Method and apparatus for protecting a transmission path between a base unit and a mobile key unit
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United States of America Patent
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Oct 22, 2002
Grant Date -
Mar 7, 2002
app pub date -
Jul 3, 2001
filing date -
Jul 4, 2000
priority date (Note) -
In Force
status (Latency Note)
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Abstract
The invention relates to a method and apparatus for protecting a transmission path between a base unit and a mobile key unit in an access control system, in particular for motor vehicles. A carrier signal which is modulated with a user signal is transmitted from a transmitting unit to a receiving unit. According to the invention, a separate reference signal, which has a fixed phase angle with respect to the carrier signal, is also transmitted. The phases of the reference signal and of the carrier signal are compared in a test circuit, and an alarm signal is emitted if a phase difference which varies with time is found.
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