A Fast Fourier Transform (FFT) based wide range millimeter wave diagnostics for spectral characterization of scattered millimeter waves in plasmas has been successfully brought into operation. The scattered millimeter waves are heterodyne down-converted and directly digitized using a fast analog-digital converter (ADC) and a compact Peripheral Component Interconnect (cPCI) computer. Frequency spectra are obtained by FFT in the time domain of the intermediate frequency signal. The scattered millimeter waves are generated during high power Electron Cyclotron Resonance Heating (ECRH) experiments on the TEXTOR Tokamak and demonstrate the performance of the diagnostics and, in particular, the usability of direct digitizing and Fourier transformation of millimeter wave signals. Major benefit of the new diagnostics is a tunable time and frequency resolution due to post-detection, near-Real-Time processing of the acquired data. This diagnostics has a wider application in astrophysics, earth observation, plasma physics and molecular spectroscopy for the detection and analysis of millimeter wave radiation, providing high-resolution spectra at high temporal resolution and large dynamic range. Such a diagnostics also has the potential to detect Electron Cyclotron Emission (ECE) and to be used in Real-Time ECE feedback control systems. © 2009 IEEE.