An Ultra-Low Power 2.4 GHz Transmitter for Energy Harvested Wireless Sensor Nodes and Biomedical Devices

We present a 2.4GHz ultra-low power energy-harvested narrowband transmitter (TX) for wireless sensing and biomedical devices. We used a 434 MHz RF energy for resonant cavity based wireless power transfer (WPT). An efficiency tracking loop maximizes the power transfer efficiency of WPT and achieves an efficiency of 11.5%. The TX comprises a voltage-controlled power oscillator (VCPO) and a loop antenna which is patterned over a flexible Parylene C substrate to achieve biocompatibility. The loop antenna acts as a radiator as well as an inductive element for the VCPO, eliminating a power amplifier stage and bulky off-chip matching network. The TX is designed for low active and sleep mode power consumption to support both continuous-data mode or deeply duty cycled burst-data mode of transmission. Implemented in 0.18 μm CMOS, the TX supports OOK and FSK modulations with the sleep mode power dissipation of 128 pW. Our active RF TX consumes an active-mode power of 70 μW at 10 Mbps while radiating -33 dBm of power, resulting in an energy efficiency of 7 pJ/bit, which is the best reported till date.