Near field communications device
View Patent ↗The present application relates to a near field communications (NFC) device having an antenna which is coupled to a load. When in the presence of an active NFC reader or similar device, the antenna generates an electromotive force (EMF), which serves as an input signal to the load. A variable potential divider is included to vary the voltage of the input signal to the load, so that if a potentially damaging over-voltage condition exists the voltage of the input signal to the load can be reduced to a safe level. The variable potential divider may be composed of an effective source impedance and an effective impedance of the load, and the impedance of the impedance matching network may be variable by means of a variable capacitance and/or a variable resistance.
1. A near field communications (NFC) device comprising an antenna for generating an electromotive force (EMF) and a load, the antenna and the load being coupled such that the EMF generated by the antenna provides an input signal for the load, wherein the NFC device further comprises a variable potential divider for varying the voltage of the input signal supplied to the load.
2. An NFC device according to claim 1 wherein the variable potential divider is made up of an effective impedance of the load and an effective impedance of a source of the EMF.
3. An NFC device according to claim 2 further comprising an impedance matching network for matching an impedance of the antenna to an impedance of the load, such that the effective impedance of the source of the EMF is dependent upon an impedance of the impedance matching network, and wherein the impedance of the impedance matching network is variable.
4. An NFC device according to claim 3 further comprising an amplitude detector for detecting the amplitude of the EMF generated by the antenna and a voltage limiter for limiting the amplitude of the voltage supplied to the load.
5. An NFC device according to claim 4 wherein the voltage limiter comprises a positive voltage limiter for limiting positive voltages and a negative voltage limiter for limiting negative voltages.
6. An NFC device according to claim 4 further comprising a processor and a controller, the processor being configured to determine whether the detected amplitude is within safe limits for the voltage limiter and to cause the controller to adjust the impedance of the impedance matching network to reduce the amplitude of the EMF generated by the antenna if the detected amplitude is outside the safe limits.
7. An NFC device according to claim 6 wherein the impedance matching network comprises a variable capacitance and a variable resistance in parallel.
8. An NFC device according to claim 7 wherein the controller is configured to make coarse adjustments to the impedance of the impedance matching network by adjusting the value of the variable resistance, and to make fine adjustments to the impedance of the impedance matching network by adjusting the value of the variable capacitance.
9. An NFC device according to claim 1 , wherein the NFC device is an NFC tag.