CA1073076A - Beat frequency interference rejection circuitry - Google Patents
Beat frequency interference rejection circuitryInfo
- Publication number
- CA1073076A CA1073076A CA278,050A CA278050A CA1073076A CA 1073076 A CA1073076 A CA 1073076A CA 278050 A CA278050 A CA 278050A CA 1073076 A CA1073076 A CA 1073076A
- Authority
- CA
- Canada
- Prior art keywords
- frequency
- signal
- beat
- swept
- transmitter
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- G—PHYSICS
- G08—SIGNALLING
- G08B—SIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
- G08B13/00—Burglar, theft or intruder alarms
- G08B13/22—Electrical actuation
- G08B13/24—Electrical actuation by interference with electromagnetic field distribution
- G08B13/2402—Electronic Article Surveillance [EAS], i.e. systems using tags for detecting removal of a tagged item from a secure area, e.g. tags for detecting shoplifting
- G08B13/2465—Aspects related to the EAS system, e.g. system components other than tags
- G08B13/2468—Antenna in system and the related signal processing
- G08B13/2471—Antenna signal processing by receiver or emitter
Abstract
ABSTRACT OF THE DISCLOSURE
A beat frequency interference rejection circuit oper-ative in an electronic security system to inhibit the provision of an alarm indication for the duration of beat interference caused by an interfering transmitter. A high pass filter is employed having a cut-off frequency which substantially blocks all frequency components produced by a resonant tag circuit while passing frequency components caused by the beat processing.
The high pass filter signal is processed to produce an inhibit signal to inhibit alarm production in the presence and for the duration of the beat interference.
A beat frequency interference rejection circuit oper-ative in an electronic security system to inhibit the provision of an alarm indication for the duration of beat interference caused by an interfering transmitter. A high pass filter is employed having a cut-off frequency which substantially blocks all frequency components produced by a resonant tag circuit while passing frequency components caused by the beat processing.
The high pass filter signal is processed to produce an inhibit signal to inhibit alarm production in the presence and for the duration of the beat interference.
Description
FIELD OF THE INVENTION
This invention relates to electronic security systems and more particularly to circuitry therefore to reject beat fre-quencies caused by an interfering transmitter signaI.
BACKGROUND OF I~E INVENTION
An electronic security system is described in U.S.
Patents 3,810,147 and 3,863,244 for detection of the unauthorized removal of items containing a resonant tag circuit. Such system employs a transmitter providing a repetitively swept range of frequencies driving an antenna which generates a swept electro-magnetic field in a zone under surveillance. A resonant tag includes a circuit resonant at a frequency within the swept band and operative in response to the applied field to resonate at its characteristic frequency which is sensed by the receiver , portion of the system and processed to provide an output alarm indication of tag presence in the surveillance zone. The receiv-er includes signal discrimination circuits for distinguishing between an actual tag and spurious signals which could be falsely detected as a tag and therefore cause a false alarm. Preferred signal processing techniques for such electronic security systems are shown in the above-cited patents.
A source of interference is sometimes present in the vicinity of a security system such as described above, which has not been distinguishable by the system from a resonant tag signal. The source of this type of interference is a fixed fre-quency transmitter producing a signal which beats with the swept ~ .
10730~6 frequency of the security system transmitter to cause a beat frequency signal which appears substantially the same as a true tag signal to the signal processing circuits of known systems.
As a result, the spurious signal produced by the fixed frequency transmitter is not discriminated against and a false alarm may result. -SU~RY OF T~E INVENTION
In accordance with the present invention, beat frequen-cy signals caused by an interfering transmitter are separately detected and employed to inhibit the security system for the duration of the beat interference, which is usually a small portion of the sweep time of the system transmitter. Resonant tags tuned to frequencies other than that of the interfering transmitter are still detectable by the security system.
The beat signal caused by interference of an interfer-ing transmitter and the system transmitter contains frequency components which extend from substantially zero frequency up to at least two times the interfering signal frequency, while frequency components of the resonant tag will not extend as high due to the inherently limited Q of the tag circuit. A high pass filter is provided which has a cut-off frequency sufPiciently high to substantially block all frequency components produced by the resonant tag while passing frequency components caused by the beat process. This high pass filter is coupled to the security system detector to receive output signal therefrom and in response to an interfering beat to provide a signal for in-hibiting the alarm for the duration of the beat interference.
,.
1073~)76 In accordance with a specific embodi~ent, there is provided, for use in an electronic security system having a transmitter producing an electromagnetic field at a frequency repetitively swept through a predetermined range, a resonant tag of resonant frequency within the swept range and receiver means to detect any resonant frequency of the tag produced by the swept field and to provide an alarm indication thereof, an interference rejection circuit for discriminating between a resonant tag signal and a spurious signal produced by a fixed frequency transmitter having a frequency within the swept range, said circuit comprising means operative in response to the detector output signal from said receiver means to detect beat frequencies caused by interference of the swept transmitter frequency and said fixed frequency of an interfer-ing transmitter, and means operative upon ~uch detection to produce a signal to inhibit said alarm indication for the duration of the beat interference.
In accordance with a further embodiment, an electronic security system comprises transmitter means for providing an electromagnetic field in a predetermined area at a frequency repetitively swept through a predetermined range. a resonant tag circuit having at least one resonant frequency w~thin the predetermined range of frequencies: receiver means for detect-ing the presence of said at least one resonant frequency from a tag circuit present in the predetermined area: means for providing an output indication of alarm actuation in response to a signal from ~aid receiver means, means operative in response to the detector output signal from said receiver means to detect beat frequencies caused by interference between the swept fre-quency of said transmitter means and a fixed frequency withinsaid swept range produced by an interfering transmitter, and means operative upon such detection to produce a signal to ,1 `
~ ~ -2a--lQ73076 inhibit said alarm indication for the duration of the beat interference.
In accordance with a still further embodiment, an electronic ~ecurity ~ystem comprises transmitter means for providing an electromagnetic field in a predetermined area at a frequency repetitively swept through a predetermined range;
a reqonant tag circuit having at least one resonant frequency within the predetermined range of frequencies, receiver means for detecting the presence of said at least one resonant fre-quency from a tag circuit pre~ent in the predetermined area,means for providing an output indi.cation of alarm actuation in respon~e to a signal from said receiver means: means operative in response to the detector output signal from ~aid receiver mean~ to detect beat frequencie~ cau~ed by interference between the swept frequency of said tran~mitter means and a fixed fre-quency within said ~wept range produced by an interfering transmitter, and means operative upon such detection to produce an output signal indication of beat interference.
In accordance with a still further embodiment, there is provided, for use in an electronic security system having means for providing in a surveillance zone an electromagnetic ~ -field of a frequency which is repetitively swept within a predetermined range, means for detecting the presence of a resonant tag circuit having a resonant frequency within said range, and means for actuating an alarm in the pre~ence of said resonant tag circuit in the surveillance zone, an interference rejection circuit for di9criminating between a -qignal indicating the presence of a re90nant tag circuit in the ~urveillance zone and ~puriou~ signal produced by a fixed frequency transmitter having a frequency within said predetermined range comprising means operative in respon~e to beat frequency ~ignals produced by ~he fixed frequency transmitter signal and the swept fre--2b-quency signal beating in the security system demodulator to provide a beat note signal representative of such beat frequency interference; and means operative in respon~e to said beat note signal to inhibit the alarm of said security sy~tem for the duration of the beat note signal.
-2c-~073~76 DESCRIPTION OF THE DRAWINGS
The inventiGn will be more fully understoad from the fol]owing detailed description taken in conjunction with the accompanying drawings in which:
Fig. 1 is a block diagram representation of an elec-tronic security system employing the invention; and Figs. 2A and 2B are plots of the frequency spectrum of a beat frequency signal and a resonant tag signal respective-ly, useful in illustrating operation of the invention.
DETAILED DESCRIPTION OF THE INVENTION
.
An electronic security system is shown in Fig. 1 and includes a transmitter 10 coupled to an antenna 12 operative to provide an electromagnetic field within a predetermined area to be controlled and which is repetitively swept over an intended frequency range. A receiving antenna 14 at the controlled area receives energy magnetically coupled from antenna 12 and is coupled to an RF front-end 16 which includes an RF band pas~
filter and RF amplifier. The output of the front-end 16 is applied to a detector 18, the output of which is coupled to a high pass filter 20 which is effective to pass only the higher frequency portion of the signal spectrum. The output of filter 20 is applied to a low pass filter 22 which is effective to remove carrier frequency components and high frequency noise.
The output of filter 22 is applied to a signal processor 24 which includes noise rejection circuitry operative to discriminate intended signals from noise and to provide an output signal 1073~)76 representative of the presence of a resonant tag circuit 15 in the controlled area. Such output signals are applied to an alarm 26 or other output utilization apparatus to denote detec-tion of a resonant tag in the controlled area. This system just described is the subject of the above-identified U.S. patents and is operative to detect tag presence in a controlled area and to provide an alarm indication thereof.
An interfering transmitter 28 and associated antenna 29 located in the vicinity of the security system produces a fixed frequency carrier within the swept frequency range of trans-mitter 10 and which can beat with the signal from transmitter 10 as it sweeps past the frequency of transmitter 28. In accor-dance with this invention, beat frequency signals caused by in-terfering transmitter 28 are discriminated against to prevent a false alarm in the presence of such interfering signals. The output signals from detector 18 are applied to a high pass filter 30 which has a cut-off frequency sufficiently high to substan-tially block all frequency components produced by the resonant tag circuit 15, while passing frequency components caused by the beat interference hetween the signals from transmitter 10 and interfering transmitter 28. The output signal from filter 30, which is representative of the beat note inter~erence, is coupled via a video amplifier 32 to a threshold detector 34.
The threshold detector includes a reference threshold defining a signal level below which no inhibiting signal is produced or needed by reason of the small interfering signal levels. Upon exceedance of the threshold level by the signals from video am-plifier 32, the threshold detector 34 provides a signal to multi-vibrator 36 which provides an inhibit pulse of fixed duration, the duration corresponding to the duration of the beat frequency ~ ~ . ...... . . ~
~073076 signal produced by beating of the fixed frequency signal of in-terfering transmitter 28 and the swept frequency signal from transmitter 10. The inhibit pulse is provided to signal proces-sor 24 for the purpose of preventing alarm actuation in the pres-ence of such inhibit pulse. Thus, the system is inhibited from producing a false alarm for the duration of the beat interfer-ence.
The frequency spectrum of the beat frequency interfer-ence is depicted in Fig. 2A and is of gererally uniform amplitude over a broad frequency range extending from zero frequency up to a minimum of twice the carrier frequency of interfering trans-mitter 28. The frequency spectrum provided by the resonance of tag 15 is illustrated in Fig. 2B and is seen to extend from the modulation frequency fm and to decrease in amplitude with increasing frequency, with the higher frequency components of the tag spectrum being much less than the higher frequency com-ponents present in the interfering frequency spectrum shown in Fig. 2A. The high pass filter 30 is provided to have a cut-off frequency above which no significant frequency components are present due to the tag circuit. In the illustrated embodiment, the tag circuit has a high frequency limit of 10 kHz. The cut-off frequency of filter 30 is 17 kHz which substantially block9 all signals produced by the resonant tag while passing frequency components produced by the beat interference.
The processing of the inhibit pulse from multivibrator 36 to prevent energizing of alarm 26 can be accomplished by well known circuitry which typically would include a gate circuit operative in the presence of the inhibit pulse to prevent appli-cation of an energizing signal to alarm 26. It will be appreci-ated that the inhibit signal derived from high pass filter 30 can be produced by different circuit means other than that illus-trated. Preferably but not necessarily the inhibit signal should be of fixed duration to minimize the effects caused by the band-width of the inhibit signal channel.
The invention is not to be limited by what has been shown or described except as what has been indicated in the ap-pended claims.
.,
This invention relates to electronic security systems and more particularly to circuitry therefore to reject beat fre-quencies caused by an interfering transmitter signaI.
BACKGROUND OF I~E INVENTION
An electronic security system is described in U.S.
Patents 3,810,147 and 3,863,244 for detection of the unauthorized removal of items containing a resonant tag circuit. Such system employs a transmitter providing a repetitively swept range of frequencies driving an antenna which generates a swept electro-magnetic field in a zone under surveillance. A resonant tag includes a circuit resonant at a frequency within the swept band and operative in response to the applied field to resonate at its characteristic frequency which is sensed by the receiver , portion of the system and processed to provide an output alarm indication of tag presence in the surveillance zone. The receiv-er includes signal discrimination circuits for distinguishing between an actual tag and spurious signals which could be falsely detected as a tag and therefore cause a false alarm. Preferred signal processing techniques for such electronic security systems are shown in the above-cited patents.
A source of interference is sometimes present in the vicinity of a security system such as described above, which has not been distinguishable by the system from a resonant tag signal. The source of this type of interference is a fixed fre-quency transmitter producing a signal which beats with the swept ~ .
10730~6 frequency of the security system transmitter to cause a beat frequency signal which appears substantially the same as a true tag signal to the signal processing circuits of known systems.
As a result, the spurious signal produced by the fixed frequency transmitter is not discriminated against and a false alarm may result. -SU~RY OF T~E INVENTION
In accordance with the present invention, beat frequen-cy signals caused by an interfering transmitter are separately detected and employed to inhibit the security system for the duration of the beat interference, which is usually a small portion of the sweep time of the system transmitter. Resonant tags tuned to frequencies other than that of the interfering transmitter are still detectable by the security system.
The beat signal caused by interference of an interfer-ing transmitter and the system transmitter contains frequency components which extend from substantially zero frequency up to at least two times the interfering signal frequency, while frequency components of the resonant tag will not extend as high due to the inherently limited Q of the tag circuit. A high pass filter is provided which has a cut-off frequency sufPiciently high to substantially block all frequency components produced by the resonant tag while passing frequency components caused by the beat process. This high pass filter is coupled to the security system detector to receive output signal therefrom and in response to an interfering beat to provide a signal for in-hibiting the alarm for the duration of the beat interference.
,.
1073~)76 In accordance with a specific embodi~ent, there is provided, for use in an electronic security system having a transmitter producing an electromagnetic field at a frequency repetitively swept through a predetermined range, a resonant tag of resonant frequency within the swept range and receiver means to detect any resonant frequency of the tag produced by the swept field and to provide an alarm indication thereof, an interference rejection circuit for discriminating between a resonant tag signal and a spurious signal produced by a fixed frequency transmitter having a frequency within the swept range, said circuit comprising means operative in response to the detector output signal from said receiver means to detect beat frequencies caused by interference of the swept transmitter frequency and said fixed frequency of an interfer-ing transmitter, and means operative upon ~uch detection to produce a signal to inhibit said alarm indication for the duration of the beat interference.
In accordance with a further embodiment, an electronic security system comprises transmitter means for providing an electromagnetic field in a predetermined area at a frequency repetitively swept through a predetermined range. a resonant tag circuit having at least one resonant frequency w~thin the predetermined range of frequencies: receiver means for detect-ing the presence of said at least one resonant frequency from a tag circuit present in the predetermined area: means for providing an output indication of alarm actuation in response to a signal from ~aid receiver means, means operative in response to the detector output signal from said receiver means to detect beat frequencies caused by interference between the swept fre-quency of said transmitter means and a fixed frequency withinsaid swept range produced by an interfering transmitter, and means operative upon such detection to produce a signal to ,1 `
~ ~ -2a--lQ73076 inhibit said alarm indication for the duration of the beat interference.
In accordance with a still further embodiment, an electronic ~ecurity ~ystem comprises transmitter means for providing an electromagnetic field in a predetermined area at a frequency repetitively swept through a predetermined range;
a reqonant tag circuit having at least one resonant frequency within the predetermined range of frequencies, receiver means for detecting the presence of said at least one resonant fre-quency from a tag circuit pre~ent in the predetermined area,means for providing an output indi.cation of alarm actuation in respon~e to a signal from said receiver means: means operative in response to the detector output signal from ~aid receiver mean~ to detect beat frequencie~ cau~ed by interference between the swept frequency of said tran~mitter means and a fixed fre-quency within said ~wept range produced by an interfering transmitter, and means operative upon such detection to produce an output signal indication of beat interference.
In accordance with a still further embodiment, there is provided, for use in an electronic security system having means for providing in a surveillance zone an electromagnetic ~ -field of a frequency which is repetitively swept within a predetermined range, means for detecting the presence of a resonant tag circuit having a resonant frequency within said range, and means for actuating an alarm in the pre~ence of said resonant tag circuit in the surveillance zone, an interference rejection circuit for di9criminating between a -qignal indicating the presence of a re90nant tag circuit in the ~urveillance zone and ~puriou~ signal produced by a fixed frequency transmitter having a frequency within said predetermined range comprising means operative in respon~e to beat frequency ~ignals produced by ~he fixed frequency transmitter signal and the swept fre--2b-quency signal beating in the security system demodulator to provide a beat note signal representative of such beat frequency interference; and means operative in respon~e to said beat note signal to inhibit the alarm of said security sy~tem for the duration of the beat note signal.
-2c-~073~76 DESCRIPTION OF THE DRAWINGS
The inventiGn will be more fully understoad from the fol]owing detailed description taken in conjunction with the accompanying drawings in which:
Fig. 1 is a block diagram representation of an elec-tronic security system employing the invention; and Figs. 2A and 2B are plots of the frequency spectrum of a beat frequency signal and a resonant tag signal respective-ly, useful in illustrating operation of the invention.
DETAILED DESCRIPTION OF THE INVENTION
.
An electronic security system is shown in Fig. 1 and includes a transmitter 10 coupled to an antenna 12 operative to provide an electromagnetic field within a predetermined area to be controlled and which is repetitively swept over an intended frequency range. A receiving antenna 14 at the controlled area receives energy magnetically coupled from antenna 12 and is coupled to an RF front-end 16 which includes an RF band pas~
filter and RF amplifier. The output of the front-end 16 is applied to a detector 18, the output of which is coupled to a high pass filter 20 which is effective to pass only the higher frequency portion of the signal spectrum. The output of filter 20 is applied to a low pass filter 22 which is effective to remove carrier frequency components and high frequency noise.
The output of filter 22 is applied to a signal processor 24 which includes noise rejection circuitry operative to discriminate intended signals from noise and to provide an output signal 1073~)76 representative of the presence of a resonant tag circuit 15 in the controlled area. Such output signals are applied to an alarm 26 or other output utilization apparatus to denote detec-tion of a resonant tag in the controlled area. This system just described is the subject of the above-identified U.S. patents and is operative to detect tag presence in a controlled area and to provide an alarm indication thereof.
An interfering transmitter 28 and associated antenna 29 located in the vicinity of the security system produces a fixed frequency carrier within the swept frequency range of trans-mitter 10 and which can beat with the signal from transmitter 10 as it sweeps past the frequency of transmitter 28. In accor-dance with this invention, beat frequency signals caused by in-terfering transmitter 28 are discriminated against to prevent a false alarm in the presence of such interfering signals. The output signals from detector 18 are applied to a high pass filter 30 which has a cut-off frequency sufficiently high to substan-tially block all frequency components produced by the resonant tag circuit 15, while passing frequency components caused by the beat interference hetween the signals from transmitter 10 and interfering transmitter 28. The output signal from filter 30, which is representative of the beat note inter~erence, is coupled via a video amplifier 32 to a threshold detector 34.
The threshold detector includes a reference threshold defining a signal level below which no inhibiting signal is produced or needed by reason of the small interfering signal levels. Upon exceedance of the threshold level by the signals from video am-plifier 32, the threshold detector 34 provides a signal to multi-vibrator 36 which provides an inhibit pulse of fixed duration, the duration corresponding to the duration of the beat frequency ~ ~ . ...... . . ~
~073076 signal produced by beating of the fixed frequency signal of in-terfering transmitter 28 and the swept frequency signal from transmitter 10. The inhibit pulse is provided to signal proces-sor 24 for the purpose of preventing alarm actuation in the pres-ence of such inhibit pulse. Thus, the system is inhibited from producing a false alarm for the duration of the beat interfer-ence.
The frequency spectrum of the beat frequency interfer-ence is depicted in Fig. 2A and is of gererally uniform amplitude over a broad frequency range extending from zero frequency up to a minimum of twice the carrier frequency of interfering trans-mitter 28. The frequency spectrum provided by the resonance of tag 15 is illustrated in Fig. 2B and is seen to extend from the modulation frequency fm and to decrease in amplitude with increasing frequency, with the higher frequency components of the tag spectrum being much less than the higher frequency com-ponents present in the interfering frequency spectrum shown in Fig. 2A. The high pass filter 30 is provided to have a cut-off frequency above which no significant frequency components are present due to the tag circuit. In the illustrated embodiment, the tag circuit has a high frequency limit of 10 kHz. The cut-off frequency of filter 30 is 17 kHz which substantially block9 all signals produced by the resonant tag while passing frequency components produced by the beat interference.
The processing of the inhibit pulse from multivibrator 36 to prevent energizing of alarm 26 can be accomplished by well known circuitry which typically would include a gate circuit operative in the presence of the inhibit pulse to prevent appli-cation of an energizing signal to alarm 26. It will be appreci-ated that the inhibit signal derived from high pass filter 30 can be produced by different circuit means other than that illus-trated. Preferably but not necessarily the inhibit signal should be of fixed duration to minimize the effects caused by the band-width of the inhibit signal channel.
The invention is not to be limited by what has been shown or described except as what has been indicated in the ap-pended claims.
.,
Claims (10)
1. For use in an electronic security system having a transmitter producing an electromagnetic field at a frequency repetitively swept through a predetermined range, a resonant tag of resonant frequency within the swept range and receiver means to detect any resonant frequency of the tag produced by the swept field and to provide an alarm indication thereof, an interference rejection circuit for discriminating between a resonant tag signal and a spurious signal produced by a fixed frequency transmitter having a frequency within the swept range, said circuit comprising:
means operative in response to the detector output.
signal from said receiver means to detect beat frequencies caused by interference of the swept transmitter frequency and said fixed frequency of an interfering transmitter; and means operative upon such detection to produce a sig-nal to inhibit said alarm indication for the duration of the beat interference.
means operative in response to the detector output.
signal from said receiver means to detect beat frequencies caused by interference of the swept transmitter frequency and said fixed frequency of an interfering transmitter; and means operative upon such detection to produce a sig-nal to inhibit said alarm indication for the duration of the beat interference.
2. The invention of claim 1 wherein said beat frequen-cy detection means includes high pass filter means having a cut-off frequency sufficient to substantially block all frequency components produced by the resonant tag while passing frequency components caused by the beat interference.
3. The invention of claim 2 wherein said alarm inhib-iting means includes circuit means operative in response to the output signal from said high pass filter means to produce an inhibit signal of a duration corresponding to the duration of the beat interference.
4. 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 resonant tag circuit having at least one resonant frequency within the predetermined range of frequencies;
receiver means for detecting the presence of said at least one resonant frequency from a tag circuit present in the predetermined area;
means for providing an output indication of alarm actua-tion in response to a signal from said receiver means;
means operative in response to the detector output signal from said receiver means to detect beat frequencies caused by interference between the swept frequency of said trans-mitter means and a fixed frequency within said swept range produced by an interfering transmitter; and means operative upon such detection to produce a signal to inhibit said alarm indication for the duration of the beat interference.
transmitter means for providing an electromagnetic field in a predetermined area at a frequency repetitively swept through a predetermined range;
a resonant tag circuit having at least one resonant frequency within the predetermined range of frequencies;
receiver means for detecting the presence of said at least one resonant frequency from a tag circuit present in the predetermined area;
means for providing an output indication of alarm actua-tion in response to a signal from said receiver means;
means operative in response to the detector output signal from said receiver means to detect beat frequencies caused by interference between the swept frequency of said trans-mitter means and a fixed frequency within said swept range produced by an interfering transmitter; and means operative upon such detection to produce a signal to inhibit said alarm indication for the duration of the beat interference.
5. The invention of claim 4 wherein said beat frequen-cy detection means includes high pass filter means having a cut-off frequency sufficient to substantially block all frequency components produced by the resonant tag while passing frequency components caused by the beat interference.
6. The invention of claim 5 wherein said inhibit sig-nal producing means includes:
Threshold means operative in response to a signal from said high pass filter means of a magnitude above a predetermined threshold level to produce an inhibit signal.
Threshold means operative in response to a signal from said high pass filter means of a magnitude above a predetermined threshold level to produce an inhibit signal.
7. The invention of claim 5 wherein said inhibit sig-nal producing means includes:
threshold means operative in response to a signal from said high pass filter means of a magnitude above a predetermined threshold level to produce a signal;
circuit means operative in response to the signal from said threshold means to provide an inhibit pulse of fixed duration corresponding to the duration of the beat frequency signal produced by beating of the interfering transmitter signal and the swept signal of said transmitter means.
threshold means operative in response to a signal from said high pass filter means of a magnitude above a predetermined threshold level to produce a signal;
circuit means operative in response to the signal from said threshold means to provide an inhibit pulse of fixed duration corresponding to the duration of the beat frequency signal produced by beating of the interfering transmitter signal and the swept signal of said transmitter means.
8. 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 resonant tag circuit having at least one resonant frequency within the predetermined range of frequencies;
receiver means for detecting the presence of said at least one resonant frequency from a tag circuit present in the predetermined area;
means for providing an output indication of alarm actuation in response to a signal from said receiver means;
means operative in response to the detector output sig-nal from said receiver means to detect beat frequencies caused by interference between the swept frequency of said transmitter means and a fixed frequency within said swept range produced by an interfering transmitter; and means operative upon such detection to produce an out-put signal indication of beat interference.
transmitter means for providing an electromagnetic field in a predetermined area at a frequency repetitively swept through a predetermined range;
a resonant tag circuit having at least one resonant frequency within the predetermined range of frequencies;
receiver means for detecting the presence of said at least one resonant frequency from a tag circuit present in the predetermined area;
means for providing an output indication of alarm actuation in response to a signal from said receiver means;
means operative in response to the detector output sig-nal from said receiver means to detect beat frequencies caused by interference between the swept frequency of said transmitter means and a fixed frequency within said swept range produced by an interfering transmitter; and means operative upon such detection to produce an out-put signal indication of beat interference.
9. For use in an electronic security system having means for providing in a surveillance zone an electromagnetic field of a frequency which is repetitively swept within a predetermined range, means for detecting the presence of a resonant tag circuit having a resonant frequency within said range, and means for actuating an alarm in the presence of said resonant tag circuit in the surveillance zone, an interference rejection circuit for discriminating between a signal indicating the pres-ence of a resonant tag circuit in the surveillance zone and spurious signal produced by a fixed frequency transmitter having a frequency within said predetermined range comprising:
means operative in response to beat frequency signals produced by the fixed frequency transmitter signal and the swept frequency signal beating in the security system demodulator to provide a beat note signal representative of such beat frequency interference; and means operative in response to said beat note signal to inhibit the alarm of said security system for the duration of the beat note signal.
means operative in response to beat frequency signals produced by the fixed frequency transmitter signal and the swept frequency signal beating in the security system demodulator to provide a beat note signal representative of such beat frequency interference; and means operative in response to said beat note signal to inhibit the alarm of said security system for the duration of the beat note signal.
10. The invention of claim 9 wherein said beat note signal means includes a high pass filter having a cut-off fre-quency sufficiently high to substantially block all frequency components produced by the resonant tag circuit while passing frequency components caused by the beat interference.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US05/777,524 US4117466A (en) | 1977-03-14 | 1977-03-14 | Beat frequency interference rejection circuit |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1073076A true CA1073076A (en) | 1980-03-04 |
Family
ID=25110489
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA278,050A Expired CA1073076A (en) | 1977-03-14 | 1977-05-10 | Beat frequency interference rejection circuitry |
Country Status (13)
| Country | Link |
|---|---|
| US (1) | US4117466A (en) |
| JP (2) | JPS53113492A (en) |
| AU (1) | AU511099B2 (en) |
| CA (1) | CA1073076A (en) |
| DE (1) | DE2725169C2 (en) |
| DK (1) | DK147169C (en) |
| ES (1) | ES459235A1 (en) |
| FR (1) | FR2384306A1 (en) |
| GB (1) | GB1580412A (en) |
| IT (1) | IT1082816B (en) |
| MX (1) | MX143644A (en) |
| SE (1) | SE427882B (en) |
| ZA (1) | ZA772858B (en) |
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|---|---|---|---|---|
| US4484182A (en) * | 1981-07-01 | 1984-11-20 | Shorrock Developments Limited | Intruder detection apparatus for functioning free of disturbance while in close proximity to high-power pulse-moldulated radars |
| US4429302A (en) * | 1981-10-08 | 1984-01-31 | I. D. Engineering, Inc. | Electronic security system with noise rejection |
| NL8202951A (en) * | 1982-07-21 | 1984-02-16 | Nedap Nv | ABSORPTION DETECTION SYSTEM. |
| US4595915A (en) * | 1984-02-06 | 1986-06-17 | Mrs. Lawrence Israel | Electronic surveillance system employing the doppler effect |
| US4565996A (en) * | 1984-02-06 | 1986-01-21 | Mrs. Lawrence Israel | Range limited coherent frequency doppler surveillance system |
| US4728938A (en) * | 1986-01-10 | 1988-03-01 | Checkpoint Systems, Inc. | Security tag deactivation system |
| NL8600738A (en) * | 1986-03-24 | 1987-10-16 | Nedap Nv | Suppression of false alarms due to touch. |
| US4779077A (en) * | 1987-04-13 | 1988-10-18 | Lichtblau G J | Continuously armed high reliability pulse train processor |
| US5508684A (en) * | 1995-03-02 | 1996-04-16 | Becker; Richard S. | Article tag |
| GB9702242D0 (en) * | 1997-02-04 | 1997-03-26 | Plessey Semiconductors Ltd | Alarm sensor and antenna arrangement |
| US9590761B2 (en) | 2011-09-23 | 2017-03-07 | Commscope Technologies Llc | Detective passive RF components using radio frequency identification tags |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3696379A (en) * | 1970-12-02 | 1972-10-03 | Knogo Corp | Apparatus for article theft detection |
| US3863244A (en) * | 1972-06-14 | 1975-01-28 | Lichtblau G J | Electronic security system having improved noise discrimination |
| US3810147A (en) * | 1971-12-30 | 1974-05-07 | G Lichtblau | Electronic security system |
| US3818472A (en) * | 1972-05-26 | 1974-06-18 | K Mauk | R.f. system for detecting unauthorized travel of articles through a selected zone |
| US3967161A (en) * | 1972-06-14 | 1976-06-29 | Lichtblau G J | A multi-frequency resonant tag circuit for use with an electronic security system having improved noise discrimination |
| NL161904C (en) * | 1973-04-13 | Knogo Corp | THEFT DETECTION SYSTEM. | |
| US3961322A (en) * | 1974-07-02 | 1976-06-01 | Lichtblau G J | Real time signal discrimination circuitry |
-
1977
- 1977-03-14 US US05/777,524 patent/US4117466A/en not_active Expired - Lifetime
- 1977-04-29 SE SE7704988A patent/SE427882B/en not_active IP Right Cessation
- 1977-04-29 DK DK189477A patent/DK147169C/en not_active IP Right Cessation
- 1977-05-10 CA CA278,050A patent/CA1073076A/en not_active Expired
- 1977-05-11 IT IT68071/77A patent/IT1082816B/en active
- 1977-05-12 GB GB20066/77A patent/GB1580412A/en not_active Expired
- 1977-05-13 ZA ZA00772858A patent/ZA772858B/en unknown
- 1977-05-20 AU AU25352/77A patent/AU511099B2/en not_active Expired
- 1977-05-23 JP JP5891677A patent/JPS53113492A/en active Pending
- 1977-05-26 FR FR7716143A patent/FR2384306A1/en active Granted
- 1977-05-27 ES ES459235A patent/ES459235A1/en not_active Expired
- 1977-06-03 DE DE2725169A patent/DE2725169C2/en not_active Expired
- 1977-09-29 MX MX170738A patent/MX143644A/en unknown
-
1983
- 1983-12-27 JP JP1983199111U patent/JPS5937910Y2/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| FR2384306A1 (en) | 1978-10-13 |
| FR2384306B1 (en) | 1983-05-13 |
| MX143644A (en) | 1981-06-17 |
| DK189477A (en) | 1978-09-15 |
| JPS59118085U (en) | 1984-08-09 |
| DK147169C (en) | 1984-10-15 |
| AU2535277A (en) | 1978-11-23 |
| JPS5937910Y2 (en) | 1984-10-20 |
| GB1580412A (en) | 1980-12-03 |
| JPS53113492A (en) | 1978-10-03 |
| ES459235A1 (en) | 1978-08-01 |
| US4117466A (en) | 1978-09-26 |
| IT1082816B (en) | 1985-05-21 |
| SE427882B (en) | 1983-05-09 |
| DE2725169C2 (en) | 1985-08-22 |
| DE2725169A1 (en) | 1978-09-21 |
| DK147169B (en) | 1984-04-30 |
| ZA772858B (en) | 1978-04-26 |
| SE7704988L (en) | 1978-09-15 |
| AU511099B2 (en) | 1980-07-31 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |