Quasi-stationary noise cancellation system
First Claim
Patent Images
1. An electronic security system comprising:
- transmitter means for providing an electromagnetic field in a predetermined area at a frequency respectively swept through a predetermined range of frequencies;
a resonant tag circuit having at least one resonant frequency within said 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;
signal processor means for providing an output indication of alarm actuation in response to a signal from said receiver means; and
noise cancellation means operative to discriminate against spurious signals in said receiver signal arising from a source having the same resonance characteristics as said tag circuit and which are quasi-stationary in time, and provide an output signal to said signal processor means free of said noise.
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Abstract
An electronic security system detects a resonant tag circuit even in the presence of a substantially identical spurious resonance. The spurious resonance is fixed or quasi-stationary in time, in relation to the time that a resonant tag circuit is present in a security system. A representation of the quasi-stationary noise is stored in an electronic memory and is subtracted from a received signal to automatically cancel the interfering noise.
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Citations
16 Claims
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1. An electronic security system comprising:
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transmitter means for providing an electromagnetic field in a predetermined area at a frequency respectively swept through a predetermined range of frequencies; a resonant tag circuit having at least one resonant frequency within said 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; signal processor means for providing an output indication of alarm actuation in response to a signal from said receiver means; and noise cancellation means operative to discriminate against spurious signals in said receiver signal arising from a source having the same resonance characteristics as said tag circuit and which are quasi-stationary in time, and provide an output signal to said signal processor means free of said noise. - View Dependent Claims (2, 3, 4)
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5. For use in an electronic security system having a transmitter producing a repetitively swept frequency band, a resonant tag of resonant frequency within the swept band, receiver means to detect any resonant frequency of the tag produced by the swept field and to provide a signal indication thereof and signal processor means for providing an alarm indication in response to the signal from the receiver means, a noise cancellation circuit operative to discriminate against spurious signals in the receiver signal arising from a source having the same resonance characteristics as the tag and which are quasi-stationary in time, comprising:
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means operative in response to a signal from said receiver means to store a representation of quasi-stationary noise; and means operative to combine said stored representation with the signal from said receiver means to substantially cancel said quasi-stationary noise, and provide a signal to said signal process means free of said noise. - View Dependent Claims (6, 7, 8, 9)
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10. For use in an electronic security system having a transmitter producing a repetitively swept frequency band, a resonant tag of resonant frequency within the swept band, receiver means to detect any resonant frequency of the tag produced by the swept field and to provide a signal indication thereof and signal processor means for providing an alarm indication in response to the signal from the receiver means, a noise cancellation circuit operative to discriminate against spurious signals in the receiver signal arising from a source having the same resonance characteristics as the tag and which are quasi-stationary in time, comprising:
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analog-to-digital converter means operative in response to the signal from said receiver means to provide a digital signal representation of quasi-stationary noise; digital memory means for storing data in accordance with said digital signal representation; digital-to-analog converter means operative in response to stored data in said memory means to provide an analog signal representation thereof; means for synchronizing the data entered into said memory means with the received signal from said transmitter; and means operative in response to the signal from said receiver means and the analog signal from said digital-to-analog converter means to cancel said quasi-stationary noise and provide an output signal to said signal processor means free of said noise. - View Dependent Claims (11, 12, 13, 14, 15, 16)
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Specification