The following description concerns a subharmonic frequency converter for applications to millimetric radio waves. The circuit structure comprises a diplexer filter realized in thin film on alumina layer set up by a pass-band section for an incoming local oscillator signal LO/2 with subharmonic frequency, and a low pass section for a signal with intermediate frequency IF. The point common to both sections is connected to a gate of a low pass filter LPS realized in microstrip on a quartz layer letting making pass the subharmonic and blocking the fundamental. The other gate of the LPS filter is connected to a head of a pair of mixer diodes in GaAs put in antiparallel on the same quartz layer; the other head of the pair is connected to ground. The diodes are at the centre of a rectangular cavity closed on one side by a short-circuit plate and on the opposite communicating with a rectangular waveguide carrying a radiofrequency signal RF.