A method or system for identifying the resonance frequency of a probe resonating in plasma

A method of identifying the resonance frequency of a probe resonating in a plasma. The method comprising obtaining at a first time, a first response signal representative of a probe resonating in a plasma. Obtaining at a second later time, a second response signal representative of the probe resonating in a plasma, wherein a perturbation is applied while the second response signal is being obtained. The first and second response signals are then combined to eliminate common background noise. The perturbation applied can be to the plasma itself i.e. power, flow rate or pressure thereof, or to the probe e.g. variation in the voltage applied thereto.