Understanding complex numbers and quadrature signals is essential for understanding DSP at both a theoretical and a practical level. Yet this strange, complex subject (based on the admittedly imaginary construct of the square root of negative one!) is among the hardest for DSP beginners to grasp – and is confusing at times even for advanced DSPers.

But don’t despair: help is on the way. In this tutorial, Rick Lyons, author of the best-selling DSP books Understanding Digital Signal Processing and Streamlining Digital Signal Processing: A Tricks of the Trade Guidebook, clears the fog around this difficult subject by providing the clearest, most intuitive explanation yet of quadrature signals and their importance in Digital Signal Processing.
QuadSignals (200K, pdf) Version 2, January 2008.