Self tuning RFID
The antenna of an RFID tag is automatically tuned by controlling both the real and imaginary components impedance “seen” by the antenna. The real component (resistive) is controlled by controlling a tap of a charge pump circuit of the tag. The imaginary (reactive) component is controlled by connecting one or more reactive elements of an antenna tuning circuit to the antenna. The real component is controlled by a first digital control loop that automatically activates successive taps on a charge pump of the RFID tag which effectively controls the resistance seen by the antenna. A second digital control loop controls the connection of the various reactive components.
1. An RFID tag having self tuning, comprising:
a charge pump having multiple taps;
an antenna;
an antenna impedance matching circuit coupled to the antenna and including a plurality of reactive components that are selectively connectable to the antenna;
a first digital control loop configured to cause the RFID tag to cycle through successive taps of the charge pump to control a resistive impedance match for the antenna and a charge pump output voltage;
a second digital control loop configured to increase the charge pump output voltage by controlling connection of the reactive components to the antenna and a reactive impedance match for the antenna; and
an indicator to the second digital control loop indicating a status of the first digital control loop cycle, wherein the reactive components of the antenna impedance matching circuit are connected to the antenna in response to the indication.
2. An RFID tag according to claim 1 wherein the first digital control loop configured to cause the RFID tag to cycle through successive taps of the charge pump until one or more of a maximum voltage is met and a predetermined voltage threshold is equaled or exceeded.
3. An RFID tag according to claim 1 wherein the reactive components are capacitors.
4. An RFID tag according to claim 1 wherein the reactive components are inductors.
5. An RFID tag according to claim 1 further comprising:
an RFID tag main storage capacitor configured to collect and store power from the charge pump and provide power to the RFID tag for its operation; and
the main storage capacitor is connected in circuit when a predetermined criteria has been met.
6. An RFID tag according to claim 5 wherein the predetermined criteria is a voltage exceeding a predetermined level.
7. A method of operating an RFID tag so that it self-tunes, the RFID tag including a charge pump having multiple taps, an antenna, an antenna impedance matching circuit including a plurality of reactive components selectively connectable to the antenna, a first digital control loop configured to cause the RFID tag to cycle through successive charge pump taps to produce a charge pump output voltage, a second digital control loop configured to control connection of the reactive components to the antenna, and an indicator to the second digital control loop indicating a status of the first digital control loop cycle, the method comprising:
(a) receiving power at the antenna;
(b) initializing the antenna impedance matching circuit to a first level of reactance;
(c) initializing a charge pump to provide a first level of resistive impedance match for the antenna and operate at a first tap;
(d) determining whether a charge pump output voltage is greater than a predetermined threshold voltage;
(e) if at step (d) the charge pump output voltage is greater than the predetermined threshold, banking in a main capacitor of the RFID tag and operating the RFID tag in a normal operation mode; and
(f) if at step (d) the charge pump output voltage is not greater than the predetermined threshold then, using said digital loops,
cycling through one or more taps of the charge pump to provide successive levels of resistive impedance match for the antenna, and
based on the cycling through the one or more taps of the charge pump, connecting the reactive components of the antenna impedance matching circuit to adjust a reactive impedance match for the antenna and increase the charge pump output voltage.
8. An RFID tag according to claim 1 wherein the second digital control loop is configured to control connection of the reactive components of the antenna impedance matching circuit to improve a match of an impedance of the antenna.
9. An RFID tag according to claim 8 wherein the second digital control loop is configured to control connection of the reactive components of the antenna impedance matching circuit to increase the output voltage of the charge pump.
10. An RFID tag according to claim 9 wherein the second digital control loop is configured to control connection of the reactive components of the antenna impedance matching circuit in response to determining that the charge pump output voltage is less than a threshold voltage.
11. An RFID tag according to claim 1 further comprising a counter that controls the connection of the reactive components of the antenna impedance matching circuit to the antenna.
12. An RFID tag according to claim 1 wherein the first control loop determines a charge pump overflow condition and wherein the indicator indicates the charge pump overflow condition.
13. A method according to claim 7 wherein controlling the antenna impedance matching circuit comprises controlling connection of the reactive components of the antenna impedance matching circuit to match an impedance of the antenna.
14. A method according to claim 7 further comprising:
incrementing a counter; and
in response to incrementing the counter, connecting a reactive component of the antenna impedance matching circuit to the antenna.
15. A method according to claim 7 further comprising determining a charge pump overflow condition and wherein controlling the antenna impedance matching circuit comprises connecting a reactive component of the antenna impedance matching circuit to the antenna in response to determining the charge pump overflow condition.