Our research activities in the field of plasma diagnostics deal with the fast and precise determination of the electron density in low-pressure plasmas. The method used, active plasma resonance spectroscopy, is based on the electrical excitation of spatially localized high-frequency plasma oscillations in the range of 1-6 GHz. Various diagnostic concepts are being developed based on the principle of the so-called multipole resonance probe. Time Domain Reflectometry (TDR) and Frequency Modulated Continuous Wave (FMCW) technology are used as measurement methods. The evaluation of the measured resonance spectra is carried out using specialized methods of fast digital signal processing. The concept is contamination-free, robust and allows high-precision measurements that can be evaluated in-situ. For this reason and because of its low investment and maintenance costs, it is particularly interesting for monitoring and controlling technical plasma processes in the industrial production of functional optical layers.