PHASE LOCKED LOOP (PLL)
TRANSMITTER DESIGN FOR
PASSIVE TELEMETRY FROM
IMPLANTED CIRCUITS

DI Stefan Sauermann, Dr. Hermann Lanmüller
Department for Biomedical Engineering and Physics, University of Vienna
1090 Vienna, Waehringerguertel 18-20, Austria

Introduction Passive telemetry is often used in implanted electronic devices, especially when low power consumption of the implant is an issue. This paper gives an introduction into phase locked loop (PLL) transmitter design and discusses possible transmitters for telemetrical data exchange and power supply.

Methods The basic transmitter circuit of Fig.1 follows slow (<0.1Hz) changes of the transmitter resonsonance frequency. That way the power efficiency remains high for changing coupling factors (k). Frequency or amplitude modulation (FM or AM) can be applied for data transmission to the implant. Phase modulation (PM) caused by impedance modulation in the implant (IIM) can be demodulated for passive telemetry.

Discussion Power efficiency of the suggested transmitter is insensitive to changes in coupling within a certain range. Unfortunately PM by IIM is very sensitive on changes in k in this mode of operation. A modified PM-detector with crystal-stabilized frequency would be more insensitive to k, and use less bandwidth. A good way to combine the advantages of power efficiency and constant frequency would be to allow frequency changes in times of high power consumption and switch to a crystal oszillator if passive telemetry is to be performed. Such a transmitter could combine the advantages of both concepts.