• Langmuir probe;
  • eedf;
  • second derivative;
  • digital filter;
  • FIR filter;
  • smoothing;
  • window function


The energy distribution, the mean energy and the density of the electrons of low pressure plasmas may be obtained from the 2nd derivative of a Langmuir probe characteristic. Often numerical methods are used for differentiation. A simple numerical technique is the non-recursive digital or FIR filtering. Because of noise this process has to comprise also smoothing elements. The best way to achieve this is the successive use of a weakly smoothing interpolation based twofold differentiating filter and a strong smoothing filter. None of the usual smoothing filters exhibits all properties needed in the evaluation of probe characteristics. Window function based FIR filters represent a class of smoothing filters which is especially suited in this context because of its increasing stop band attenuation. A new smoothing filter based on a new parameterized window function is introduced. The testing of this new filter is performed and hints for finding of the filter parameters are given (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)