Abstract: A Langmuir probe system for measuring plasma internal discharge parameters in a radio frequency excited plasma processing system includes an electrically tuned resonant circuit. The electrically tuned resonant circuit includes a semiconductor variable capacitor. Specifically, an inductor and FET are connected in parallel to form a resonant circuit used to electrically tune the Langmuir probe. The tuning circuit is placed within a moveable, electrically floating, probe housing and is electrically tuned to improve tuning accuracy and to reduce detuning during operation.