A Tutorial on Frequency Stability Fundamentals

Local Oscillator (LO) stability is paramount for optimal performance in RF communication systems. Frequency fluctuations in the LO can not only result in reciprocal mixing in narrowband communication systems, but it can also cause carrier mismatches between the transmitter and receiver, resulting in higher guard times and Bit Error Rates (BER); even in the absence of adjacent channels. While traditional communication systems rely on high-quality crystal oscillators and Phase-Locked Loops (PLLs) to maintain frequency stability, the power and area constraints, and sometimes battery-less nature of IoT motes urge the elimination of the crystal reference and PLL. Consequently, there is a growing demand for low-power oscillators that are stable enough to meet the requirements of modern communication standards such as IEEE 802.15.4 and Bluetooth Low Energy (BLE). The stability of an oscillator's frequency can be evaluated in both frequency and time domains using metrics such as phase noise, jitter, and Allan variance. This tutorial paper provides a comprehensive summary of the key metrics employed in the measurement of frequency stability, along with their conversion methods. By elucidating these critical measures, this paper aims to enhance understanding and facilitate the designers to better comprehend the subtle nuances of frequency stability.