An ideal signal has no phase noise, harmonics nor spurious frequency components associated with it. Unfortunately the ideal signal does not exist in the real world, and these factors degrade the spectral purity of all real world signals to some degree. High-performance microwave signal generators are designed to maximize spectral purity, with very low levels of phase noise, harmonics and spurs, while maintaining general purpose flexibility. The levels of spectral purity performance are one of the key differences between high-performance, mid-range and low-end or entry-level signal generators. This paper discusses the factors that make up spectral purity specifications to help you make more informed comparisons of microwave signal generator performance for test and measurement applications.