Oscillators and the characterization of frequency stability: an introduction

This paper provides an introduction to basic concepts commonly used in time and frequency metrology, and is addressed to other scientific communities. Thus no attempt is made to provide an exhaustive review. Instead, attention is focused on the most important tools used by physicists and engineers involved in time and frequency metrology. We first explain the principles of the oscillator through an example. Then we introduce the concepts of frequency reference, oscillating loop, frequency stability and accuracy. Finally, we define the power spectrum density of frequency fluctuations and the Allan variance as means to characterize the stability of frequency standards.