It is well known that the noise figure of a receiving system is directly proportional to the conversion power-loss ratio of the mixer. The paper is concerned with a mixer circuit whose loss, under certain conditions, can theoretically approach zero. A current-driven lattice mixer, using four matched semiconductor diodes, is analysed in detail, assuming a diode law as observed in practice. The design formulas are derived under matched conditions for the conversion power loss and the input and load impedances of the mixer. These formulas are expressed as functions of the termination at the image frequency, the diode quality factor and the local-oscillator current drive. It is finally concluded that losses below 1 dB are theoretically possible in a mixer using a lattice configuration of practical diodes. This conclusion is supported by the results obtained, mainly at low frequencies, using experimental circuits constructed for exploratory purposes.