System for preventing reading of undesired RF signals

• US 5,504,485 A
• Filed: 07/21/1994
• Issued: 04/02/1996
• Est. Priority Date: 07/21/1994
• Status: Expired due to Term

• Current Assignee: Amtech Systems LLC (Roper Technologies, Inc.)
• Original Assignee: Amtech Corporation (McClarin Plastics LLC)
• Inventors: Landt, Jeremy A., Speirs, Donald F., Koelle, Alfred R.
• Primary Examiner(s): Sotomayor, John B.
• Application Number:
  US08/278,276
• Time in Patent Office: 621 Days
• Field of Search: 342/42, 342/44, 342/50, 342/51
• US Class Current: 342/42
• CPC Class Codes: G01S 13/758 using a signal generator powered by the interrogation signal
• CPC Class Codes: G06K 7/0008 General problems related to the reading of electronic memory record carriers, independent of its reading method, e.g. power transfer

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.

Claims

1. In combination in a system including a reader and a transponder displaced from the reader for obtaining information associated with the transponder,

means at the reader for generating a low-frequency signal;

means at the reader for varying the frequency of the low-frequency signal in a predetermined pattern of frequency variations individual to the reader;

means disposed at the transponder and responsive to the low frequency signal for supplying a transponder clock signal which contains a predetermined pattern of frequency variations which relates to the predetermined pattern of frequency variations individual to a reader;

means disposed at the transponder and responsive to the transponder clock signal for producing an RF signal;

means at the reader for receiving and decoding the RF signal;

means at the reader for extracting the transponder clock signal from the received RF signal and for identifying frequency variations in the transponder clock signal;

means at the reader for comparing the frequency variations in the extracted transponder clock signal with the predetermined pattern of frequency variations in the low-frequency signal to verify whether or not the received RF signal is from a transponder whose clock signal has been supplied by itself.

3. The combination set forth in claim 1, further comprising:

means disposed at the transponder and responsive to the low frequency signal for supplying power to the transponder.

2. In combination in a system including a reader and a transponder displaced from the reader for obtaining information associated with the transponder,

means at the reader for generating a low-frequency signal;

means at the reader for varying the frequency of the low-frequency signal in a predetermined pattern individual to the reader;

means disposed at the transponder and responsive to the low frequency signal for supplying a transponder clock signal;

means disposed at the transponder and responsive to the transponder clock signal for producing an RF signal at a frequency higher than the frequency of the low-frequency signal in a pattern individual to the transponder;

means at the reader for receiving the higher frequency RF signal from the transponder;

means at the reader for generating a message decoding clock signal;

means at the reader and responsive to the message decoding clock signal for decoding the received higher frequency RF signal.

4. The combination set forth in claim 2, further comprising:

means disposed at the transponder and responsive to the low frequency signal for supplying power to the transponder.