EP0707295B1 - Electronic article surveillance system - Google Patents

Abstract

An electronic security system to prevent theft of articles uses a hanger (9), to which the article is attached, that is located within an electromagnetic field. The hanger contains an inductance and capacitance coupled as a resonator device and is located between transmitting (8) and receiving (10) aerials. A voltage controlled oscillator (2) is controlled to produce a range of frequencies that are mixed (3) with the output of a second oscillator (18) and a multiplier (19). The receiver output is mixed with a half the frequency of the VCO output and is used in the detector. The presence of the hanger triggers the alarm.

Description

The invention relates to an installation for electronic Article monitoring according to the preamble of claim 1.

Electronic article surveillance systems, the presence one equipped with an oscillating circuit (coil and capacitor) Fuse trailer in a surveillance zone prove and use in particular in sales facilities find are known in the art. Since the respective resonance frequency of the individual fuse tags a manufacturing tolerance of typically ± 10% subject, it is necessary to transmit the frequency via a to vary (sweep) certain area to make sure be able to prove all security trailers. To usually finds a voltage controlled oscillator (VCO) Use that delivers the desired frequency range. Be Input is with a sine, triangle or stair voltage acted upon, and at the exit is one of the Input voltage proportional frequency. Alternatively, you can digital-analog converters are also used for frequency generation. Of the Oscillator is connected to an output stage, which (in Usually in pulse mode) a transmitting antenna feeds in turn generates a magnetic field in the surveillance zone. A separate receiving antenna is used to receive the interference used a security tag in the surveillance zone generated. This antenna is on a receiving circuit connected, the whole swept by the transmitter Frequency range is sensitive, while the output of the Receiver is connected to a detector, which at Presence of a security tag in the surveillance zone triggers an alarm so that the personnel of the Sales facility or the like prevent theft if necessary can. The detection is usually carried out in such a way that if the received power changes - as a result of the Presence of a surveillance trailer - at one certain transmission frequency, i.e. constant time position an alarm is given during the sweep cycles.

A system of the generic type is known from EP 565 481 A1 known. On the one hand, the probability of detection the security trailer by increasing the sensitivity to be able to improve, on the other hand, the probability to reduce a false alarm, the Receiver a mixer, whose first input with the Reception coil is connected. The second entrance is with the transmission frequency. The result is on Output of the mixer to a DC voltage signal, the in the presence of a security tag in the surveillance zone changes. This voltage becomes a bandpass filter and an amplifier and finally an analog-to-digital converter of the detector.

A disadvantage is that the output frequency the mixer must remain relatively low. Low frequencies however, it is difficult to amplify and filter. The The result is high manufacturing costs and a high one Sensitivity to interference signals.

Based on the prior art, the invention is Underlying problem, a system for electronic article surveillance to create, which is characterized by low sensitivity against interference signals and a high probability of detection distinguished.

According to the invention the problem is characterized by Part of claim 1 solved.

The main idea is to reinforce the finally (intermediate) fed to the detection device Generate signal according to the overlay principle. While on first input of the mixer the one received by the antenna the signal corresponding to the transmission frequency is present second input with the sum or difference between the Transmission frequency and a second, fixed frequency acted upon. Because at the exit of the mixer except the two Input frequencies the sum and the difference of the frequencies of the two input signals can be applied in both cases a time constant, independent of the transmission frequency, arbitrarily high and therefore easily amplifiable intermediate frequency be won. If at the second input of the mixer the Sum frequency from the transmission signal and the fixed frequency is at the outlet of the mixer Differential frequency constant over time, i.e. from the wobble frequency independently. The one at the output is analog Total frequency constant if at the second input of the mixer the difference between the transmission frequency and a fixed one Frequency is present. This tension is now the Intermediate frequency amplifier supplied, which is only a narrow Range around the frequency to be amplified while all other frequencies - and therefore also unwanted ones Interference signals - are suppressed.

The advantages of the invention are primarily that the - any high - intermediate frequency easily amplified is, so that a high sensitivity and probability of detection The result is. Because of the selective gain is the sensitivity to interference signals significantly reduced.

Specifically, it proves itself to be used to generate the transmission frequency second mixer as appropriate. The output of the Circuit for generating the variable frequency - that is voltage-controlled oscillator or the digital-to-analog converter - connected to an input of this second mixer, while the other input with a generator fixed frequency. The frequencies of the Generators are obviously chosen such that the at the output of the mixer, the transmitting antenna supplied sum or, which is preferred, difference frequency corresponds to the resonance frequency of the fuse trailer. For Generation of the intermediate frequency of the receiver is the Output of the circuit for generating the variable frequency with the second input of the mixer of the receiver connected.

In principle, it would be conceivable for the generator to have a fixed frequency to work at the intermediate frequency. Then that would Circuit for generating the variable frequency at the Sum or difference frequency from the transmission frequency and the Frequency of the fixed generator oscillate. Because of the high Gain of the intermediate frequency amplifier would be unwanted radiation (and thus interference, possibly even overdrive) in these disadvantageous consequence because the generator fixed frequency exactly at the frequency that the intermediate frequency amplifier operates reinforced. Hence the use of two frequency dividers equal division ratio (or multiplier) suggested. One of them is between the exit of the second mixer and the transmitting antenna switched during the others the output of the variable generation circuit Frequency with the second input of the mixer of the receiver connects. As a result, the fixed frequency generator works now at a multiple (or fraction) of the frequency of the repeater, so that no disturbing Irradiations are to be feared.

It is obvious that any combinations between frequency shift, division ratio and intermediate frequency are conceivable. If the intermediate frequency is designated by f, the division ratio of the frequency dividers by N and the wobble range by s, the circuit for generating variable frequency runs through a frequency range which corresponds to the following expression: N · (f + s), if the difference frequency of the input frequencies of the mixer of the transmitter is fed to the transmitting antenna. The output of the fixed frequency generator then has an output voltage with the frequency N · f on. The difference frequency produced by the transmitter mixer is N · (f + s) - N · f = N · s, because the difference frequency is sent. The transmitter's frequency divider then produces the desired sweep range: Ns / N = s.

The output of the frequency divider connected between the circuit for generating a variable frequency and the mixer of the receiver then provides an output frequency which N (f + s) / N = f + s corresponds. Finally, is the intermediate frequency supplied by the receiver mixer f + s - s = f, the desired intermediate frequency.

The fixed frequency generator can also have a lower frequency Have oscillator, the output signal frequency multiplied becomes. It should be noted that the here relevant multiplication factor - if possible - not with the Division factor of the frequency divider that corresponds to the Mixers of the transmitter or receiver are connected because otherwise the oscillator and intermediate frequency match and thus unwanted radiation into the intermediate frequency amplifier become possible.

Since the presence of a backup trailer in the Monitoring zone not only the amplitude, but also the Phase of the received signal changes is the use a phase-sensitive synchronous detector in the detection device prefers. The first of its two entrances is with the recipient, i.e. the output of the intermediate frequency amplifier, connected while his second Input is connected to the fixed frequency generator. Of the The synchronous detector is thus enabled, the phase difference between the radiated and the received Examine signal. The only available from phase information independent of the respective wobble frequency obtained from the fixed frequency generator.

The synchronous detector is preferably a known, so-called IQ detector for use. He has one Output at which the in-phase component between the and reception signal is present. At a second exit the part of the. shifted by 90 ° relative to the transmission signal received signal. The decision about the trigger An alarm can now be based on two pieces of information (Amplitude and phase) and is less Error probability subject to conventional Systems that only evaluate the amplitude.

In the following, an embodiment of the invention will be described the drawing explained in more detail. It shows in schematic Representation of a system for electronic article surveillance.

The system according to the invention has a digital-to-analog converter as a wobble generator (1), for example Sawtooth, triangle, sine or staircase output signal generated that a voltage controlled oscillator (2) is supplied. Its output frequency is proportional to its input voltage. In the illustrated Embodiment is the frequency of the voltage controlled Oscillator (2) varies over a range that is double Width of the intended in the surveillance zone Frequency range includes. In addition, the output frequency of the voltage controlled oscillator (2) compared to the Variation range of the voltage controlled oscillator (2) around shifted a fixed amount (for example 90 MHz). With a normal electronic article surveillance system the resonance frequency of the fuse tags is 8.2 MHz ± 10%, so that the transmission frequency between 7.4 MHz and 9.0 MHz is to be varied. The wobble generator (1) varies Output frequency over the double range (between 14.8 and 18 MHz), and with an offset (of 90 MHz). The Output frequency of the voltage controlled oscillator varies therefore between 90 + 14.8 MHz (104.8 MHz) and 90 + 18 MHz (108 MHz).

A separate oscillator (18) that operates at 30 MHz and is preferably quartz crystal controlled, controls the tripling Frequency multiplier (19), which is therefore a Frequency of 90 MHz generated. The outputs of the frequency multiplier (19) and the voltage-controlled oscillator (2) are connected to a mixer (3) whose output frequency the input frequencies, their sum and their difference corresponds. The 90 MHz of the Frequency tripler (19), frequencies in the range between 104.8 and 108 MHz, corresponding to the voltage controlled Oscillator (2), frequencies in the range between 194.8 and 198 MHz as the sum frequency and finally frequencies in Range between 14.8 and 18 MHz as the difference frequency between the voltage controlled oscillator (2) and the Frequency tripler (19). The output of the mixer (3) is connected to a low pass filter (4), which frequencies cuts off above 20 MHz. At the output of the low pass filter (4) are therefore only frequencies in the range between 14.8 and 18 MHz, which is the difference in frequency of the voltage controlled oscillator (2) and the frequency tripler (19) correspond.

The output of the low pass filter (4) is at the input of a Frequency divider (5) connected, which halves the frequency. Its output voltage is therefore in the range between 7.4 and 9.0 MHz. However, since the frequency divider (5) also produces higher harmonics of its input frequency, is its output to another low pass filter (6) connected, which only passes frequencies below 9.0 MHz.

The output of the low pass filter (6) then becomes one Power amplifier (7) supplied, the signal to one for operation a transmitting antenna (8) suitable level amplified.

The output of the voltage controlled oscillator (2), the Frequency is in the range between 104.8 and 108 MHz, is also connected to a frequency divider (13) the frequency halved in the range between 52.4 and 54 MHz. Of the Output of the frequency divider (13) is via an (optional) Low pass filter (14), which eliminates the higher harmonics, connected to the mixer (12) of the receiver.

The one picked up by a receiving antenna (10), possibly from a fuse trailer (9) with a resonant circuit originating signals are via a preamplifier (11) also the mixer (12) of the receiver fed. At the outlet of the mixer (12) are therefore Input frequencies, as well as their sum and difference. in the these are specific frequencies in the range between 52.4 and 54 MHz from the low pass filter (14) and frequencies in the range 7.4 to 9 MHz from the preamplifier (11). Since the voltage controlled oscillator (2) for generating both Input frequencies of the mixer (12) is their difference constant. Is the output frequency of the voltage controlled Oscillator (2), for example 104.8 MHz, is the Output of the low pass filter (14) 52.4 MHz and the input frequency of the receiver is 7.4 MHz. The sum of these Frequencies is 59.8 MHz and their difference is 45 MHz. Is the output frequency of the voltage controlled Oscillators (2) analog 108 MHz, are at the output of the low pass filter (14) 54 MHz and at the output of the preamplifier (11) 9.0 MHz. The sum of these frequencies is 63 MHz, while their differences are 45 MHz. Obviously the difference frequency is always 45 MHz, regardless of the output frequency of the voltage controlled oscillator (2). The system according to the invention therefore does what is known Take advantage of the overlay principle.

The output of the mixer (12) of the receiver is at the Intermediate frequency amplifier (15) connected, the only Signals in a narrow band range (in the specific 45 MHz ± 5 kHz) amplified. Because of the combination of the overlay principle with the narrowband nature of the intermediate frequency amplifier (15) all interference frequencies suppresses that more than 10 kHz from the transmit frequency differ. The result is much better suppression of interference signals that lower the detection threshold and thus an improvement in the probability of detection allowed.

Due to the high gain of the intermediate frequency amplifier (15) Care should be taken to ensure that no part of the rest of the system corresponding to its amplification frequency 45 MHz can be generated. This is realized in that the voltage controlled oscillator (2) by twice the Intermediate frequency of 45 MHz, i.e. 90 MHz shifted upwards is generated and twice the wobble bandwidth. Farther the frequency of the fixed oscillator (18) is tripled, so that it never corresponds to the intermediate frequency. Also the one from the voltage-controlled oscillator (2) to the mixer (12) frequency supplied to the receiver does not correspond to any Case of intermediate frequency.

Those skilled in the art will recognize that the process of doubling the Sweep frequency and the addition of an offset with one itself subsequent frequency halving the generation of the Intermediate frequency successful at unwanted points prevents.

It should be noted that the output of the frequency tripler (19) connected to a further frequency doubler (20) which is thus 180 in the illustrated embodiment MHz delivers. This frequency, like that Output voltage of the intermediate frequency amplifier (15), the Synchronous detector (16) supplied. The latter internally shares the from the frequency doubler (20) supplied 180 MHz by 4 µm the in-phase and quadrature information at its outputs between the sent and the received To provide signals. At an output of the synchronous detector (16), the in-phase part lies between Send and receive signal on, while at the other output Quadrature component - that is to say by 90 ° relative to the transmitted signal moved - is present. The amplitude of the received Signals is known to be the square root of the sum the squares of both output voltages of the synchronous detector (16). These two voltages are a circuit (17) supplied when certain thresholds are exceeded triggers an alarm.

Since the phase of the emitted by the transmitting antenna (8) Signals through the phase of the voltage controlled oscillator (2) and that of the frequency multiplier (19) determined and the phase of the two outputs of the synchronous detector from the voltage controlled oscillator (2) depends, any change in phase and / or amplitude of the received signal - due to the presence of a fuse trailer (9) in the surveillance zone - detected using both outputs of the synchronous detector (16) be, so that the system according to the invention in the result a much better distinction between Fuse tags, other objects and faults enables.

Claims (6)

1. An apparatus for electronic article surveillance, with a transmitter emitting an electromagnetic field into a zone subject to surveillance, and with a receiver which is associated with the transmitter and whose output signal is supplied to a detecting device activating an alarm signal when the presence of a security tag (9) in the surveillance zone is established, wherein the security tag (9) includes an oscillator circuit comprised of an inductor and a capacitor, wherein the frequency of the transmitter is varied over a range covering the resonant frequency of the oscillator circuit of the security tag (9), and wherein the receiver includes a mixer (12) whose first input is connected to a receive antenna (10) and whose second input is in communication with the transmitter, characterized in that a signal resides at the second input of the mixer (12) whose frequency corresponds to the sum of, or the difference between, the transmit frequency and a fixed frequency, and that an intermediate-frequency amplifier (15) is connected to the output of the mixer (12) whose output is connected to the detecting device and which amplifies only a narrow band range by a frequency that corresponds to the difference between, or the sum of, the two input frequencies of the mixer (12).
2. The apparatus as claimed in claim 1,
   characterized in that the output of a circuit for generating the variable frequency, in particular a voltage-controlled oscillator (2) or a digital-to-analog converter (1), is connected to a second mixer (3) which controls the transmit antenna (8) and whose second input is connected to a generator of the fixed frequency, and that the output of the circuit (2) for generating the variable frequency is connected to the second input of the mixer (12) of the receiver.
3. The apparatus as claimed in claim 2,
   characterized in that a frequency divider (5) is connected between the output of the second mixer (3) and the transmit antenna (8), and that a further frequency divider (13) is connected between the output of the circuit (2) for generating the variable frequency and the second input of the mixer (12) of the receiver.
4. The apparatus as claimed in claim 2 or 3,
   characterized in that the generator of the fixed frequency includes an oscillator (18) whose output signal is supplied to a frequency multiplier (19).
5. The apparatus as claimed in any one of the claims 1 to 4,
   characterized in that the detecting device includes a phase-sensitive synchronous detector (16) whose first input is connected to the output of the intermediate-frequency amplifier (15) and whose second input is connected to the generator of the fixed frequency.
6. The apparatus as claimed in claim 5,
   characterized in that the synchronous detector (16) includes an output at which the equiphase component between transmit and receive signal resides, and that it includes an output at which the fraction of the receive signal shifted through 90° relative to the transmit signal resides.

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