signals for switching antenna units
12 between "on" and "off" states.
[0016] Switchable antenna units
12 are each connected to preamp
14 by a common signal bus
24. preamp
14 is any conventional amplifier, and serves to boost the signal to an intermediate
level for further processing. The signals from antenna units
12 are conveyed directly from preamp
14 to signal processor
18. Signal processor
18 may be as simple as an amplitude detector (see U.S. Patent No. 4,438,389 issued to
A. DeSa), or may include other signal conditioning devices, such as an automatic gain
control, a bandpass filter, a variable gain amplifier (see the Thomas et al. patent),
an analog-to-digital converter (see the Darilek patent), etc., or any combination
thereof. The processed signals are generally conveyed to controller
16 which performs data analysis (e.g., amplitude ratios, phase comparisons, etc.), and
outputs appropriate information to output device
22, typically an alphanumeric display.
[0017] Referring now to Figure 3, the detailed construction of the switchable antenna units
12 is shown. Each switchable antenna unit
12 includes antenna means
26 and switching means
28. In the preferred embodiment, antenna means
26 is a loop-type antenna or induction coil, comprising a first winding
30, which receives ambient electromagnetic signals. In practice, first winding
30 is wrapped around a ferrite core, and a second winding
32 is physically wound over (and thus inductively coupled to) first winding
30. Therefore, the windings may conveniently be referred to as inner winding
30 and outer winding
32. Those skilled in the art will appreciate, however, that windings
30 and
32 could be placed side by side, both wrapped around the same core, or in any other
manner which would inductively couple them together. Due to their physical proximity,
inner and outer windings
32 might together be construed as antenna means
26 but, as noted below, outer winding
32 is actually part of switching means
28.
[0018] Each inner winding
30 has two terminals
34 and
36, terminal
34 (the output of antenna means
26) being connected to common signal bus
24, and terminal
36 being connected to switching means
28. Similarly, each outer winding
32 has two terminals
38 and
40, connected to ground and switching means
28, respectively. Winding
30 provides signal reception to bus
24, and winding
32 serves to attenuate the signal when that particular antenna unit
12 is unselected.
[0019] Switching means
28 is designed to switch antenna means
26 between "on" and "off" states, and could alternatively be described as enabling or
disabling means. In the off state, terminal
36 of inner winding
30 is effectively open, which attenuates any signal that might otherwise be passed to
bus
24. In the on state, terminal
36 is connected to ground, which allows the signal to flow to bus
24. In the preferred embodiment, switching means
28 includes an NPN transistor
42 whose collector is connected to terminal
36 of inner winding
30. The emitter of transistor
42 is connected to ground, and its base is connected, via a current limiting resistor
44, to the controller which, in the preferred embodiment, is a microprocessor
46.
[0020] Outer winding
32 is connected to switching means
28 in a manner similar to inner winding
30. One terminal
38 of outer winding
32 is connected to a common ground, and the other terminal
40 is connected to the collector of a PNP transistor
48. The emitter of PNP transistor
48 is connected to ground. The base of transistor
48 is connected to resistor
44, which is in turn connected to microprocessor
46. It is anticipated that a microprocessor such as that sold under part number MC68HC11
by Motorola, of Austin, Texas, would be sufficient for the purposes contemplated herein.
[0021] Operation of switchable antenna units
12 is straightforward. A given switchable antenna unit is selected for signal reception
when its control line
49 is biased positive with respect to ground. Conversely, a given unit is unselected
when its control line
49 is biased negative with respect to ground.
[0022] For an unselected unit, transistor
42 is turned off, effectively leaving inner winding
30 in an open state which naturally attenuates signals that would otherwise be efficiently
transmitted to bus
24. Transistor
48, on the other hand, is turned on, shorting outer winding
32. This also effectively shorts out inner winding
30, since the coefficient of coupling between the two windings is high, resulting in
further attenuation of signals emanating from the unselected antenna unit. In this
respect, outer winding
32 is properly considered part of switching means
28, rather than part of antenna means
26, since outer winding
32 does not present any signal to bus
24 and serves only to attenuate the signal in inner winding
30.
[0023] For the selected unit, transistor
42 receives a (positive) reference voltage from microprocessor
46, which turns transistor
42 on and allows the signal from inner winding
30 to pass to common bus
24. At the same time, transistor
48 is turned off, leaving outer winding
32 in an open state which has a negligible effect on signal reception. It should be
noted that switchable antenna unit
12 might include only inner winding
30 without outer winding
32. In such a case, switching means
28 would include transistor
42 only, and not transistor
48. For attenuation purposes, however, it is desirable to include an outer winding
32 which is also responsive to switching means
28.
[0024] In order to optimize the efficiency of switchable antenna units
12, it is advisable to provide transistors with a low dynamic saturation resistance
and low collector to base capacitance. A low saturation resistance (from collector
to emitter when the transistor is on) minimizes transistor noise in comparison to
the desired antenna signal. A low collector to base capacitance is desirable for maximizing
attenuation of the unwanted signal when switchable antenna unit
12 is in the off state. A transistor sold under part number NR041 by National Semiconductor
of Santa Clara, California, has proven acceptable for the frequency range of 10 kilohertz
to 1 megahertz.
[0025] It is also beneficial to keep the capacitance between inner and outer windings
30 and
32 as low as possible in order to improve attenuation of the signal when the antenna
unit is unselected. This may be accomplished by increasing the separation between
the windings which decreases the capacitance; however, this also decreases the mutual
inductance between the windings, which is undesirable since attenuation improves with
increased mutual inductance. Therefore, it is necessary to find an intermediate separation
which optimizes both the capacitance between the windings and their mutual inductance.
The inventor has found that a separation distance in the range of 0.7 to 1.5 millimeters
is optimal for antenna windings approximately 5 centimeters long and about 1 centimeter
in diameter.
[0026] Some alternative embodiments of the present invention are shown in Figures 4 through
6. With reference to Figure 4, a switchable antenna unit
50 utilizing metal oxide semiconductor field-effect transistors (MOSFET's) is explained.
In this embodiment, the transistors of Figure 3 have been replaced with n-type MOSFET's
52. Each MOSFET
52 essentially functions in the same manner as transistor
42, but is less preferable since its resistance (in the on state) is generally higher
than that of a bipolar transistor having approximately the same capacitance (in the
off state). Additionally, switching transients are usually larger with MOSFET's than
with bipolar transistors.
[0027] Figure 4 illustrates another design alternative which may be used with either MOSFET's
or bipolar transistors, namely, the use of two semiconductor devices of the same type
as opposed to the use of complimentary devices. This requires the provision of two
control lines for each switchable antenna unit. In switchable antenna unit
50, one control line
54 is connected to MOSFET
52 which regulates inner winding
30, and the second line
56 is connected to MOSFET
52′ which controls outer winding
32. In this configuration, it is presumed that the control signals emanating from microprocessor
46 are complementary, e.g., if line
54 is at zero volts, then line
56 is at the positive reference voltage. Those skilled in the art will appreciate that
a single control line setup (such as that shown in Figure 3) using MOSFET's may be
afforded by making MOSFET
52′ a p-type MOSFET. Alternatively, the two control line setup (with complementary control
signals) may be used with bipolar transistors, in which case transistors
42 and
48 of Figure 3 would both be NPN transistors.
[0028] Referring now to Figure 5, a balanced antenna unit
60 is shown. Balanced antenna unit
60 still utilizes transistors
42 and
48, but additionally includes transistors
42′ and
48′ in a symmetric configuration. The terminals
38 and
40 of outer winding
32 are connected to the collectors of PNP transistors
48′ and
48, respectively. The terminals
34 and
36 of inner winding
30 are similarly connected to the collectors of NPN transistors
42′ and
42, respectively. Now, however, inner winding
30 has been split, creating two intermediate output lines
62 and
64 which are connected to common signal buses
24a and
24b, respectively. Signal buses
24a and
24b are connected to a balanced transformer
66 whose output is directed to preamp
14. The emitters of all the transistors are connected to ground, and the bases of all
the transistors are connected to micro processor
46 via resistors
44. Balanced antenna unit
60 is useful in reducing noise from switching transients; the noise is attenuated since
it occurs as a common mode input to balanced input transformer
66. The design shown in Figure 5 is, however, more costly than the preferred embodiment
of Figure 3.
[0029] Another alternative embodiment is shown in Figure 6, to which attention is now directed.
Figure 6 illustrates the use of a bridged antenna unit
70 having two identical windings
72 and
74. One terminal of winding
72 is attached to the collector of NPN transistor
42, and one terminal of winding
74 is connected to a terminal of a variable capacitor
76. The other terminals of windings
72 and
74 are joined to provide a common output to signal bus
24. When variable capacitor
76 is properly adjusted (to the collector-base capacitance of transistor
42), antenna unit
70 forms a bridged circuit in the off state. In this state, the signals from windings
72 and
74 cancel each other out, resulting in a negligible output to common signal bus
24. The primary disadvantage of this structure is the great care which must be taken
in adjusting variable capacitor
76 in order to achieve a balanced circuit.
[0030] Although the invention has been described with reference to specific embodiments,
this description is not meant to be construed in a limiting sense. Various modifications
of the disclosed embodiment, as well as alternative embodiments of the invention,
will become apparent to persons skilled in the art upon reference to the description
of the invention. For example, the antenna units may be tuned by the addition of a
capacitor across the common signal bus if frequency selectivity were required (although
this is deemed undesirable since it would affect the attenuation of unselected antenna
signals). It is therefore contemplated that the appended claims will cover such modifications
that fall within the true scope of the invention.
1. A switchable antenna unit for receiving and conveying electromagnetic signals to
a signal bus, comprising:
antenna means having a first terminal for connection to the signal bus, and having
a second terminal; and
switching means connected to said second terminal of said antenna means for selectively
attenuating signals received by said antenna means, said switching means having an
"on" state and an "off" state whereby, when said switching means is in said "on" state,
said antenna means conveys said received signals to said first terminal and, when
said switching means is in said "off" state, said received signals are attenuated
by said switching means.
2. The switchable antenna unit of Claim 1 wherein said antenna means includes a first
winding, and said switching means includes a second winding inductively coupled to
said first winding.
3. The switchable antenna unit of Claim 1 wherein said switching means causes said
second terminal of said antenna means to be connected to ground when said switching
means is in said "on" state, and said switching means causes said second terminal
of said antenna means to be effectively open when said switching means is in said
"off" state.
4. The switchable antenna unit of Claim 1 wherein said switching means has a control
line for connection to an external controller.
5. The switchable antenna unit of Claim 4 wherein said switching means includes a
transistor having a collector, a base, and an emitter, said collector being connected
to said second terminal of said antenna means, said base being connected to said control
line, and said emitter being connected to ground.
6. The switchable antenna unit of Claim 4 wherein said switching means includes a
metal oxide semiconductor field-effect transistor having a drain, a gate, and a source,
said drain being connected to said second terminal of said antenna means, said gate
being connected to said control line, and said source being connected to ground.
7. The switchable antenna unit of Claim 4 wherein said antenna means includes a first
winding, and said switching means includes a second winding inductively coupled to
said first winding.
8. The switchable antenna unit of Claim 4 wherein:
said antenna means comprises a first winding having a first terminal for connection
to the common signal bus, and having a second terminal; and
said switching means comprises:
a transistor having a collector, a base, and an emitter, said collector being connected
to said second terminal of said first winding, said base being connected to said control
line, and said emitter being connected to ground,
a second winding having a first terminal connected to said first terminal of said
first winding, and having a second terminal, and
a capacitor having a first terminal connected to said second terminal of said second
winding, and having a second terminal connected to ground.
9. The switchable antenna unit of Claim 7 wherein:
said switching means further includes first and second transistors each having a collector,
a base, and an emitter;
said collector of said first transistor being connected to said second terminal of
said first winding;
said collector of said second transistor being connected to said second terminal of
said second winding;
said bases of said first and second transistors being connected to said control line;
and
said emitters of said first and second transistors being connected to ground.
10. An apparatus for selectively outputting signals received by an antenna array,
comprising:
a common signal bus;
a plurality of switchable antenna units, each comprising:
antenna means having a first terminal connected to said common signal bus, and having
a second terminal, and
switching means connected to said second terminal of said antenna means for selectively
attenuating signals received by said antenna means, said switching means having an
"on" state and an "off" state whereby, when said switching means is in said "on" state,
said antenna means outputs said received signals to said common signal bus and, when
said switching means is in said "off" state, said received signals are attenuated
by said switching means, said switching means further having a control line; and
control means connected to each said control line for selectively switching each said
switching means between "on" and "off" states.
11. The switchable antenna unit of Claim 10 wherein said switching means causes said
second terminal of said antenna means to be connected to ground when said switching
means is in said "on" state, and said switching means causes said second terminal
of said antenna means to be effectively open when said switching means is in said
"off" state.