The genesis of SIS mixers - the legacy of John Tucker in radio astronomy

In 1979, John Tucker showed that if the nonlinearity of a mixer diode is so sharp that its conductance changes significantly over a voltage increment hf/e, then its mixer characteristics are no longer described by classical mixer theory (e.g, as normally used for Schottky diode mixers). Under these conditions a resistive diode mixer can have conversion gain and/or negative port impedances. Tucker´s quantum mixer theory provided the theoretical foundation for a revolution in low-noise coherent receivers for radio astronomy in the millimeter and sub-millimeter bands.