Cryogenic Hybrid Coupler for Ultra-Low-Noise Radio Astronomy Balanced Amplifiers

Modern heterodyne ultra low noise receivers used for radio astronomy have evolved to provide very wide instantaneous bandwidth. Some of the configurations used in present cryogenic front-ends, such as sideband separating mixers and balanced amplifiers, require 90° hybrids as a part of the intermediate frequency (IF) circuitry. A very common choice for the band of operation of these hybrids is 4-12-GHz band. There are devices commercially available covering this band with good ambient temperature characteristics, but their cryogenic performance degrades to unacceptable levels. This paper describes the design, construction and measurement of a multioctave stripline hybrid for the 4-12 GHz band specially conceived to operate satisfactorily when cooled down to 15 K (-258°C). The materials and mechanical construction have been carefully selected and the result is a very compact, reliable and low thermal mass device, capable to withstand extreme thermal cycling. The coupling and reflection characteristics of the prototype show very low temperature dependence. The utility of the hybrid developed is demonstrated in a cryogenic balanced amplifier. The noise temperature obtained using this configuration (6 K) shows clear advantage (about 33%) over the classical isolator-amplifier combination normally used in heterodyne radio astronomy receivers. Besides, a superior insensitivity of the amplifier's noise performance to the input termination mismatch is obtained.