Multimode Cavity Enabling Backscatter Communication for mm-Sized Implants in Freely Moving Animals

Backscatter (BackCom) communication using a cavity poses a significant challenge due to the bandpass filtering behavior of the cavity, which restricts the range of separation between uplink and downlink frequencies, potentially leading to self-jamming issues, requiring a high-sensitivity transceiver and a high quality-factor narrow-band filter. This paper proposes a new method for BackCom using a multimode rectangular cavity with dimensions of 49.6 cm × 49.6 cm × 30.4 cm, which resonates at 427.5 MHz in TE011mode and 570 MHz in TM110mode. A double-sided square loop coil, sized 1.2 mm × 1.2 mm on an FR4 substrate, serves as the miniaturized device (MD) antenna for freely moving small animal implants. The MD integrates an on-chip circuit implemented in a 0.35 μm CMOS technology, featuring a 5-stage differential rectifier with a power management unit and a low-power on-chip frequency generator. The frequency generator eliminates a power-hungry synthesizer by deriving the 142.5 MHz modulation signal from the 570 MHz downlink via frequency division. The multimode cavity as a reader receives an uplink signal of -20 dBm at 427.5 MHz in TE011mode, with a downlink input power of 24 dBm at 570 MHz in TM110mode applied to the cavity. This approach shows a sufficient separation of 142.5 MHz between the uplink and downlink modes, which eliminates self-jamming, relaxes filter requirements, and enables using the full cavity bandwidth for potentially higher data rates.