Walton in U.S. Pat. No. 3,752,960 issued Aug. 14, 1973, U.S. Pat. No. 4,223,830 issued Sept. 23, 1980, U.S. Pat. No. 4,236,068 issued Nov. 25, 1980, and U.S. Pat. No. 4,388,524 issued June 14, 1983 and Kip et al. in U.S. Pat. No. 4,196,418 issued Apr. 1, 1980 have disclosed examples of credit card identifiers in which the identification is obtained by placing the card near a reader station. The user need not remove the card from the wallet, handbag, or the like and readinng occurs in a fraction of a second. Each card bears a unique code with the coding typically being entered at the time of manufacture, although in some circumstances field change of the code is possible. In some systems the number of available codes is limited. Typical uses of such identifiers include access control, personal banking and transaction identification, and vehicle and freight car identification.
Such identification systems require the card identifiers to transmit a signal to the reader or otherwise emit an identification code which can be detected by the reader. This signal generation in some cases has been effected by incorporating a battery in the card to power a signal generating circuit. In other instances the identifier derives its power from the interrogating signal and utilizes that power to energize a radio frequency oscillator and signal generator for generating a signal which in turn is radiated back to the reader for identification purposes. It is desirable that the power requirements from the reader to the card be minimal so that (a) the radio frequency radiation from the reader has minimum impact on other users of the radio frequency spectrum and thereby minimizes the possibility of governmental objection to the system, (b) the reader can be maintained compact and inexpensive, and (c) the distance at which the card can be energized and read can be greater.
It is the purpose of the present invention to provide a proximity system with a reader and portable identifier combination which is simple in design, has a large code population, wherein the identifier can readily be powered by the signal from the reader, and in which the power requirements of the card are minimal.