Probe (5) to be inserted in-line in an AC plasma processing system (3) 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 (2) 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. Three or more scalar values representative of the magnitude of the alternating signals and their combination serve as the variable inputs to determine complex reflection coefficient or impedance using known formulas. Use of a sign bit detector or assumptions with respect to the processing plasma (2) is disclosed for complete characterization of the plasma in an efficient manner.