Signal mixer

A class AB mixer combines a signal i.e. a square wave, from a local oscillator (26) with two unidirectional pulse trains (23, 25) which, in combination, represent an input signal in phase and amplitude. Input circuitry (20) generates the pulse trains by splitting alternate half wave cycles of the input signal into two components and inverting the polarity of one of said components. The pulse trains have the same polarity and revert substantially to zero voltage and current between pulses, thereby to reduce switching noise in the mixer. The RF input signal may be applied via an RF amplifier 21 and a driver 20 which may be of the class AB or B type. The input signal may alternatively be applied via a transformer to respective bases of two drive transistors. The bases of the mixer transistors Q2, Q3 may be connected to a common bias voltage or respectively connected to bases of Q4, Q1.