Controlled External Powering of Miniaturized Chronically Implanted Biotelemetry Devices

Radio frequency energizing of implanted transmitters presents an interesting possibility to make feasible comparative measurements from sites within the body over very long periods of time. Disturbances in the energy transport may, however, cause the implant carrier frequency to change, which is seen as artifacts or drift at the demodulator end. Here, this problem was solved by feeding back the demodulator output level to control the amplitude of the external energizing oscillator. The closed loop modifies the frequency characteristic of the transmitter-receiver system in a predictable way. The artifacts due to implant movements can be minimized by careful application and by analysis of the field characteristics inside and outside the energizing solenoid, thus positioning the implant adequately.