AbstractA new flexible, broadband ECE diagnostic (FLECE) has been implemented on the DIII-D tokamak. This instrument features a spectral range of 50-300 GHz and a frequency response from quasi-DC up to 1 MHz. Electron cyclotron (EC) emission from the plasma is split off from the input beamline of the Michelson interferometer and directed by Gaussian optics to an InSb detector. Spectral filters such as dichroic plates and Fabry–Perot etalons are introduced into the optical path to tailor the measurement to desired frequency ranges. Applications of this instrument include fast third-harmonic electron temperature measurement, identification of mode numbers of MHD instabilities in conjunction with the heterodyne radiometer, and spectral measurement of high-harmonic emission by energetic electrons. Result from this diagnostic will be presented. Calculations of electron temperature derived from Fabry–Perot measurements of third-harmonic EC emission are in agreement with measurements from the heterodyne radiometer. The instrument has detected short bursts of high harmonic emission (>200 GHz) associated with MHD reconnections in the plasma. These bursts have been observed during the thermal quench stage of major disruptions and during the beta collapses associated with resistive wall modes.