Self-tuning of RFID tags using nonuniform impedance steps

Abstract

An RFID tag tuning circuit may be capable of adjusting the impedance matching between an RFID integrated circuit (IC) and an antenna on an RFID tag to increase the amount of power that the IC can extract from an incident RF wave. The tuning circuit switches a variable impedance coupling the antenna and the IC between several different impedance settings, where each impedance setting differs from an adjacent impedance setting by a respective impedance step size and at least one impedance step size has a different value than another impedance step size. The tuning circuit may switch the variable impedance by incrementing through a counter, decrementing through the counter, or performing some search algorithm. The tuning circuit may also initialize the variable impedance based on a default impedance setting or a random impedance setting derived from a random counter.

We claim: 
     
       1. A Radio Frequency Identification (RFID) integrated circuit (IC) comprising:
 an antenna port; 
 a tuning block coupled to the antenna port, the tuning block having a plurality of discrete elements, wherein a first one of the discrete elements has a first weight and a second one of the discrete elements has a second weight different from the first weight; and 
 a tuning circuit including: 
 a controller configured to:
 detectreceive a first transferred power and a second transferred power from the antenna port, wherein the first transferred power is received when operating the tuning block with the first element but not the second element coupled to the antenna port and the second transferred power is received when operating the tuning block with the second element but not the first element coupled to the antenna port; and 
 detect a second transferred power when operating the tuning block with at least the second element coupled to the antenna port; 
 compare the first and second transferred powers; and 
 based on the comparison, couplecause at least one of the first and second elements to be coupled to the antenna port for subsequent IC operations based on whether the first transferred power is higher or lower than the second transferred power. 
 
 
     
     
       2. The IC of  claim 1 , wherein the plurality of discrete elements are capacitors. 
     
     
       3. The IC of  claim 1 , wherein the tuning circuit controller is further configured to:
 sequentially increment or decrement through a plurality of impedance settings by coupling causing the plurality of discrete elements to be coupled to the antenna port; 
 determine a maximum transferred power; and 
 store an impedance setting associated with the maximum transferred power. 
 
     
     
       4. The IC of  claim 3 , wherein the tuning circuit controller is further configured to:
 roll over to a minimum impedance setting or a maximum impedance setting subsequent to incrementing to the maximum impedance setting or decrementing to the minimum impedance setting, respectively; 
 continue sequentially incrementing or decrementing upon rolling over to the minimum impedance setting or the maximum impedance setting, respectively; and 
 stop incrementing or decrementing when the impedance setting is equal to the stored impedance setting. 
 
     
     
       5. The IC of  claim 3 , wherein the tuning circuit controller is further configured to:
 start the sequence based on a default impedance setting or a value in a randomizing register upon powering up; and 
 roll over to a minimum impedance setting or a maximum impedance setting subsequent to incrementing to the maximum impedance setting or decrementing to the minimum impedance setting, respectively. 
 
     
     
       6. The IC of  claim 5 , wherein the tuning circuit is configured to detect the second transferred power when operating the tuning block with the first element and the second element coupled to the antenna port. 
     
     
       7. The IC of  claim 1 , wherein:
 the tuning circuit is further comprises a peak follower configured to detect measure a voltage from the first transferred power by measuring a voltage, and 
 the controller is configured to receive the first transferred power by receiving the voltage. 
 
     
     
       8. A Radio Frequency Identification (RFID) integrated circuit (IC) comprising:
 an antenna port; 
 a tuning block coupled to the antenna port, the tuning block having a first discrete element having a first weight and a second discrete element having a second weight different from the first weight; and 
 a tuning circuit comprising: 
 a controller configured to:
 couplecause the first element but not the second element to be coupled to the antenna port; 
 subsequently couple at least cause the second element but not the first element to be coupled to the antenna port; and 
 then couple cause at least one of the first and second elements to be coupled to the antenna port to maximize transferred power for subsequent IC operations. 
 
 
     
     
       9. The IC of  claim 8 , wherein the first and second discrete elements are capacitors. 
     
     
       10. The IC of  claim 8 , wherein the tuning circuit controller is further configured to:
 sequentially increment or decrement through a plurality of impedance settings by coupling causing at least one of the first and second elements to be coupled to the antenna port; 
 determine a maximum transferred power; and 
 store an impedance setting associated with the maximum transferred power. 
 
     
     
       11. The IC of  claim 10 , wherein the tuning circuit controller is further configured to:
 roll over to a minimum impedance setting or a maximum impedance setting subsequent to incrementing to the maximum impedance setting or decrementing to the minimum impedance setting, respectively; 
 continue sequentially incrementing or decrementing upon rolling over to the minimum impedance setting or the maximum impedance setting, respectively; and 
 stop incrementing or decrementing when the impedance setting is equal to the stored impedance setting. 
 
     
     
       12. The IC of  claim 10 , wherein the tuning circuit controller is further configured to:
 start the sequence based on a default impedance setting or a value in a randomizing register; and 
 roll over to a minimum impedance setting or a maximum impedance setting subsequent to incrementing to the maximum impedance setting or decrementing to the minimum impedance setting, respectively. 
 
     
     
       13. The IC of  claim 8 , wherein the tuning circuit controller is configured to subsequently couple cause the first and second elements to be coupled to the antenna port before maximizing transferred power for subsequent IC operations. 
     
     
       14. A method to adjust power transfer in a Radio Frequency Identification (RFID) integrated circuit (IC), the method comprising:
 detectingreceiving a first transferred power when coupling a first element but not a second element to an antenna port of the IC, wherein the first element has a first weight and the second element has a second weight different from the first; 
 detectingreceiving a second transferred power when coupling at least the second element but not the first element to the antenna port; 
 comparing the first and second transferred powers; and 
 based on the comparison, coupling at least one of the first and second elements to the antenna port for subsequent IC operation. 
 
     
     
       15. The method of  claim 14 , wherein the first element and the second element are capacitors. 
     
     
       16. The method of  claim 14 , further comprising:
 sequentially incrementing or decrementing through a plurality of impedance settings by coupling at least one of the first element and the second element to the antenna port; 
 determining a maximum transferred power from the antenna; and 
 storing an impedance setting associated with the maximum transferred power. 
 
     
     
       17. The method of  claim 16 , further comprising:
 rolling over to a minimum impedance setting or a maximum impedance setting subsequent to incrementing to the maximum impedance setting or decrementing to the minimum impedance setting, respectively; 
 continuing sequentially incrementing or decrementing upon rolling over to the minimum impedance setting or the maximum impedance setting, respectively; and 
 stopping incrementing or decrementing when the impedance setting is equal to the stored impedance setting. 
 
     
     
       18. The method of  claim 16 , further comprising:
 starting the sequence based on a default impedance setting or a value in a randomizing register; and 
 rolling over to a minimum impedance setting or a maximum impedance setting subsequent to incrementing to the maximum impedance setting or decrementing to the minimum impedance setting. 
 
     
     
       19. The method of  claim 14 , further comprising:
 detectingreceiving the second transferred power when coupling the first element and the second element to the antenna port. 
 
     
     
       20. The method of  claim 14 , further comprising detecting receiving the first transferred power by measuring a voltage.