A real time digital spectral analysis system, applicable to broad band plasma fluctuations, is presented. The system digitally records plasma potential fluctuations on a pair of matched electrostatic probes, and subsequently outputs the cross power spectral density and phase via an on‐line computer graphics display. The spectral analysis system and its experimental accuracy are discussed. Using the Lawrence Berkeley Laboratory ten amp neutral beam ion source, several examples demonstrating the systems application to plasma wave detection are given.