An aliasing sampler probe for detecting plasma RF voltage and current employs a sampling signal with a sampling rate slower that the RF fundamental frequency selected to produce an aliasing waveform at an aliasing frequency that is several orders of magnitude below the RF fundamental frequency. In one embodiment, the RF power is applied at 13.56 MHz. and sampling pulses have a sampling rate of 2.732 MHz to produce replicas of the RF voltage and current waveforms at an aliasing frequency of about 100 KHz. The aliasing replicas preserve phase and harmonic information with an accuracy that is not available from other sampling techniques.