United States Patent  [ii] Patent Number: 4,658,252
Rowe  Date of Patent: Apr. 14, 1987
U.S. Patent Apr. 14,1987 Sheet 1 of3 4,658,252
 ENCODER/DECODER FOR CARD ENTRY SYSTEM
 Inventor: Don H. Rowe, Portola Valley, Calif.
 Assignee: GTE Government Systems
Corporation, Stamford, Conn.
 Appl. No.: 882,438
 Filed: Jul. 7,1986
Related U.S. Application Data
 Continuation of Ser. No. 640,060, Aug. 13, 1984, abandoned.
 Int. CI.* G09C 1/00
 U.S. a 340/825.31; 340/825.34;
 Field of Search 340/825.3-825.39,
340/825.54; 235/435, 439; 343/6.5 R-6.8 LC;
310/313 R, 313 B; 333/193, 150, 30
 References Cited
U.S. PATENT DOCUMENTS
3,344,629 10/1967 Burney 70/280
3,377,616 4/1968 Auer 340/825.54 X
3,568,102 3/1971 Tseng 333/30
3,706,094 12/1972 Cole et al 343/6.55 SS
3,836,876 9/1974 Marshall et al 333/30
4,006,436 2/1977 Heeks 333/30
4,058,217 11/1977 Vaughan et al 209/11
Primary Examiner—Ulysses Weldon
Attorney, Agent, or Firm—Douglas M. Gilbert
A surface acoustic wave (SAW) filter device configured as a tapped time delay is mounted on a plastic card. The SAW filter acts as a nondestructive storage medium for an identification code in an access control system. The identification code is imbedded in the SAW device by selectively connecting certain taps to the filter output terminals. The identification code is read out of the SAW filter device and decoded for verification of the code. To read the code a sweep frequency oscillator excites the filter to launch an acoustic wave. The filter ouput mixes with the input sweep signal and the lower sideband (f*-fa) is extracted to obtain a discrete line spectra each line representing the connected tap of the filter. The line spectra signal is mixed with another version of the input sweep signal to obtain a time-based amplitude response of the discrete line spectra. This analog signal is converted to a digital signal which corresponds to the binary code of the connections to the SAW filter.
22 Claims, 6 Drawing Figures