A method of analysing RF signals from a plasma system, comprising the steps of: (a) receiving from the plasma system one or more signal channels, wherein the or each signal channel comprises a data source representing signals and noise from the plasma system across a frequency range; (b) identifying, in a first RF signal sample from one of said signal channel(s), a first frequency value, F, at which a local maximum is found in the frequency domain; (c) determining a first complex frequency-domain signal component at said first frequency value, F in said first signal sample; (d) identifying, in said first RF signal sample or in a further RF signal sample from one of said signal channel(s), a further frequency value F, where N is an integer representing an Nth order harmonic of said first frequency F, and wherein N = 1 denotes a 1st order harmonic at the first frequency F; (e) determining a further complex frequency-domain component at said further frequency value in the RF signal sample in which said further frequency value is identified; (f) transforming the first complex frequency-domain component by adjusting its phase by an angle θ to a predetermined phase angle φ, to provide a phase-adjusted first complex signal component; (g) transforming the further complex frequency-domain component by adjusting its phase by an angle equal to N x θ, to provide a phase-adjusted further complex signal component; (h) iteratively repeating steps (b) to (g) in respect of a plurality of signal samples from the same plasma system, wherein in each iteration the value of θ is chosen to give a constant phase angle φ for the phase-adjusted first complex signal component across different iterations, and wherein within any iteration the value of θ chosen in step (f) is employed in the adjustment of step (g); (i) aggregating or averaging the phase adjusted first complex signal components obtained in each iteration; (j) aggregating or averaging the phase-adjusted further complex signal components obtained in each iteration. (a) receiving from the plasma system one or more signal channels, wherein the or each signal channel comprises a data source representing signals and noise from the plasma system across a frequency range; (b) identifying, in a first RF signal sample from one of said signal channel(s), a first frequency value, F, at which a local maximum is found in the frequency domain; (c) determining a first complex frequency-domain signal component at said first frequency value, F in said first signal sample; (d) identifying, in said first RF signal sample or in a further RF signal sample from one of said signal channel(s), a further frequency value F, where N is an integer representing an Nth order harmonic of said first frequency F, and wherein N = 1 denotes a 1st order harmonic at the first frequency F; (e) determining a further complex frequency-domain component at said further frequency value in the RF signal sample in which said further frequency value is identified; (f) transforming the first complex frequency-domain component by adjusting its phase by an angle θ to a predetermined phase angle φ, to provide a phase-adjusted first complex signal component; (g) transforming the further complex frequency-domain component by adjusting its phase by an angle equal to N x θ, to provide a phase-adjusted further complex signal component; (h) iteratively repeating steps (b) to (g) in respect of a plurality of signal samples from the same plasma system, wherein in each iteration the value of θ is chosen to give a constant phase angle φ for the phase-adjusted first complex signal component across different iterations, and wherein within any iteration the value of θ chosen in step (f) is employed in the adjustment of step (g); (i) aggregating or averaging the phase adjusted first complex signal components obtained in each iteration; (j) aggregating or averaging the phase-adjusted further complex signal components obtained in each iteration.