Approaching noise performance limits of Schottky-barrier mixers at millimeter waves

An ultra low-noise Schottky mixer was developed for 80 to 120 GHz. When the mixer is cooled to 20 K, its double sideband noise temperature is 50 K over the whole frequency band. At the best operation points noise temperature is 35 to 40 K. A combination of this mixer and a cryogenic FET-amplifier at 1.4 GHz yields a receiver noise temperature of 62 K at 115 GHz. Studies were carried out with a 30:1 scale model, at 2.5 to 12 GHz: fundamental and 2nd and 3rd harmonic bands. At fundamental frequencies, whisker length and shape are found to be important. A formula for the whisker inductance was found. At second harmonic frequencies the backshort and RF-filter dominate. An equivalent circuit of the embedding impedance derived from the scale model measurement results was used with diode data to calculate the theoretical noise temperature of the mixer and the receiver. A good agreement with experiments is found.