Antenna driving device for transponder

#The present invention relates to an antenna driving device for a transponder for generating a magnetic field which is magnetically coupled to a transponder incorporated and used in an automobile key.

#A transponder is actuated by electromagnetic energy in a radio signal generated by an interrogator and transmits various data to the interrogator, for example, as disclosed in Japanese unexamined patent publication JP-A 5-281347(1993). With a transponder incorporated in an automobile key, the key can be automatically identified, which is useful in preventing the automobile from being stolen. FIG. 10 shows a fundamental configuration associated with such a transponder. In an interrogator, an antenna coil 1 and a resonant capacitor 2 are connected in series, and a carrier signal source 4 is connected thereto through a switch 3. When the switch 3 is closed, a magnetic field 5 is generated at the antenna coil 1 in accordance with a signal from the carrier signal source 4 to excite an antenna coil 7 of a transponder 6. The transponder 6 accumulates energy from the magnetic field 5 as electric power by a receiving circuit 8, and transmits a predetermined certain identification code and the like from the antenna coil 7 to the antenna coil 1 using the accumulated electric energy.

#In upper and lower halves of FIG. 11, there are respectively shown an electrical configuration of a driving circuit which drives the antenna coil 1 to supply electric power to the transponder 6, and signal waveforms during control over the magnetically coupled transponder as shown in FIG. 10. The switch 3 consists of an NPN transistor 9 and a PNP transistor 10. When a carrier signal from the carrier signal source 4 is supplied as a rectangular wave having an oscillation waveform at a frequency which is close to the resonant frequency of the antenna coil 1 and the resonant capacitor 2, the transistor 9 and the transistor 10 alternately conduct to provide a waveform which resembles a sine wave at a point S. The supply voltage Vcc is on the order of 5 V, and a current of, for example, 200 mA flows to the series resonant circuit of the antenna coil 1 and the resonant capacitor 2. A magnetically coupled transponder is provided with a halt period during which the carrier signal is turned off and a signal from the transponder is received or data are transmitted to the transponder by turning the carrier signal on and off.

#FIG. 1 is an electrical circuit diagram and waveform diagrams showing a fundamental configuration of an embodiment of the invention;

#FIG. 2 is an electrical circuit diagram showing an electrical configuration of the embodiment in FIG. 1;

#FIG. 3 is a flowchart showing an operation of a microcomputer shown in FIG. 2;

#FIG. 1 shows a basic configuration of an embodiment of the invention in the upper half and operational waveforms of constituent parts thereof in the lower half. In an interrogator, a DC power supply 20 can constitute a closed loop configured by being connected to a series resonant circuit including an antenna coil 21 and a resonant capacitor 22 between Vcc as one output and GND as another output. A one switching element 23 is connected between one end of the series resonant circuit and the Vcc of the DC power supply 20, and another end of the resonant circuit is grounded to the GND. An another switching element 24 is connected between the one end of the resonant circuit and the GND of the DC power supply. The one switching element 23 and the another switching element 24 are implemented using semiconductor switching elements such as bipolar transistors and FETs.

#The one switching element 23 is controlled such that it is alternately in ON and OFF states as indicated by the operational waveform S1. The another switching element 24 is controlled to be in the phase opposite to that of the one switching element 23 as indicated by the operational waveform S2. Thus, an output waveform like a sine wave is obtained at a point S. The antenna coil 21 outputs a magnetic field 25 corresponding to the waveform at the point S. This magnetic field 25 is received by an antenna coil 27 of a transponder 26 to be used as electric power to operate a receiving circuit 28.

#When both of the one switching element 23 and another switching element 24 are put in a cutoff state which is an OFF state at the interrogator, the resonant circuit formed by the antenna coil 21 and the resonant capacitor 22 no longer forms a closed loop, which results in rapid attenuation of the output waveform at the point S. Thus, a repeat cycle in transmitting data to the transponder 26 by repeating the continuation and halt of the carrier signal for control can be shortened, thereby increasing the data transmission speed.