Antenna driver with scalable output impedance

1. An apparatus, comprising:

an antenna;

a modulator operatively connected to the antenna, the modulator configured to send a signal to a wireless device via the antenna, the modulator comprising finger elements configured to produce an output impedance;

a controller connected to the antenna, the controller comprising:

a current detector configured to determine an amplitude of a response received at the antenna from the wireless device, the response comprising a response carrier component; and

a driver configured to scale, via a switching operation on the finger elements, the output impedance responsive to the amplitude; and

a carrier cancellation path coupled to the modulator, the carrier cancellation path configured to provide a carrier cancellation component to the controller to facilitate removal of the response carrier component from the response.

2. The apparatus of claim 1 , where the response carrier component is phase inverted with respect to the carrier cancellation component.

3. The apparatus of claim 1 , where the current detector is configured to determine changes to the amplitude at a time scale longer than a modulation rate of the wireless device.

4. The apparatus of claim 3 , where the current detector comprises a low pass filter configured to implement the time scale.

5. The apparatus of claim 1 , where the amplitude is responsive to a load of the antenna.

6. The apparatus of claim 5 , where the driver is configured to maintain the amplitude via scaling the output impedance.

7. The apparatus of claim 1 , where the finger elements comprise transistor fingers connected in parallel.

8. The apparatus of claim 7 , where the driver is configured to engage an individual one of the transistor fingers in response to a change in the amplitude.

9. The apparatus of claim 1 , where:

the modulator comprises two paths; and

the modulator is configured to encode data into the signal via differential phase modulation across the two paths.

10. A device, comprising:

an antenna including a load, the load dependent on a distance between the antenna and a wireless device;

a modulator, operatively connected to the antenna, the modulator configured to send, via the antenna, a signal to the wireless device, the modulator comprising finger elements configured to produce an output impedance; and

a controller coupled to an output of the antenna, the controller comprising:

a current detector configured to determine a carrier amplitude of a response from the wireless device; and

a driver configured to scale, via a switching operation on the finger elements, the output impedance responsive to the carrier amplitude to maintain a ratio between the output impedance and the load of the antenna.

11. The device of claim 10 , where the load is configured to be modulated at a sub-carrier frequency by the wireless device.

12. The device of claim 10 , where the driver is configured to implement a loop algorithm to maintain the ratio within a determined tolerance.

13. The device of claim 10 , where the current detector is configured to determine changes to the carrier amplitude at a time scale longer than a modulation period of the wireless device.

14. The device of claim 13 , where the current detector comprises a low pass filter configured to implement the time scale by filtering contributions from changes occurring over durations shorter than the time scale.

15. The device of claim 10 , where:

the modulator comprises multiple paths; and

the modulator is configured to encode data into the signal via differential phase modulation across the multiple paths.

16. A method comprising:

generating a signal with a modulator, the modulator comprising finger elements configured to produce an output impedance;

transmitting the signal to a wireless device via an antenna coupled to the modulator;

determining an amplitude of a response received at the antenna from the wireless device, the response comprising a response carrier component;

performing a switching operation on the finger elements to scale the output impedance responsive to the amplitude; and

providing, via a carrier cancellation path, a carrier cancellation component to control circuitry for the modulator to facilitate removal of the response carrier component from the response.

17. The method of claim 16 , further comprising encoding data into the signal via differential phase modulation across multiple paths of the modulator.

18. The method of claim 16 , where determining the amplitude of the response comprises filtering the response at a modulation frequency of the wireless device.

19. The method of claim 16 , where performing the switching operation on the finger elements comprises disengaging a finger transistor in the modulator.

20. The method of claim 16 , where performing the switching operation on the finger elements comprises engaging a finger transistor in the modulator.