Abstract: A probe to be inserted in-line in an AC plasma processing system allows accurate, real time determination of plasma parameters such as power and complex impedance over a broad dynamic range. Any need to know power output from a source is avoided and signals are selected to optimize accuracy such that only two alternating signals need be sensed for many applications. Signals are selected such that magnitudes of simple alternating signals can be easily measured as scalar values in a fashion that affords the use of these values to completely characterize the power actually delivered to the processing plasma and its complex impedance in real time. Plasma characterization can be limited to only specific frequencies for more accurate determination. Microstrip directional couplers are used to sense signals from the power transmission. These signals are then utilized to derive simple alternating signals representative of power or voltage.