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A new fast-sweep, broadband frequency-modulated reflectometer on the DIII-D tokamak has produced routine, reliable density profiles with high spatial (⩽1 cm) and temporal resolution (∼100 μs). The system utilizes a solid-state microwave source and an active quadrupler, covering the full Q-band (33–50 GHz) and providing high output power (20–60 mW). The rf source frequency is linearized using an arbitrary function generator and the temperature of the source is actively controlled to reduce rf frequency drifts. The system hardware allows the rf frequency to be swept fullband in 10 μs, but, due to digitization limits, the sweep time used currently is 75–100 μs. The reliability of the reconstructed profiles was improved by a combination of fast frequency sweep, which reduces density fluctuation effects on the measurements, and advanced signal analysis based on digital complex demodulation, which improves phase accuracy. The fast-sweep system has resolved fast-changing edge density profiles during edge localized modes with unprecedented resolution. © 1997 American Institute of Physics.