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arXiv · 1205.4494

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

M. P. Westig·M. Justen·K. Jacobs·J. Stutzki·M. Schultz·F. Schomacker·C. E. Honingh
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Abstract

This article presents a heterodyne experiment which uses a 380-520 GHz planar circuit balanced Nb-$\mathrm{Al_2O_3}$-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 $hν/k_B$ 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 $T_{LO}$. In a second experiment we use only one of two SIS mixers of the balanced mixer chip, in order to verify the influence of near-carrier LO noise power on a single-ended heterodyne mixer measurement. We find an IF frequency dependence of near-carrier LO noise power. The frequency-resolved IF noise temperature slope is flat or slightly negative for the single-ended mixer. This is in contrast to the IF slope of the balanced mixer 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. Our findings imply that near-carrier LO noise has the largest impact on the sensitivity of a receiver system which uses mixers with a low IF band, for example superconducting hot-electron bolometer (HEB) mixers.

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M. P. Westig, M. Justen, K. Jacobs, J. Stutzki, M. Schultz, F. Schomacker, C. E. Honingh. 2012-08-27. 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. https://doi.org/10.1063/1.4764324

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