U.S. patent number 3,810,147 [Application Number 05/214,361] was granted by the patent office on 1974-05-07 for electronic security system.
Invention is credited to George Jay Lichtblau.
United States Patent |
3,810,147 |
Lichtblau |
May 7, 1974 |
ELECTRONIC SECURITY SYSTEM
Abstract
An electronic security system especially adapted for use in
retail stores and employing a multi-frequency resonant tag circuit
having distinct frequencies for detection and deactivation. A
transmitting system provides an electromagnetic field within a
controlled area at a frequency which is swept through a range
including the detection frequency of the resonant circuit. In the
presence of a tag circuit within the controlled area and operative
at the detection frequency, pulses are detected by a receiver which
includes noise rejection circuitry for discriminating true signals
from noise. The receipt of a predetermined number of signal pulses
within a prescribed interval of time causes an alarm actuation. The
resonant circuit is formed by printed circuit techniques as a
relatively small tag which can be affixed to an article of
merchandise. The tag includes a fusible link integrally formed as
part of the circuit and which can be fused upon application of an
electromagnetic field of predetermined magnitude at the
deactivation frequency of the resonant circuit. Deactivation of the
tag destroys the resonant properties of the tag at the detection
frequency such that a deactivated tag produces no alarm when
passing through a controlled area.
Inventors: |
Lichtblau; George Jay (New
York, NY) |
Family
ID: |
22798779 |
Appl.
No.: |
05/214,361 |
Filed: |
December 30, 1971 |
Current U.S.
Class: |
340/572.3;
334/39; 340/572.4; 340/572.5; 334/8 |
Current CPC
Class: |
G08B
13/2471 (20130101); G08B 13/2488 (20130101); G08B
13/2414 (20130101) |
Current International
Class: |
G08B
13/24 (20060101); G01s 009/56 () |
Field of
Search: |
;340/280,258R,152T
;343/6.5SS,6.5R,726-728 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Caldwell; John W.
Assistant Examiner: Wannisky; William M.
Attorney, Agent or Firm: Weingarten, Maxham &
Schurgin
Claims
1. An electronic security system comprising:
transmitter means for providing an electromagnetic field in a
predetermined area at a frequency repetitively swept through a
predetermined range;
a multi-resonant tag circuit having a first resonant frequency
within said predetermined range of frequencies and a second
frequency outside of said predetermined range of frequencies;
receiver means for detecting the presence of said first resonant
frequency from a tag circuit present in said predetermined area;
and
deactivation transmitter means for providing an electromagnetic
field in a predetermined area at said second frequency thereby to
induce a current flow in a fusible link of said tag circuit
sufficient to destroy the resonant properties of said tag circuit
at said first resonant frequency.
2. An electronic security system according to claim 1 wherein said
receiver means includes:
an RF front-end for receiving the electromagnetic field from said
transmitter means and for providing an RF output signal in response
thereto;
detector means operative in response to said output signal to
provide output pulses in response to the presence of the first
resonant frequency of said tag circuit in said electromagnetic
field;
means for suppressing noise received with said output pulses;
and
means operative in response to said output pulses for providing an
output indication representing the presence of a tag circuit having
said first
3. An electronic security system according to claim 2 wherein said
noise suppression means includes:
means for passing only the signal spectrum of said RF output
signal; and
means for digitally processing said pulses from said detector means
to provide output pulses only in response to pulses of
predetermined rate and
4. An electronic security system according to claim 1 wherein said
transmitter means includes:
an oscillator for providing a modulation signal at a predetermined
frequency;
a voltage controlled oscillator operative in response to said
oscillator to provide a carrier frequency varied through a
predetermined frequency range in response to the modulation signal
of said oscillator;
an amplifier coupled to said voltage controlled oscillator; and
an antenna coupled to said amplifier for providing said
electromagnetic
5. An electronic security system according to claim 1 wherein said
means for electrically deactivating said tag circuit includes:
an antenna for providing said electromagnetic energy at said second
resonant frequency; and
transmitter means for driving said antenna to provide said
electromagnetic
6. An electronic security system according to claim 5 wherein said
antenna includes a balanced loop antenna matched to the output of
said transmitter
7. An electronic security system according to claim 5 wherein said
antenna includes a balanced loop antenna having one terminal
coupled to the output of said transmitter means, said loop antenna
being balanced thereto by a balanced lead structure, the other end
of said loop antenna being
8. An electronic security system according to claim 1 wherein said
receiver means includes:
detector means operative in response to said first resonant
frequency in a received electromagnetic field from said transmitter
means to provide output pulses in response thereto;
filter means for suppressing noise in the presence of such
pulses;
means for amplifying said output pulses;
means for shaping said amplified pulses; and
means for digitally processing said shaped pulses and to provide an
alarm indication only in response to a predetermined number of
shaped pulses
9. An electronic security system according to claim 8 wherein said
filter means includes:
a low pass filter operative to pass the signal spectrum of received
pulses;
a notch filter operative to remove the signal component caused by
said modulating frequency; and
a bandpass filter operative to remove noise components outside of
said
10. An electronic security system according to claim 8 wherein said
means for digitally processing said detected pulses includes:
means operative in response to said shaped pulses to provide first
output pulses of predetermined width greater than the width of
expected received pulses;
staircase generating means operative in response to said first
output pulses to generate a staircase signal which increases in
amplitude in the presence of each of said first output pulses;
means operative in response to said first output pulses to provide
counter pulses of a width slightly less than the period of expected
signals;
gate means operative in response to said first output pulses for
providing a gate signal to said counter means defining an interval
of time within which a predetermined number of counter pulses must
be received;
counter means operative in response to said counter pulses to
provide an output indication in response to the presence of a
predetermined number of counter pulses within an interval of time
specified by the gate signal of said gate means;
said staircase generating means being operative to provide a reset
signal upon the exceedance of a predetermined threshold level by
said staircase signal to reset said counter means, said staircase
generating means and
11. An electronic security system according to claim 1 wherein said
receiver means includes:
detector means operative to provide output pulses in response to
the presence of said tag circuit in said electromagnetic field;
and
a cross correlation filter operative in response to said output
pulses to provide an output signal for indicating the presence of a
tag circuit only when the spectral content of said output pulses is
of predetermined
12. An electronic security system according to claim 1 wherein said
receiver means includes:
means for detecting pulses produced by said multi-resonant tag
circuit being swept by the electromagnetic field of said
transmitter means;
means for discriminating signal pulses from said tag circuit from
spurious signals; and
means for providing an alarm indication in response to receipt of a
predetermined number of said signal pulses occurring within a
13. An electronic security system according to claim 1 wherein said
multi-resonant tag circuit includes:
a first tuned circuit resonant at said first frequency within said
predetermined range;
a second tuned circuit resonant at said second frequency outside of
said predetermined range; and
a fusible link in circuit with said tuned circuits and operative to
fuse upon application of electromagnetic energy of predetermined
power at said
14. An electronic security system according to claim 13 wherein
said multi-resonant tag circuit includes:
a planar substrate of electrically insulative material, said first
and second tuned circuits being formed on said substrate in planar
circuit
15. An electronic security system according to claim 1 wherein said
multi-resonant tag circuit includes:
a planar substrate of electrically insulative material;
a first conductive path formed on a surface of said substrate in a
configuration to define a first inductor;
a second conductive path formed on said substrate in a
configuration to define a second inductor;
a plurality of pairs of conductive areas each pair formed of
conductive areas in alignment on respective opposite surfaces of
said substrate, the conductive areas on the substrate surface
containing said first and second conductive paths being
electrically connected thereto at selected points to define a
plurality of capacitors for said tag circuit; and
a conductive path of predetermined size provided in circuit with
said tag circuit on said substrate and operative to fuse upon
application of
16. For use in an electronic security system which includes means
for providing an electromagnetic field of a frequency which is
swept within a predetermined range, means for detecting a first
frequency within said range, and means for providing an
electromagnetic field at a second frequency outside of said range,
a multi-frequency resonant tag circuit comprising:
a first tuned circuit resonant at said first frequency within said
predetermined range;
a second tuned circuit resonant at said second frequency outside of
said predetermined range; and
a fusible link in circuit with said first and second tuned circuits
and operative to fuse upon application of an electromagnetic field
of predetermined power at said second frequency to thereby destroy
the
17. The invention according to claim 16 wherein said
multi-frequency resonant tag circuit includes:
a planar substrate of electrically insulative material, said first
and second tuned circuits being formed on said substrate in planar
circuit
18. The invention according to claim 17 wherein said fusible link
is also formed in planar circuit configuration with said first and
second tuned
19. The invention according to claim 16 wherein said
multi-frequency resonant tag circuit includes:
a planar substrate of electrically insulative material;
a first conductive path formed on a surface of said substrate in a
configuration to define a first inductor;
a second conductive path formed on said substrate in a
configuration to define a second inductor; and
a plurality of pairs of conductive areas fromed on said substrate
each pair having conductive areas in alignment on respective
opposite surface of said substrate to define a capacitor, the
conductive areas on the substrate surface containing said first and
second conductive paths being electrically connected to said paths
at selected points to define said
20. The invention according to claim 16 wherein said
multi-frequency resonant tag circuit includes:
a planar insulative substrate having formed on one surface
thereof
a first conductive area centrally formed on said substrate
surface;
second and third conductive areas formed in adjacent spaced
relation along one side of said substrate surface;
a first conductive path arranged on said substrate surface between
and in electrical connection with said first and second conductive
areas;
a second conductive path formed on said substrate surface between
and in electrical connection with said second and third conductive
areas;
and wherein the opposite surface of said substrate includes:
fourth, fifth and sixth conductive areas each in alignment and
substantially coextensive with said respective first, second and
third conductive areas formed on said one substrate surface;
a conductive path connecting said forth and fifth conductive
areas;
said first and second conductive paths and plurality of conductive
areas comprising said first and second tuned circuits; and
wherein said fusible link includes a conductive path
interconnecting said fifth and sixth conductive areas and
dimensioned to fuse upon application of an electromagnetic field of
predetermined power at said second
21. The invention according to claim 16 wherein said first and
second tuned
22. The invention according to claim 16 wherein said first and
second tuned
23. An electronic security system according to claim 1 wherein:
said multi-resonant tag circuit has a third resonant frequency
operative when the resonant properties of said tag circuit at said
first resonant frequency have been destroyed by provision of an
electromagnetic field thereto;
and further including:
means for detecting the presence of said third resonant frequency
from said
24. The invention according to claim 16 wherein said
multi-frequency resonant tag circuit includes:
a third tuned circuit resonant at a third frequency when the
resonant properties of said tag circuit at said first frequency
have been destroyed by application of said electromagnetic field at
said second frequency.
Description
FIELD OF THE INVENTION
This invention relates to electronic security systems and more
particularly to a radio frequency system for the reliable detection
of a resonant tag circuit within a controlled area and for
electronic destruction or alteration of the tag to permit passage
of articles through the controlled area.
BACKGROUND OF THE INVENTION
Electronic security systems have been proposed for detecting the
unauthorized removal of articles from an area under protection.
Such systems have been proposed especially for use in retail stores
to prevent the theft of articles from the store and minimize the
considerable losses occasioned by shoplifting. Electronic security
systems generally include an electromagnetic field provided in a
controlled area through which merchandise must pass in leaving the
store. A resonant circuit is attached to articles of merchandise
and the presence of the resonant circuit in the controlled area is
sensed by a receiving system to denote the unauthorized removal of
an article. The resonant circuit is removed by store personnel from
an article properly leaving the store to permit passage of the
article through the controlled area without alarm activation.
One prior art system is shown in U.S. Pat. No. 3,500,373 wherein a
single resonant circuit is affixed to an article and its presence
in a controlled area detected by interrogation with a swept
transmitted frequency which includes the resonant frequency of the
circuit. Upon interrogation the resonant circuit absorbs energy
which is sensible by a receiver to produce pulses for alarm
actuation. This system is, however, quite susceptible to noise
which can cause erroneous alarm actuation. A further deficiency of
such a system is that the resonant circuit must be physically
accessible to be removed prior to passage of the associated article
through the controlled area. The accessibility of the resonant
circuit also permits its removal by a thief and the system can
thereby be circumvented.
Another system of the prior art is shown in U.S. Pat. No. 3,624,631
in which a single resonant circuit includes a fusible link. The
resonant circuit is again interrogated by a swept frequency, the
presence of the circuit in a controlled area causing energy
absorption at the resonant frequency which is detected by a
receiver for subsequent alarm actuation. Upon application of a
swept frequency of higher energy than that employed for detection,
the fusible link of the resonant circuit can be destroyed to
deactivate the tuned circuit such that no detection is possible.
The utility of this later system in a commercial setting is
diminished by several factors. Since deactivation is accomplished
by a swept frequency transmitter, the energy level of the radiated
field must be very low to meet requirements of the Federal
Communications Commission (FCC). The fusible link must therefore be
extremely small and made of a material to allow fusing at such low
power levels. As a result, the single resonant circuit is
relatively costly and difficult to manufacture. Moreover, since the
resonant circuit is both detected and deactivated at the same
frequency, the energy level of the applied field must be carefully
controlled to prevent spurious alarm responses in the receiver of
the system or neighboring systems.
SUMMARY OF THE INVENTION
In accordance with the present invention, an electronic security
system is provided which is substantially immune to noise and
wherein a multi-frequency resonant tag circuit is provided having
distinct frequencies for detection and deactivation. The resonant
tag circuit is operative at a first frequency to permit detection
by electromagnetic interrogation thereof, and is operative at a
second frequency to permit the deactivation thereof by an applied
electromagnetic field which destroys the resonance of the circuit
at its detection frequency. The system is sufficiently sensitive to
permit radiation at a detection frequency at levels below the
minimum radiation requirements of the Federal Communications
Commission (FCC), thereby eliminating the requirement for an
operating license. It is a feature of the invention that the
deactivation frequency of the resonant tag can be at one of the
frequencies designated by the FCC to have unlimited radiated power
levels. As a result, sufficient power is readily provided to cause
tag deactivation, and further, operation at these designated
frequencies does not require an operating license. The tag circuit
is composed wholly of passive elements and is typically formed by
printed or etched circuit techniques as a small tag or card adapted
to be attached to articles being protected. A fusible link is
provided which preferably is an integral element of and of the same
conductive material as the resonant circuit, permitting the
inexpensive and facile manufacture of the tag circuit.
In brief, the system embodying the invention comprises transmitting
apparatus for providing an electromagnetic field at a controlled
area at a frequency swept through a predetermined range which
includes the detection frequency of the tag circuit. Receiving
apparatus is provided for detecting pulses caused by the presence
of the tag circuit in the radiated field, the pulses being
processed to discriminate true signals from noise and to provide an
alarm or other suitable output indication in response to a
predetermined number of valid signal pulses within a specified
interval of time. Deactivation of the tag circuit is accomplished
by a separate transmitter operative at the tag deactivation
frequency to cause destruction of a fusible link in the tag
circuit, and is accomplished at a single transmitted frequency and
with sufficient power to alter tap resonance within short and
commercially realistic duration.
DESCRIPTION OF THE DRAWINGS
The invention will be more fully understood from the following
detailed description taken in conjunction with the accompanying
drawings, in which:
FIG. 1 is a block diagram representation of a security system
according to the invention;
FIG. 2 is a block diagram representation of the transmitter of FIG.
1;
FIG. 3 is a pictorial view of one side of a resonant tag circuit
embodying the invention;
FIG. 4 is a pictorial view of the opposite side of the resonant tag
circuit of FIG. 3;
FIG. 5 is a schematic diagram of the equivalent electrical circuit
of the resonant tag of FIGS. 3 and 4;
FIG. 6 is a block diagram representation of an embodiment of the
invention employing amplitude modulation detection;
FIGS. 7A thru 7E are a plot of spectral diagrams useful in
illustrating operation of the embodiment of FIG. 6;
FIGS. 8A and 8B are a plot of signal diagrams useful in
illustrating operation of the invention;
FIG. 9 is a block diagram representation of the digital processing
circuitry of FIG. 1;
FIGS. 10A thru 10F are a plot of signal diagrams useful in
illustrating operation of the circuitry of FIG. 9;
FIG. 11 is a block diagram representation of an embodiment of the
invention employing phase modulation detection;
FIGS. 12A thru 12C are a plot of spectral diagrams useful in
illustrating operation of the embodiment of FIG. 11;
FIG. 13 is a diagrammatic representation of the cross correlation
filter of the apparatus of FIG. 11; and
FIG. 14 is a diagrammatic representation of a deactivation system
embodied in the invention.
DETAILED DESCRIPTION OF THE INVENTION
An electronic security system according to the invention is
depicted in block diagram form in FIG. 1 and includes a transmitter
10 coupled to an antenna 12, typically a loop antenna operative to
provide an electromagnetic field within a predetermined area to be
controlled. A receiving antenna 14, also typically a loop antenna,
is arranged at the controlled area to receive energy radiated by
transmitting antenna 12 and to couple received energy to an RF
front-end which includes an RF bandpass filter 16 and RF amplifier
18. The output of amplifier 18 is applied to a detector 20, the
output of which is, in turn, coupled to noise rejection circuitry
22. Output signals from noise rejection circuitry 22 are amplified
by amplifier 24 and applied to pulse shaping circuitry 26 and
thence to digital processing circuitry 28, the output of which is
operative to actuate an alarm 30 or other output utilization
apparatus.
The transmitter 10 is illustrated in greater detail in FIG. 2 and
includes an oscillator 32 for providing a modulation signal to a
voltage controlled oscillator 34, the output of which is applied to
a power amplifier 36 which, in turn, drives transmitting antenna
12. Oscillator 32 provides a regularly recurring waveform at a
predetermined frequency, typically 1 KHz. The voltage controlled
oscillator 34 provides an appropriate carrier frequency which is
varied in frequency under the influence of the modulation signal
from oscillator 32. For example, the carrier frequency can have a 5
MHz center frequency varied with a 15 percent frequency deviation
at a 1 KHz rate. The modulation signal provided by oscillator 32
can be any regularly recurring waveform such as a sine wave,
triangle wave, sawtooth, ramp or exponential signal. Amplifier 36
preferably provides a driving current to antenna 12 of a magnitude
which varies inversely with frequency and as a result, the signal
received by antenna 14 is of an amplitude which is constant with
changes in frequency.
In the absence of a resonant circuit in the controlled area, the
electromagnetic field provided by antenna 12 is sensed by receiving
antenna 14 but no output indication is provided by the receiving
system as no pulse response is present to trigger the alarm. When a
resonant tag is present in the controlled area, the tag will become
maximally coupled to the receiving antenna 14 each time the
transmitted sweep frequency passes through the resonant detection
frequency of the tag, giving rise to a sensible change in
electrical characteristics at the receiving antenna to cause
detection of tag presence.
A multi-frequency resonant circuit embodied on a card or tag
adapted to be affixed to items of merchandise and the like is
illustrated in FIG. 3. The circuit is formed by printed or etched
circuit techniques and includes a first conductive path 40 arranged
in a generally rectangular path on a surface of an insulative
substrate 42 and terminating in respective conductive areas 44 and
46 disposed in adjacent spaced relationship near one edge of
substrate 42. A second conductive path 48 is formed as a
rectangular spiral on substrate 42 and terminates at its outer end
at a junction 50 with path 40, and at its inner end at a conductive
area 52 centrally of the spiral. The opposite surface of substrate
42 is illustrated in FIG. 4 and includes a conductive area 54 in
alignment and generally coextensive with conductive area 52 on the
substrate surface depicted in FIG. 3, and a pair of conductive
areas 56 and 58 in alignment and generally coextensive with areas
44 and 46 on the other surface. The conductive areas 56 and 58 are
interconnected by a conductive path 62. As will be further
described hereinbelow, path 62 is dimensioned to fuse upon
energization by a predetermined electromagnetic field to alter the
resonant properties of the tag circuit.
The conductive paths 40 and 48 serve as respective inductors of the
resonant circuit and these paths are wound with opposite sense to
provide a negative mutual coupling coefficient between the
inductors. The conductive areas 52 and 54 spaced by the interposed
substrate 42 serve as a first capacitor, while second and third
capacitors are formed by the conductive areas 44 and 46 on one
surface of substrate 42 and cooperative with paths 56 and 58 on the
opposite substrate surface. The equivalent electrical circuit of
the tag is illustrated in schematic form in FIG. 5 and exhibits two
resonant frequencies, one employed for detection of tag presence
and the other for alteration or destruction of the detection
frequency. The inductor L1 is the outer loop 40, while the inductor
L2 is the inner loop 48. Capacitor C2 is formed by conductive areas
52 and 54, while conductive areas 44 and 56 and 46 and 58 serve as
capacitors C1 and C3 respectively.
The two loops in series, composed of inductors L1, L2, C2 and C1,
are tuned to a detection frequency which may be in any convenient
portion of the spectrum. Typically, a detection frequency of 5 MHz
is employed. The outer loop, composed of inductor L1 and capacitors
C1 and C3 is tuned to a destruction frequency which preferably is
one of the frequencies allocated by the FCC for industrial,
scientific, and medical purposes known as the ISM frequencies.
These ISM frequencies offer the advantage of unlimited radiated
power and with no requirement for an operator's license. The ISM
frequencies are 13.56, 27.12, 40.00 and 905 MHz, and in the
illustrated embodiment a frequency of 27.12 MHz is employed.
Destruction of the resonant properties of the tag is readily
accomplished by application of energy at the destruction frequency
to cause fusing of link 62 such that the tag is no longer sensible
by the receiving system. Sufficient power can be applied to the
resonant tag at the destruction frequency to permit fusing of the
link therein even though the actual resonant point of the tag
circuit varies somewhat from the nominal destruction frequency.
The tag circuit can be fabricated by various printed and etched
circuit techniques well known in the art. Extremely low cost
fabrication can be accomplished by use of a continuous production
process to provide a strip of tags which can be separated for use.
Typically, in such a continuous process, an insulative strip of
material is employed having a conductive foil laminated or extruded
on each side thereof. The tag circuit is repetitively printed onto
respective sides of the strip and the nonprinted areas are etched
away to complete the continuous roll of tags, which can then be cut
or otherwise severed from the strip. The strip is typically high
density polyethylene of one to two mils thickness having formed on
each side thereof an aluminum foil of, typically, one mil
thickness.
The resonant circuit depicted in FIGS. 3 through 5 has been found
particularly effective in the present system; however, it will be
appreciated that the resonant circuit may take a variety of
configurations to provide the intended destruction and detection
frequencies. In the tag circuit illustrated, the provision of three
capacitors allows fabrication of the tag without electrical
connection between respective sides of the circuit board. A dual
resonant circuit with only two capacitors can be employed by use of
a circuit connection between opposite board surfaces, such as by
means of a plated-through hole in board 42 interconnecting
respective circuit patterns. The fusible link 62 may be placed in
circuit otherwise than as illustrated in accomplish deactivation of
the tag at the detection frequency, or, as an alternative, to cause
complete deactivation of the resonant circuit. The mutual coupling
coefficient M between the coils of the resonant circuit can be of
either positive or negative sense. When the coupling coefficient is
negative, the respective tuned circuits comprising the overall tag
can be separately tuned and thus simplify the design of specific
tag circuits having intended resonance characteristics. Since the
sensitivity of a tuned circuit is related to the total number of
coil turns of the circuit inductors, the provision of a positive
coupling coefficient will result in greater sensitivity since the
number of turns of the circuit inductors, being wound with the same
sense, will be additive. In the circuit of FIG. 5, for example, if
the mutual coupling (M) is negative, the detection frequency is
determined by inductor L1 in series with inductor L2 and capacitors
C2 and C1, while the deactivation frequency is determined by
inductor L1 in series with capacitors C1 and C3. If, however, the
mutual coupling is positive, the resonant frequencies are caused by
total interaction of the several inductors and capacitors.
The fusible link can also be placed within the tag circuit such
that, upon destruction by an applied electromagnetic field, the tag
becomes wholly inoperative. Alternatively, the link can be placed
such that upon destruction, the resonant properties of the tag at
the detection frequency are destroyed and the tag made operative at
a third frequency different than both the detection and destruction
frequencies. This third frequency can be ignored during system
operation as the system does not operate at the third frequency. In
an alternative embodiment, however, the third frequency, which
becomes operative upon destruction of the fusible link, may be
utilized to ascertain that a tag present at a controlled area has
been electrically altered. This latter embodiment may be employed
for example at an exit station of a store where the security system
is operative to detect tag presence by sensing the detection
frequency in order to determine whether items are being improperly
removed from the store. Detection of a tag at the third frequency
is indicative of an electrically altered tag and the absence of
third frequency detection can indicate the presence of an item from
which a tag has been removed. The use of such a third tag frequency
therefore can provide an added level of security.
The present system is operative to detect amplitude, frequency or
phase modulation provided by the presence of a resonant tag within
an applied electromagnetic field. An embodiment of the invention
employing amplitude modulation is depicted in FIG. 6. The RF
front-end is as described above in connection with FIG. 1 and
including a receiving antenna 14, RF bandpass filter 16 and RF
amplifier 18. Signals from the RF amplifier are applied to a full
wave detector 64, the output of which is applied to a low pass
filter 66. The output of filter 66 is coupled to a notch filter 68
which, in turn, is coupled to a bandpass filter 70. The output of
bandpass filter 70 is then processed in the same manner as in the
embodiment of FIG. 1. More particularly, the output of filter 70 is
applied via a video amplifier 24 to pulse shaping circuitry 26 and
then to digital processing circuitry 28, the output of which is
operative to energize a suitable alarm 30.
The operation of the system of FIG. 6 is best described in
conjunction with the spectral diagrams of FIG. 7 and the signal
diagrams of FIG. 8. The carrier frequency illustrated in FIG. 8A
varies in a regularly recurring manner through a predetermined
frequency range, typically .+-. 15% of the carrier center
frequency. After processing by RF bandpass filter 16 (FIG. 1) the
received signal has a spectral content 72 as depicted in FIG. 7A
centered about the carrier fc. After full wave detection by
detector 64, the spectrum present is as in FIG. 7B and includes a
carrier spectrum 74 centered about twice the carrier frequency 2
fc, and a signal spectrum 76 which includes the modulation
frequency fm. Noise components 78 are also present, as illustrated.
The signal output from detector 64 is a series of bipolar pulses,
depicted in FIG. 8B, each of which occurs upon traversal of the
resonant detection frequency of the tag circuit by the swept
carrier. It will be noted that adjacent pulses are of opposite
sense in response to respective traversal of the resonant detection
frequency in positive and negative directions by the swept
carrier.
The low pass filter 66 is operative to remove the double carrier
spectrum 74, as illustrated in FIG. 7C, leaving the signal spectrum
76, and carrier frequency fc and associated noise component 78.
Notch filter 68 is operative to remove the frequency component at
twice the modulation frequency (2 fm) caused by the response of RF
bandpass filter 16 to the received swept carrier frequency and
provides the signal spectrum of FIG. 7D. The bandpass filter 70 is
operative to pass only the signal spectrum 76, as shown in FIG. 7E,
rejecting noise above and below this spectrum. After amplification
in video amplifer 24, signals are applied to pulse shaping
circuitry 26 which typically includes pulse height and pulse width
discriminating circuits to provide a train of pulses of
standardized height and width for subsequent digital processing.
Digital processing circuitry 28 is operative to discriminate true
signal pulses from noise and spurious signals and to energize an
alarm 30 upon detection of a predetermined number of valid signal
pulses within a selected interval of time.
The pulse processing circuitry 28 is depicted in greater detail in
FIG. 9. Signals from pulse shaping circuitry 26 are applied to a
first monostable multivibrator 80, the output of which is applied
simultaneously to a staircase generator and comparator 82, a second
monostable multivibrator 84 and a third monostable multivibrator
86. The output of multivibrator 84 is applied to a counter 88, the
output of which is a pulse train for alarm energization. Staircase
generator 82 is operative to apply a reset signal to counter 88 and
to multivibrator 86. Multivibrator 86 is operative to apply a reset
signal to counter 88 and to staircase generator 82.
Operation of the circuitry of FIG. 9 will be described in
conjunction with the signal diagrams of FIG. 10. The pulses from
pulse shaping circuitry 26 are depicted in FIG. 10A. These pulses
are applied to a monostable multivibrator 80 which is operative to
produce output pulses, as depicted in FIG. 10B, of predetermined
width T1 which is selected to exceed the duration of expected
received signals. The standardized pulse signals from multivibrator
80 are applied to respective inputs of a staircase generator and
comparator 82 and monostable multivibrators 84 and 86. The
multivibrator 84 has a period equal to approximately the modulation
rate, and in response to pulses from multivibrator 80, provides
pulses as shown in FIG. 10D to advance the counter 88 by one count
for each pulse applied thereto. The pulses T2 from multivibrator 84
commence upon the leading edge of corresponding pulses from
multivibrator 80 and terminate slightly before the commencement of
a subsequent pulse from multivibrator 80, and are operative to
inhibit spurious input signals from advancing counter 88 at a rate
faster than that of expected signals.
The output from staircase generator and comparator 82 is depicted
in FIG. 10C and is seen to be a staircase voltage which rises
during the duration of each pulse from multivibrator 80. A
threshold circuit is provided within the staircase generator and
comparator 82 and upon exceedance of a predetermined threshold
level by the staircase signal, circuit 82 is operative to provide
an output pulse employed to reset counter 88 and multivibrator 86
and also to reset the staircase generator itself. Noise pulses
which occur at a repetition rate greater than the modulation rate
cause the staircase voltage to rise to a level above the threshold
to reset the system. The counter is thus reset before it has
advanced to its final count so that an alarm will not be provided
in response to fast noise inputs. For true signals occurring at the
modulation rate, the staircase voltage does not exceed the
threshold level within the interval of time in which a
predetermined number of signal pulses is received for
processing.
The multivibrator 86 specifies the maximum time in which the
predetermined number of pulses must be received in order to provide
an output pulse to the alarm circuitry. For purposes of
illustration, 32 pulses are employed in the present embodiment to
cause alarm actuation. The output signal from multivibrator 86 is
depicted in FIG. 10E and is effectively a long gate which commences
upon receipt of the first pulse from the tag circuit and which
terminates at a selected time after the occurrence of the 32nd
pulse. The multivibrator 86 thus has a period longer than 32 times
the modulation rate, and the output pulse thereof is operative to
reset counter 88 to its initial state. The counter will not
generate an output pulse to the alarm circuitry if 32 pulses from
multivibrator 84 are not received within the interval specified by
the gate provided by multivibrator 86. Counter 88 provides an
output pulse, depicted in FIG. 10F, upon receipt of 32 pulses from
multivibrator 84 at the modulation rate, and, as described above,
noise signals will not, statistically, be operative to cause a
counter output for alarm actuation.
A system for phase detection of the presence of the resonant tag
within an applied electromagnetic field is illustrated in FIG. 11.
Signals from the RF front-end (FIG. 1) are applied to a phase
detector 90 such as a phase-locked loop, the output signal of which
is a series of demodulated pulses which occur each time the
transmitted carrier frequency sweeps through the resonant detection
frequency of the tag. The phase shift resulting from the tag being
present is detected by comparing the phase of the received signals
with that of a local oscillator which is phase synchronized to the
transmitted signal. In the phase-locked loop, the received signal
is automatically synchronized to the local oscillator and the phase
of the transmitted signal is compared with that of the local
oscillator providing an error signal which represents the
derivitive of the phase error between received and local oscillator
signals.
The output of the phase detector 90, depicted in FIG. 12A,
comprises a signal spectrum 91 which includes the modulation
frequency fm together with noise components 93. Output signals from
detector 90 are applied to a bandpass filter 92 which passes the
signal spectrum 91, removing noise components above and below this
spectrum, as depicted in FIG. 12B. The filtered output of bandpass
filter 92 is applied to cross correlation filter 94 which is
operative in response to spectral energy of predetermined
configuration to provide an output signal, and to provide
effectively no output signal for other input conditions. The output
of the cross correlation filter is typically as shown in FIG. 12C
and is a pulse of predetermined spectral content 95 in response to
a unique spectral input condition. The cross correlation filter,
itself well known in the art, is shown in FIG. 13 and includes a
tapped delay line 96, each output tap of which is connected via a
respective resistor 98 to an input of summing circuit 100. The
resistors 98 are illustrated as adjustable resistors and are each
of a value selected to provide a predetermined weighted signal to
summing circuit 100. In response to input pulses of predetermined
spectral characteristics an output pulse is provided by summing
circuit 100 which is then processed as described above by amplifier
24, pulse shaping circuitry 26 and digital processing circuitry
28.
The apparatus for providing an electromagnetic field for
destruction or alteration of the resonant tag circuit is shown in
FIG. 14 and includes a transmitter 102 operative to provide an
output at the destruction frequency of the tag circuit, and which
is coupled to a balanced loop antenna 104 which is arranged to
provide the destruction field for a tag in the vicinity of this
field. The transmitter 102 is typically of 50 to 100 watts output
power and is matched to the loop antenna 104 by means of a balanced
lead configuration. One output terminal of transmitter 102 is
coupled to a lead of loop antenna 104. The other output terminal of
transmitter 102 is coupled to a lead 106 which follows the path of
the antenna and which is connected to the antenna at a selected
point 108. This lead 106 serves as an impedance matching loop and
follows the configuration of the antenna loop to prevent flux
leakage between the input leads and the associated antenna. The
other end of antenna 104 is grounded via a variable capacitor 110.
The geometry of the input leads provides effectively a tapped
inductor having an intended input impedance to match the output
impedance of transmitter 102 to the input impedance of antenna
104.
In an alternative embodiment of the invention, subcarrier
modulation can be employed such that different subcarrier
frequencies can be provided in systems operating in proximity to
one anoher to prevent crosstalk between such systems. It will also
be appreciated that although the invention finds primary
utilization as a security system for stores and other
establishments of trade, it is also useful in various other
environments. For example, the system can be employed for automatic
identification by use of a multi-resonant tag circuit such as
described above but which does not include a fusible link. The
resonant frequencies of the tag are selected to be within a
frequency range swept by an interrogating signal. The resonant
frequencies of the interrogated tag are sensed by a receiver such
as described above but operative at each resonant frequency, the
detection of all tag frequencies being employed to identify the
presence of the tag in a prescribed area. It should be evident that
the invention can be implemented by various circuits other than
those disclosed to accomplish the intended purpose. Accordingly, it
is not intended to limit the invention by what has been
particularly shown and described except as indicated in the
appended claims.
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