A fast-sweep broadband FM reflectometer system has been successfully developed and operacted at the DIII-D tokamak, producing reliable density Profiles with excellent spatial (1 $leq$ cm) and temporal resolution (~100 $mu$ s). The system uses a solid-state microwave oscillator and an active quadrupler, covering full Q-band frequencies (33~50 GHz) and providing relatively high output power (20~60 mW). The system hardware allows fu11band frequency sweep in 10 $mu$ s, but due to digitization rate limit on DIII-D, sweep time was limited to 75~100 $mu$ s. Fast frequency sweep has helped to reduce density fluctuation effects on the reflectometer phase measurements, thus improving reliability for individual sweeps. The fast-sweep system with high spatial and temporal resolution has allowed to measure fast-changing edge density profiles during plasma ELMS and L-H transitions, thus enabling fast-time sca1e physics studies.