A 490 GHz planar circuit balanced Nb-Al$_\mathbf{2}$O$_{\mathbf{3}}$-Nb quasiparticle mixer for radio astronomy: Application to quantitative local oscillator noise determination

摘要

This article presents a heterodyne experiment which uses a 380-520 GHz planarcircuit balanced Nb-$\mathrm{Al_2O_3}$-Nbsuperconductor-insulator-superconductor (SIS) quasiparticle mixer with 4-8 GHzinstantaneous intermediate frequency (IF) bandwidth to quantitatively determinelocal oscillator (LO) noise. A balanced mixer is a unique tool to separatenoise at the mixer's LO port from other noise sources. This is not possible insingle-ended mixers. The antisymmetric IV characteristic of a SIS mixer furtherhelps to simplify the measurements. The double-sideband receiver sensitivity ofthe balanced mixer is 2-4 times the quantum noise limit $h\nu/k_B$ over themeasured frequencies with a maximum LO noise rejection of 15 dB. This workpresents independent measurements with three different LO sources that producethe reference frequency but also an amount of near-carrier noise power which isquantified in the experiment as a function of the LO and IF frequency in termsof an equivalent noise temperature $T_{LO}$. In a second experiment we use onlyone of two SIS mixers of the balanced mixer chip, in order to verify theinfluence of near-carrier LO noise power on a single-ended heterodyne mixermeasurement. We find an IF frequency dependence of near-carrier LO noise power.The frequency-resolved IF noise temperature slope is flat or slightly negativefor the single-ended mixer. This is in contrast to the IF slope of the balancedmixer itself which is positive due to the expected IF roll-off of the mixer.This indicates a higher noise level closer to the LO's carrier frequency. Ourfindings imply that near-carrier LO noise has the largest impact on thesensitivity of a receiver system which uses mixers with a low IF band, forexample superconducting hot-electron bolometer (HEB) mixers.