Wireless Transceivers for Implantable Microsystems.

Abstract

In this thesis, we present the first-ever fully integrated mm3 low-power biomedical transceiver with 1 meter of range that is powered by a mm2 thin-film battery. The transceiver is targeted for biomedical implants where size and energy constraints dictated by application make design challenging. Despite all the previous work in RFID tags, form factor of such radios is incompatible with mm3 biomedical implants. The proposed transceiver bridges this gap by providing a compact low-power solution that can run off small thin-film batteries and can be stacked with other system components in a 3D fashion. On the sensor-to-external side, we proposed a novel FSK architecture based on dual-resonator LC oscillators to mitigate unwanted overlap of two FSK tones’ phase noise spectrum. Due to inherent complexity of such systems, fourth order dual-resonator oscillators can exhibit instable operation. We mathematically modeled the instability and derive design conditions for stable oscillations. Through simulation and measurements, validity of derived models was confirmed. Together with other low-power system blocks, the transmitter was successfully implanted in live mouse and in-vivo measurements were performed to confirm successful transmission of vital signals through organic tissue. The integrated transmitter achieved a bit-error-rate of 10-6 at 10cm with 4.7nJ/bit energy consumption. On the external-to-sensor link, we proposed a new protocol to lower receiver peak power, which is highly limited due to small size of mm3 microsystem battery. In the proposed protocol, sending same data multiple times drastically relaxes jitter requirement on the sensor side at the cost of increased power consumption on the external side without increasing peak power radiated by the external unit. The receiver also uses a dual-coil LNA to improve range by 22% with only 11% area overhead. An asynchronous controller manages protocol timing and limits total monitoring current to 43nA. The fabricated receiver consumes 1.6nJ/bit at 40kbps while positioned 1m away from a 2W source.