Education I&M: Seeing Phase Truncation Spurs in the Output of Direct Digital Synthesizers

When making measurements, it is often important to have a signal generator that outputs accurate sine waves that can be used as inputs to the system under test. One nice, conceptually simple way to implement such an instrument is to make use of direct digital synthesis. Synthesizers that use this technique, described below, have many nice features. For example, they can switch between different frequencies quickly, and they produce very nice output. Unfortunately, their output contains small spectral components at frequencies other than the desired one. There are, for example, components due to the nonlinearity of the digital to analog converter used as part of the synthesizer, components due to clock feedthrough, and components due to phase truncation spurs [1], [2]. In this lecture/lab note we examine phase truncation spurs.