Free-running 2.4 GHz Ring Oscillator-based FSK TX/RX for Ultra-small IoT Motes

We demonstrate narrowband FSK communication between radios with free-running 2.4GHz ring oscillators requiring neither frequency synthesizer nor external frequency reference (e.g., crystal oscillator). After a single calibration and in an open lab environment, a free-running low-IF ring RX is able to demodulate coded binary FSK signals sent by a free-running ring TX (FSK tone separation 5.5MHz) with input power as low as -67dBm (yielding estimated 5.5m range) to better than 6.5% chip error rate (CER)/1% packet error rate (PER) as specified by the IEEE 802.15.4 standard. By solely relying on ring oscillators for 2.4GHz tone generation, this work enables 600x chip area reduction when comparing a 10um × 10um ring oscillator (oscillator only, no current DAC) with a 250um × 250um 4.7nH LC tank (inductor only, no capacitor DAC). This work also enables 4x power reduction when comparing 105uW ring oscillator power vs. 400uW LC tank power.