System for preventing reading of undesired RF signals

Details System for preventing reading of undesired RF signals System for preventing reading of undesired RF signals

1994-07-21

1996-04-02

Sotomayor, John B.

Communications: directive radio wave systems and devices (e.g.,

Radar transponder system

342 44, G01S 1376

Patent

active

055044859

ABSTRACT:
The invention provides a reader-transponder system that prevents reading of undesired transponder signals. The reader generates a low-frequency signal that is used to supply power to the transponder. The same low-frequency signal also is used to generate a clock signal for the transponder. The reader varies the frequency of the low-frequency power signal according to a code that is unique to the reader. Because the transponder clock signal is derived from the encoded low-frequency power signal, it also varies in accordance with the reader's code. The coded transponder clock signal is contained in the modulation of the RF signal produced by the transponder. When the reader receives and decodes the RF signal, it also extracts the transponder clock signal from the received RF signal and compares variations in the extracted transponder clock signal to the code that was applied to the low-frequency power signal transmitted by the reader. If the pattern of variations in the transponder clock signal matches the reader's code, then the reader senses that the RF signal it has received is from a transponder being powered by that reader and no other.

REFERENCES:
patent: 3646580 (1972-02-01), Fuller et al.
patent: 4724427 (1988-02-01), Carroll
patent: 4857893 (1989-08-01), Carroll
patent: 5053774 (1991-10-01), Schuermann et al.
patent: 5166676 (1992-11-01), Milheiser
patent: 5355513 (1994-10-01), Clarke et al.
patent: 5410315 (1995-04-01), Huber

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