We present an inexpensive, combined intermodulation-numerical differentiation system that provides fast on-line analogic EEDF measurements with the advantage of convolution inversion. The measured signal only requires numerical smoothing for the calculation of higher order derivatives, i.e. we do not need to smooth the probe characteristic. We present results of this technique applied to a surface wave sustained coaxial discharge. With the frequency constraints imposed by large capacitances in the system-experiment interfacing devices, the measurement of the second derivative at one single point is accomplished in 200 mu s. Without averaging over multiple data points and with the limitation of a 12-bit acquisition board, a 3-decade EEDF range is achieved. This is made possible by taking advantage of the optimum transfer function provided by the analogic 3-point differentiation technique and using a finite amplitude stepped probing signal.
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