The following description regards a frequency converter (1) for applications on millimetric radio waves. The circuit structure comprises a diplexer filter realized in thin film on an alumina layer connected to a balun set up by a quartz plate on which two mixing diodes in GaAs have been mounted, placed in the centre of a rectangular cavity (2) closed on one side by a short circuit plate and on the opposite side communicating with a rectangular waveguide (4) transporting a radio frequency signal RF. On the upper side of the plate three parallel metallic bands have been put in the sense of the width of the cavity, between the bands the diodes are connected in series. The most external bands are welded to the walls of the cavity (2), the central one on the other hand is connected to the point common to both sections of the diplexing filter by means of a metallic strap in the air welded to a microstrip. The ports of the local oscillator signals LO and an intermediate frequency IF are set up by two connected coaxial connectors (7, 8) by means of glass-bead (5, 6) to respective microstrips of the diplexer filter.