ANALYSING RF SIGNALS FROM A PLASMA SYSTEM
Samples representing signals and noise from a plasma system across a frequency range are collected. A first complex frequency-domain signal component is identified from a sample corresponding to a frequency value F at which a local maximum signal is found. This first component is phase-adjusted by a variable angle θ to a predetermined phase angle φ, and stored. A further complex component is identified corresponding to a frequency F(N) representing an Nth order harmonic of F. This further component is phase-adjusted by an angle N×θ, and stored. The procedure is repeated to build up sets of phase-adjusted first and further components, with θ chosen in each iteration for the first component to give a constant phase angle φ, and within any iteration the value of θ used for the first component is employed in the adjustment of the further component. The aggregated, phase-adjusted components exhibit increased signal-to-noise.
1 . 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(N), 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×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.
2 . The method of claim 1 wherein the signal channel in which the first frequency value is identified is not the same as the signal channel in which the further frequency value is identified.
3 . The method of claim 1 , in which the signal channel in which the first frequency value is selected is one of a voltage signal channel, a current signal channel, and an optical signal channel, and the signal channel in which the further frequency value is identified is a different one of a of a voltage signal channel, a current signal channel, and an optical signal channel.
4 . The method of claim 1 , wherein on each iteration, steps (d), (e) and (g) are carried out more than once such that a plurality of further frequency values are identified in step (d) each representing a harmonic of said first frequency F found within a signal channel from the plasma system, and for each such harmonic a respective further complex frequency-domain component is determined in step (e) which is transformed in step (g) to provide a respective phase-adjusted further complex signal component, and wherein step (i) is repeated separately for each such phase adjusted further complex signal component.
5 . The method of claim 4 , wherein on each iteration, steps (d), (e) and (g) are carried out at least once for a further frequency value identified in said first RF signal sample, and at least once for a further frequency value identified in a further RF signal sample from a different signal channel.
6 . The method of claim 5 , wherein the first frequency value is identified as a fundamental frequency in a voltage signal channel and wherein at least one further frequency value is a harmonic of said fundamental frequency in said voltage signal channel of order N>1, and further wherein another of said further frequency values is identified as a harmonic (N>=1) of the same fundamental frequency within a signal sample from a current signal channel or an optical signal channel.
7 . The method of claim 5 , wherein the first frequency value is identified as a fundamental frequency in a current signal channel and wherein at least one further frequency value is a harmonic of said fundamental frequency in said current signal channel of order N>1, and further wherein another of said further frequency values is identified as a harmonic (N>=1) of the same fundamental frequency within a signal sample from a voltage signal channel or an optical signal channel.
8 . The method of claim 6 , wherein the first frequency value and plurality of further frequency values include at least the first and second order harmonics from a current signal channel and the first and second order harmonics from a voltage signal channel.
9 . The method of claim 1 , wherein:
the step (b) of identifying a first frequency value comprises identifying a bin number b1 of a discrete Fourier transform in which a local maximum of signal magnitude is located or expected, and
the step (d) of identifying a further frequency value comprises determining the identity of a bin number b2=N×b1, subject to modulo arithmetic where the modulus is the total number of bins, and selecting a bin which lies within N bins adjacent to bin b2.
10 . The method of claim 9 , wherein the selection of a bin which lies within N bins adjacent to bin b2 comprises identifying a bin within said range where a local maximum of signal magnitude is found.
11 . The method of claim 9 , wherein the selection of a bin which lies within N bins adjacent to bin b2 comprises selecting a bin in which a frequency multiple F(N) will be found where the frequency F is known with a precision greater than the bin size.
12 . The method of claim 1 , wherein the value of the first frequency F drifts between successive iterations and wherein on each iteration the first frequency value F is identified as a local maximum signal within an expected frequency range.
13 . The method of claim 1 , further comprising the initial steps of receiving at least one RF signal sample from a signal channel and performing a transform of said at least one RF signal sample to the frequency domain.
14 . The method of claim 13 , wherein said transform is a discrete Fourier transform, and preferably a fast Fourier transform.
15 . The method of claim 1 , wherein steps (b) to (j) are repeated based on a different first frequency F′, at which a local maximum is found in the frequency domain, where F′ and F are not harmonics of one another.
16 . A computer program product comprising machine-readable instructions which, when executed in a processor provided with data representative of one or more RF signals of a plasma system, are effective to carry out the method of claim 1 .
17 . A system for analysing RF signals from a plasma system, comprising one or more processing circuits programmed to execute the method of claim 1 .
18 . The system of claim 16 , wherein said one or more processing circuits are implemented as a field programmable gate array and wherein said programming of said circuits comprises configuring said field programmable gate array with a logic function implementing said method.