Method and apparatus for detecting RF field strength
A power detector for use in an RF receiver. The detector includes a power reference generator and a power quantizer. The power reference generator develops a power reference current, voltage, or signal as a function of a power transferred via a received RF signal. The power quantizer is responsive to the power reference current, voltage, or signal to develop a digital field power value indicative of the power reference current, voltage, or signal.
1. A method for execution by a radio frequency identification (RFID) tag, the method comprises:
extracting power from a signal to produce an extracted power;
when the extracted power exceeds a power ok threshold:
generating a sample of the extracted power;
comparing a current sample of the extracted power to a previous sample of the extracted power; and
when the current sample is greater than the previous sample, changing impedance of an antenna circuit to affect the extracted power from the signal.
2. The method of claim 1 further comprises:
receiving the signal for an RFID reader.
3. The method of claim 1 , wherein the antenna circuit comprises:
an antenna; and
a variable capacitance circuit.
4. The method of claim 3 , wherein the changing the impedance of the antenna circuit comprises:
changing capacitance of the variable capacitance circuit.
5. The method of claim 3 , wherein the extracting power from the signal further comprises:
utilizing an initial capacitance value of the variable capacitance circuit.
6. The method of claim 1 further comprises:
determining a polarity of the comparison of the current sample of the extracted power to the previous sample of the extracted power;
when the polarity is positive, changing the impedance of the antenna circuit in a first direction; and
when the polarity is negative, changing the impedance of the antenna circuit in a second direction.
7. The method of claim 1 further comprises:
when the current value of the reference voltage is less than the previous value of the reference voltage, changing the impedance of the antenna circuit to affect the extracted power from the signal.
8. The method of claim 1 , wherein generating the sample comprises:
generating a reference voltage from the extracted power as the sample.
9. The method of claim 1 , wherein generating the sample comprises:
rectifying the signal to produce a rectified signal;
low pass filtering the rectified signal to produce a filtered signal; and
sampling and holding the filtered signal to produce the sample.
10. The method of claim 1 further comprises:
storing a capacitance setting associated with the changing of the impedance of the antenna circuit.
11. A radio frequency identification (RFID) tag comprises:
an antenna circuit;
a regulator operably coupled to the antenna circuit and operable to:
extract power from a signal to produce an extracted power; and
generate a power ok signal when the extracted power exceeds a power ok threshold;
a tuning circuit operable, when the extracted power exceeds the power ok threshold to:
generate a sample of the extracted power;
compare a current sample of the extracted power to a previous sample of the extracted power; and
when the current sample is greater than the previous sample, change impedance of an antenna circuit to affect the extracted power from the signal.
12. The RFID tag of claim 11 further comprises:
the antenna circuit receiving the signal for an RFID reader.
13. The RFID tag of claim 11 , wherein the antenna circuit comprises:
an antenna; and
a variable capacitance circuit.
14. The RFID tag of claim 13 , wherein the tuning circuit changes the impedance of the antenna circuit by:
changing capacitance of the variable capacitance circuit.
15. The RFID tag of claim 13 , wherein the rectifier extracts power from the signal further by:
initially utilizing an initial capacitance value of the variable capacitance circuit.
16. The RFID tag of claim 11 , wherein the tuning circuit is further operable to:
determine a polarity of the comparison of the current sample of the extracted power to the previous sample of the extracted power;
when the polarity is positive, change the impedance of the antenna circuit in a first direction; and
when the polarity is negative, change the impedance of the antenna circuit in a second direction.
17. The RFID tag of claim 11 , wherein the tuning circuit is further operable to:
when the current value of the reference voltage is less than the previous value of the reference voltage, change the impedance of the antenna circuit to affect the extracted power from the signal.
18. The RFID tag of claim 11 , wherein the tuning circuit comprises:
a voltage reference circuit operable to generate a voltage reference that is representative of the extracted power, wherein the voltage reference is the sample;
a comparison circuit operable to compare a current voltage reference with a previous voltage reference; and
a change circuit operable to change the impedance of the antenna circuit based on an output of the comparison circuit.
19. The RFID tag of claim 11 , wherein the tuning circuit comprises:
a rectifier operable to rectify the signal to produce a rectified signal;
a low pass filter operable to filter the rectified signal to produce a filtered signal; and
a sample and hold circuit operable to sample and hold the filtered signal to produce the sample.
20. The RFID tag of claim 11 further comprises:
memory operable to store a capacitance setting associated with the changing of the impedance of the antenna circuit.