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Current-voltage curves of a NbN HEB mixer taken at different LO powers, with a LO frequency of 5.3 THz, at a bath temperature of 4.2 K, where the optimum operating region is indicated. The inset shows a distribution of the electron temperature and the normalized local resistivity calculated for the long NbN bridge (with a critical temperature of 9.3 K).
Schematic picture of the measurement setup, where the hot/cold loads and the beam splitter are built into a vacuum unit, directly attached to the HEB cryostat. Switching between the hot and cold load is done by rotating a mirror.
Measured DSB receiver noise temperature as a function of the current of the HEB obtained at a bias voltage of 0.6 mV and an IF of 1.5 GHz for five different LO frequencies. In the inset: measured receiver output powers, responding to hot and cold loads, vs current in the HEB at 5.3 THz (dots) and the polynomial fit (lines).
Measured minimal DSB receiver noise temperature of the HEB mixer at different LO frequencies (squares) and fitted curves for three different values of , using Eq. (2) (lines). The noise temperature contributed by only quantum noise is also shown. The inset shows all the curves at optimum LO power at the five different frequencies.
Summary of the data for five LO frequencies : optical loss of the Mylar beam splitter at 300 K (, calculated), heat filter at 4 K (, measured), uncoated Si lens at 4 K (, reflection loss calculated; absorption loss is negligible based on our measurements), coupling between antenna and HEB (, calculated), DSB conversion loss including optical loss (, measured) and DSB receiver noise temperature (, measured).
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