ABSTRACT
Frequency metrology first became a topic of interest around 1920, when the burgeoning radio industry began erecting radio transmitters all over the world. These transmitters had to stay near their assigned frequencies to avoid interference with signals from other stations. This chapter provides an overview of frequency measurements. It focuses on the measurement of the electrical signals produced by oscillators. The chapter begins by discussing the concepts of accuracy and stability, which are essential to understanding oscillator specifications. It then describes the various types of oscillators used as frequency standards, including quartz and atomic oscillators, and oscillators disciplined to agree with an external reference signal. The chapter also describes the methods and techniques used to measure frequency and calibrate oscillators. It provides a brief look at the likely future of frequency metrology. The accurate measurement of optical frequencies had historically been difficult, involving large and complex chains of frequency-doubled and frequency-mixed lasers.
