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Encoder/decoder for card entry system

  • US 4,658,252 A
  • Filed: 07/07/1986
  • Issued: 04/14/1987
  • Est. Priority Date: 08/13/1984
  • Status: Expired due to Fees
First Claim
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1. In an access control card entry system, apparatus for storing binary identification key codes and for recovering such key codes, said apparatus comprising:

  • a surface acoustic wave (SAW) delay line device mounted on a control card, said device having an input terminal, an output terminal and a plurality of delay line taps, for imbedding a binary identification key code therein by connecting selected ones of said taps to said output terminals;

    frequency generating means having an output terminal connected to said input terminal of said SAW device, for generating a periodic swept frequency signal varying in frequency from fo to ff and including at least the operating frequency range of said SAW device;

    oscillator means having an output, said oscillator means for providing a frequency reference signal of frequency fo ;

    a first, second, and third frequency conversion means, each having first and second inputs and an output for generating an output signal whose frequency is equal to at least the difference in frequency of signals applied to said first and second inputs;

    said first input of said first frequency conversion means connected to said SAW output terminal, said second input of said first frequency conversion means connected to said output terminal of said frequency generating means, for producing a periodic output signal having a unique frequency component corresponding to each connected tap of said SAW device;

    said first input of said second frequency conversion means connected to said output terminal of said frequency generating means and said second input of said second frequency conversion means connected to said output of said oscillator means, said second frequency conversion means for down converting the frequency of said periodic swept frequency signal;

    said first input of said third frequency conversion means connected to said output of said first frequency conversion means, and said second input of said third frequency conversion means connected to said output of said second frequency conversion means, said third frequency conversion means for generating a periodic pulse waveform with each pulse corresponding to said selected ones of said taps and said pulse waveform having a timed relationship between pulses corresponding to the time delay between said selected ones of said taps;

    means for detecting the presence or absence of pulses in said periodic pulse waveform at each interval of time corresponding to the interval of time between adjacent time delay taps, and means for generating a binary pulse signal corresponding to said binary code.

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