Abstract: An oscillator for very high microwave or millimeterwave frequencies employs a pair of negative-resistance semiconductive devices, each mounted in its own resonant cavity. The two cavities are coupled together by an iris in their common wall. An output waveguide is coupled symmetrically to both sides of the common wall to load both cavities equally. An adjustable mode-control element projects into the cavity to the near vicinity of one of the devices to induce the oscillator to start in the desired mode. Additional dielectric or metallic tuners in the cavities provide a wide variation of frequency.