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Radio signalling systems

  • US 4,162,448 A
  • Filed: 03/16/1977
  • Issued: 07/24/1979
  • Est. Priority Date: 03/16/1977
  • Status: Expired due to Term
First Claim
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1. A radio signalling system including a number of portable tokens each capable of transmitting signals in the form of a carrier radio frequency which is common to the tokens, with spaced modulation pulses superimposed on the carrier and defining a carrier burst between successive of said pulses, the number of carrier frequency cycles in a carrier burst between successive modulation pulses being different for different tokens, and a receiver capable of distinguishing between received signals from tokens with different characteristic numbers of cycles in their bursts, said receiver comprising a counter of clock pulses derived from the cycles in the burst of received carrier frequency and a comparator of the count achieved by the counter which comparator is arranged to compare the achieved count with a pre-set count identifying tokens to which access is to be given to a security area, wherein said comparator is arranged to compare the achieved count with the count achieved in the preceding burst of carrier frequency received.

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