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An electronic system includes a reader and a remotely powered and remotely interrogated sensor transponder. The sensor transponder includes a reader and a remotely powered and remotely interrogated sensor transponder. The sensor transponder includes a sensor and a radiation receiving device. Data fr

An electronic system includes a reader and a remotely powered and remotely interrogated sensor transponder. The sensor transponder includes a reader and a remotely powered and remotely interrogated sensor transponder. The sensor transponder includes a sensor and a radiation receiving device. Data from the sensor is conditioned to provide sensor data ratiometric with magnitude of excitation voltage provided by the radiation receiving device.

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What is claimed is: 1. An electronic system, comprising a reader and a remotely powered and remotely interrogated sensor transponder, wherein said sensor transponder includes a sensor and a radiation receiving device, wherein said radiation receiving device is for receiving radiation, wherein said

What is claimed is: 1. An electronic system, comprising a reader and a remotely powered and remotely interrogated sensor transponder, wherein said sensor transponder includes a sensor and a radiation receiving device, wherein said radiation receiving device is for receiving radiation, wherein said radiation receiving device is connected to deliver an excitation voltage derived from said radiation to said sensor, wherein said sensor provides a sensor output, wherein said sensor output is conditioned to provide a ratiometric output, wherein said ratiometric output is equal to a ratio of said sensor output to magnitude of said excitation voltage. 2. An electronic system as recited in claim 1, wherein said radiation receiving device includes at least one from the group consisting of a coil and an antenna. 3. An electronic system as recited in claim 2, wherein said coil includes a tap. 4. An electronic system as recited in claim 3, wherein said excitation voltage is voltage at said tap. 5. An electronic system as recited in claim 3, wherein said coil includes multiple taps, wherein said excitation voltage is voltage at one of said taps, wherein said one tap is dynamically selected depending on loading to provide impedance matching and efficient energy transfer. 6. An electronic system as recited in claim 2, wherein said sensor transponder further includes a capacitive divider, wherein said capacitive divider includes a capacitive divider output. 7. An electronic system as recited in claim 5, wherein said excitation voltage is voltage at said capacitive divider output. 8. An electronic device as recited in claim 1, wherein said sensor transponder further includes a receiver resonant tank circuit and a power-using device, said receiver resonant tank circuit for receiving electromagnetic radiation for powering said power-using device, said receiver resonant tank circuit including said radiation receiving device, wherein said radiation receiving device includes a first end and a second end, wherein said receiver resonant tank circuit further includes an impedance matching circuit, wherein said impedance matching circuit is connected to said radiation receiving device to provide greater current to said power-using device than would be available to said power-using device if said power-using device were connected between said first and said second end. 9. An electronic device as recited in claim 8, wherein said impedance matching circuit comprises a tap between said first and said second end. 10. An electronic device as recited in claim 9, wherein said tap is provided at a location between said first end and said second end so said power-using device does not substantially degrade Q factor of said receiver resonant tank circuit. 11. An electronic device as recited in claim 8, wherein said impedance matching circuit comprises a plurality of taps between said first and said second end, wherein connection is switchably provided to one of said plurality of taps to most closely impedance match to impedance of said power using device. 12. An electronic device as recited in claim 8, wherein said impedance matching circuit comprises a capacitive divider. 13. An electronic device as recited in claim 12, wherein said capacitive divider provides an output set so said power-using device does not substantially degrade Q factor of said receiver resonant tank circuit. 14. An electronic device as recited in claim 8, wherein said impedance matching circuit provides an output so impedance of said power-using device approximately matches impedance presented by said radiation receiving device at said output. 15. An electronic device as recited in claim 8, wherein said impedance matching circuit provides an output so power transfer to said receiver resonant tank circuit from said electromagnetic radiation is not substantially degraded for expected power consumption of said power-using device. 16. An electronic device as recited in claim 8, wherein said impedance matching circuit provides an output so power transfer to said power-using device from said receiver resonant tank circuit is optimized for expected power consumption of said power-using device. 17. An electronic device as recited in claim 8, wherein said sensor transponder further includes a processor, wherein said impedance matching circuit has an output that can be dynamically varied during operation under the control of said processor, so power transfer to said receiver resonant tank circuit from said electromagnetic radiation is optimized for power actually being consumed by said power-using device and so power transfer to said power-using device from said receiver resonant tank circuit is optimized for expected power consumption of said power-using device. 18. An electronic system as recited in claim 1, further comprising a housing containing said sensor transponder. 19. An electronic system as recited in claim 18, wherein said housing is hermetically sealed. 20. An electronic system as recited in claim 19, wherein said housing comprises a metal enclosure and wherein said radiation receiving device is tuned to receive radiation at a frequency sufficiently low so a substantial portion of said radiation is able to penetrate through said metal enclosure. 21. An electronic system as recited in claim 20, wherein said frequency is less than 125 kHz. 22. An electronic system as recited in claim 21, wherein said frequency is less than about 44 kHz. 23. An electronic system as recited in claim 22, wherein said frequency is about 4 kHz. 24. An electronic system as recited in claim 18, wherein said housing is fabricated of a material suitable for implanting in living tissue. 25. An electronic system as recited in claim 24, further comprising an orthopedic implant, wherein said sensor transponder senses position with respect to said orthopedic implant. 26. An electronic system as recited in claim 1, wherein all power for operating said sensor transponder is derived from power radiated from said reader and received by said radiation receiving device. 27. An electronic system as recited in claim 26, wherein said sensor transponder further includes a rechargeable energy storage device, wherein said rechargeable energy storage device is connected to be recharged with power derived from said radiation receiving device. 28. An electronic system as recited in claim 27, wherein said energy storage device is connected to provide a higher power to said sensor than is available from said radiation receiving device. 29. An electronic system as recited in claim 1, wherein said sensor transponder further includes a processor connected to receive power derived from said radiation receiving device. 30. An electronic system as recited in claim 29, wherein said sensor is connected to provide data derived from said ratiometric output to said processor. 31. An electronic system as recited in claim 30, wherein said sensor transponder further comprises an analog/digital converter. 32. An electronic system as recited in claim 29, wherein said processor includes an integrated clock. 33. An electronic device as recited in claim 32, wherein said integrated clock comprises an RC clock. 34. An electronic device as recited in claim 33, wherein said RC clock is connected to encode RC clock data with data derived from said sensor. 35. An electronic system as recited in claim 1, wherein said sensor transponder further includes a transmitting device connected to receive power derived from said radiation receiving device and connected for transmitting data derived from said sensor to said reader. 36. An electronic system as recited in claim 35, wherein said transmitting device includes at least one from the group consisting of a switched reactance circuit and a transmitter. 37. An electronic system as recited in claim 36, wherein said switched reactance circuit comprises a reactive component connected to switchably affect electromagnetic radiation radiated from said reader. 38. An electronic system as recited in claim 36, wherein said sensor transponder further includes a data receiving device and a processor, wherein said data receiving device is connected for receiving digital data derived from said reader and for providing said digital data to said processor. 39. An electronic system as recited in claim 38, wherein said data receiving device includes a demodulator. 40. An electronic system as recited in claim 38, further comprising an RF transceiver, wherein said transmitter and said data receiving device are included in said RF transceiver. 41. An electronic system as recited in claim 38, wherein said processor is connected to receive said digital data for performing at least one from the group including: reprogramming said processor, triggering data logging, initiating transmission of stored data, and initiating calibration. 42. An electronic system as recited in claim 1, wherein said reader includes a radiation transmitting device, wherein said reader comprises a circuit to detect changes in loading of said radiation transmitting device, as a result of switching of said switched reactance circuit in said sensor transponder. 43. An electronic device as recited in claim 1, wherein said reader includes an RF receiver, wherein said sensor transponder includes an RF transmitter for transmitting data to said reader. 44. An electronic system as recited in claim 1, wherein said sensor transponder further includes a non-volatile memory connected for storing sensor data. 45. An electronic system as recited in claim 1, further comprising a plurality of said remotely powered and remotely interrogated sensor transponders, wherein each said sensor transponder is connected for receiving all power for operating said sensor transponder derived from power radiated from said reader, wherein each said sensor transponder has an address and wherein each said sensor transponder has a system to transmit data so as to avoid collisions. 46. An electronic system as recited in claim 45, wherein said system to avoid collisions includes a random timing generator. 47. An electronic system as recited in claim 45, wherein each of said plurality of sensor transponders includes a memory for data logging. 48. An electronic system as recited in claim 45, wherein each of said plurality of sensor transponders includes an energy storage device. 49. An electronic system as recited in claim 45, wherein each of said plurality of sensor transponders includes a device for two way communication. 50. An electronic system as recited in claim 1, wherein said sensor comprises at least one from the group consisting of a displacement sensor, a pressure sensor, a force sensor, a torque sensor, and a temperature sensor. 51. An electronic system as recited in claim 50, wherein said displacement sensor comprises a variable reluctance transducer. 52. An electronic system as recited in claim 1, further comprising a member subject to corrosion, wherein said sensor is located to detect corrosion of said member. 53. An electronic system as recited in claim 1, wherein said sensor transponder is configured to report at least one from the group consisting of a change in said ratiometric output and ratiometric output within an acceptable limit. 54. An electronic system as recited in claim 1, wherein said sensor transponder further includes a rectifier for rectifying said excitation voltage and a rectifier for rectifying said sensor output, wherein said ratiometric output is equal to a ratio of said rectified sensor output to magnitude of said rectified excitation voltage. 55. An electronic system as recited in claim 54, wherein said excitation voltage is digitized and wherein said sensor output is digitized to provide sensor data ratiometric with magnitude of excitation voltage provided by said radiation receiving device. 56. An electronic system as recited in claim 1, wherein said sensor is configured so said sensor output is a difference between two voltages and wherein said excitation voltage is a sum of said two voltages.