Noise limitation in helium-cooled parametric amplifiers

Abstract

The minimum noise temperature of a parametric amplifier depends on the varactor junction capacitance, spreading resistance and its thermal temperature. The time-dependent junction capacitance remains nearly constant by cooling. But the spreading resistance remains constant with cooling only if the varactor is degenerately doped and has an abrupt p-n junction. Actually the junction region is slightly graded and therefore not degenerate. Thus the spreading resistance increases two or three times by cooling from room temperature to 4°K. The higher the thermal resistance of the varactor the more the spreading resistance is heated by the pump power. The measured thermal resistance of some diffused varactors is nearly the same at room and helium temperatures, while the thermal resistance of an epitaxial varactor increases sharply at very low temperatures. So the optimum pump frequency for minimum noise may be much lower than calculated from previous theories which neglect the pump heating. This is because the pump heating decreases with decreasing pump frequency and is of greater importance at low ambient temperatures. Considering the increase and the heating of the spreading resistance the minimum diode noise temperature of a helium cooled parametric amplifier at 4 Gc/s is calculated to be 3°–4°K. This agrees with measured data.