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Determining input loss in SIS receivers

Authors:
C.E. Honingh, S. Haas, D. Hottgenroth, K. Jacobs, J. Stutzki
Abstract:
We present measurements on two high frequency S(uperconductor) I(nsulator) S(uperconductor) heterodyne receivers at 660 GHz and 810 GHz. Their noise temperatures are 163 K and 860 K, respectively. The contribution of the high frequency (RF) input loss to this number is measured with the commonly used method of intersecting lines. The result is compared to a more rigorous analysis using the full quantum theory of mixing. We will show that in this frequency range the method of intersecting lines in general leads to an overestimation of the RF input loss. Even at 660 GHz, with a well matched SIS junction, the input noise contribution is overestimated by about 25 percent. This is due to the summation of the contributions due to the vacuum fluctuations, which is the highest contribution, the mismatch at the intermediate frequency (IF), and the fact that the mixer output noise is not completely independent of local oscillator (LO) power. At 810 GHz, due to a much stronger loss-induced mismatch of the SIS junction, the mixer output noise varies considerably with LO-power. The large loss of approximately 8 dB directly in front of the mixer enhances this contribution to the measured RF input loss. The contribution of the 1F-mismatch is likewise enhanced. Together with the contribution of the vacuum fluctuations, this can lead to an overestimation of the RF input noise by more than 100 percent, depending on the actual mismatch of the mixers. We therefore conclude that, for the severe mismatch that occurs in IVb based SIS mixers above 700 GHz, the method of intersecting lines gives incorrect results and the full analysis should be applied.
Categories:
SIS
Year:
1997
Session:
2
Full-text:
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Page Number(s):
92-100