A reliable technique for wide band measurements of the spectral linewidth of superconducting oscillators integrated on‐chip with superconductor‐insulator‐superconductor (SIS) detectors has been developed. The spectral linewidth of flux‐flow oscillators (FFO) based on the unidirectional and viscous flow of magnetic vortices in a long overdamped Josephson tunnel junction was measured in the frequency range 250–580 GHz, and a linewidth as low as 200 kHz was obtained at 450 GHz. Also stable frequency locking of a FFO to very high (≤60th) harmonics of an external microwave reference source has been demonstrated. The proposed technique may improve the sensitivity, frequency resolution, and stability of the fully superconducting integrated submillimeter wave receiver. ©1996 American Institute of Physics.