A ULP Long-Range Active-RF Tag with Automatic Antenna-Interface Calibration Achieving 20.5% TX Efficiency at -22dBm EIRP, and -60.4dBm Sensitivity at 17.8nW RX Power

For event-driven loT applications such as long-range remote control and livestock monitoring, an ultra-low-power (ULP) active-RF Tag capable of communicating over a long distance will be attractive to enhance the battery lifespan and save maintenance costs. The recent wake-up receivers (RXs) using an envelope-detector (ED)-first architecture demonstrated <-60dBm sensitivity with a nano-Watt power budget [1–3], yet the transmitter (TX) output power and efficiency limited the communication distance and battery lifespan. Communicating with an RFID reader [4] at a 160m distance entails a tag sensitivity of only -43dBm for the downlink, while the tag equivalent isotropically radiated power (EIRP) has to be -16dBm to uphold the same distance for the uplink (Fig. 31.6.1 top-left). As a result, the TX power budget has to be 4 orders of magnitude higher than that of the RX. Besides, the ED-first RX [1] typically requires a high-Q off-chip matching network to improve the sensitivity. This not only adds the system form-factor but also restricts the operating frequency from switching between different channels, which can serve as a signature to avoid false wake-up [5]. The single-loop-antenna-based transceiver (TRX) architecture in [2] realizes a passive voltage gain by utilizing the antenna-TRX interface, eliminating the bulky off-chip filter. However, the resonant frequency $(\mathsf{f}_{\mathsf{RSN}})$ of the antenna-TRX interface is sensitive to the process and temperature variations of the capacitor, which requires manual tuning. On the TX side, the functionreused VCO-PA in [2] utilizes the high-Q loop antenna to reduce the TX power budget. The Class-B VCO-PA topology has an inferior power efficiency since the $-g_{\mathsf{m}}$ transistors stay in the triode region for a long time at a large output swing, increasing the tank loss. Furthermore, the input capacitor $(C_{\mathsf{ED}})$ from the ED incurs a large fixed capacitance in the LC tank of the VCO-PA, compromising its tank $\mathsf{Q}$ over the frequency tuning range.