A 490 GHz planar circuit balanced Nb-Al2O3-Nb quasiparticle mixer for radio astronomy: Application to quantitative local oscillator noise determination

This article presents a heterodyne experiment which uses a 380–520 GHz planar circuit balanced Nb-Al2O3-Nb superconductor-insulator-superconductor (SIS) quasiparticle mixer with 4–8 GHz instantaneous intermediate frequency (IF) bandwidth to quantitatively determine local oscillator (LO) noise. A balanced mixer is a unique tool to separate noise at the mixer's LO port from other noise sources. This is not possible in single-ended mixers. The antisymmetric IV characteristic of a SIS mixer further helps to simplify the measurements. The double-sideband receiver sensitivity of the balanced mixer is 2–4 times the quantum noise limit hν/kB over the measured frequencies with a maximum LO noise rejection of 15 dB. This work presents independent measurements with three different LO sources that produce the reference frequency but also an amount of near-carrier noise power which is quantified in the experiment as a function of the LO and IF frequency in terms of an equivalent noise temperature TLO. Two types of LO ...