Improved Multi-octave 3 dB IF Hybrid for Radio Astronomy Cryogenic Receivers

Modern mm and sub-mm 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, like sideband separating mixers and balanced amplifiers, need 90o 3 dB hybrids at the IF, typically in the 4-12 GHz band. There are commercially available devices covering this band with good ambient temperature characteristics, but poor cryogenic performance. We describe the design, construction and measurement of a multioctave stripline hybrid for the 4-12 GHz band specially conceived to perform reliably when cooled to 15 K. The coupling and reflection show very little temperature dependence. A balanced cryogenic amplifier was assembled with two 3 dB hybrid units and available amplifiers (~4.5 K noise temperature) designed and built in-house for ALMA. This device is critically compared with a single ended amplifier and with an amplifier with an input isolator. The latter is the typical arrangement of the IF of radio astronomy receivers. The balanced option shows an advantage of 2.8 K in noise with less sensitivity to input mismatches.