Wireless sensor including an RF signal circuit
A radio frequency identification (RFID) tag includes an antenna structure, a tank circuit, and a processing module that includes a tuning circuit. The processing module requires a processing module power level to operate according to a protocol. The tuning circuit requires a tuning circuit power level to adjust input impedance. The processing module power level is greater than the tuning circuit level. The processing module measures a first power level from a received RF signal. When the first power level is greater than the tuning circuit power level but less than the processing module power level, the processing module adjusts the input impedance to increase efficiency of power measurement and measures a second power level from the received RF signal. When and the second power level is greater than the processing module power level, the processing module uses the second power level to operate according to the protocol.
1. A radio frequency identification (RFID) tag comprises:
an antenna structure operable to receive a radio frequency (RF) signal, wherein the antenna structure includes a sensing element, an antenna, and a tuning circuit, and wherein exposure to an environmental condition causes a change in input impedance of the antenna structure while receiving the RF signal, and wherein a desired input power level occurs when the antenna structure is in resonance with the RF signal;
a memory; and
a processing module operably coupled to the antenna structure and the memory, wherein the processing module is operable to:
measure a first input power level from the RF signal;
when the first input power level is less than the desired input power level:
adjust the input impedance of the antenna structure to increase the first input power level;
measure a second input power level from the RF signal at the adjusted input impedance; and
when the second input power level substantially matches the desired input power level:
determine a power difference between the first input power level and the second input power level;
generate a digital value based on the power difference; and
store the digital value in the memory.
2. The RFID tag of claim 1 , wherein the tuning circuit comprises:
a capacitor; and
a variable inductor, wherein the processing module is operable to adjust the variable inductor to adjust the input impedance.
3. The RFID tag of claim 1 , wherein the tuning circuit comprises:
an inductor; and
a variable capacitor, wherein the processing module is operable to adjust the variable capacitor to adjust the input impedance.
4. The RFID tag of claim 1 , wherein the processing module is further operable to adjust the input impedance by:
setting the input impedance to an initial impedance.
5. The RFID tag of claim 1 , wherein the processing module is further operable to adjust the input impedance by:
determining an adjustment based on a difference between a current measured power level and a previously measured power level.
6. The RFID tag of claim 1 , wherein the processing module is further operable to adjust the input impedance in response to an RFID reader command.
7. The RFID tag of claim 1 , wherein the processing module is further operable to adjust the input impedance by:
using a measurement of the first input power level to adjust the input impedance.
8. A method for execution by a radio frequency identification (RFID) tag, the method comprises:
measuring, by a processing module of the RFID tag, a first input power level from a received RF signal, wherein an antenna structure of the RFID tag includes a tuning circuit, an antenna, and a sensing element, wherein exposure to an environmental condition causes a change in input impedance of the antenna structure while receiving the RF signal, and wherein a desired input power level occurs when the antenna structure is in resonance with the RF signal; and
when the first power level is less than the desired input power level:
adjusting, by the processing module, the input impedance of the antenna structure to increase the first input power level;
measuring, by the processing module, a second input power level from the received RF signal at the adjusted input impedance; and
when the second input power level substantially matches the desired input power level:
determining, by the processing module, a power difference between the first input power level and the second input power level;
generating, by the processing module, a digital value based on the power difference; and
storing, by the processing module, the digital value in memory of the RFID tag.
9. The method of claim 8 , wherein the adjusting the input impedance includes:
setting, by the processing module, the input impedance to an initial impedance.
10. The method of claim 8 , wherein the adjusting the input impedance includes:
determining, by the processing module, an adjustment based on a difference between a current measured power level and a previously measured power level.
11. The method of claim 8 , wherein the adjusting the input impedance includes:
using, by the processing module, a measurement of the first input power level to determine the adjusting of the input impedance.