US Patent 7466120 Communicating with an implanted wireless sensor

Aspects of the present invention determine the resonant frequency of a sensor by obtaining sensor signals in response to three energizing signals, measuring the phase of each sensor signal, and using a group phase delay to determine the resonant frequency. The phase difference between the first and second signal is determined as a first group phase delay. The phase difference between the second and third signal is determined as a second group phase delay. The first group phase delay and second group phase delay are compared. Based on the comparison, the system may lock on the resonant frequency of the sensor or adjust a subsequent set of three energizing signals.