Performance Investigation of a Quasi-Optical NbN HEB Mixer at Submillimeter Wavelength

In this paper, the performance of a quasi-optical phonon-cooled NbN superconducting hot electron bolometer (HEB) mixer is thoroughly investigated. The variation of the receiver noise temperature and the conversion gain around 600 GHz with respect to DC bias voltage and LO pumping level has been measured. The lowest receiver noise temperature is 750 K and reduced to about 250 K after correcting the quasi-optical losses before the HEB mixer. We compared the measured results with the predictions according to one dimensional distributed hot spot model. A reasonable match between theoretical model and experiment is observed. In addition, the direct response of the HEB mixer (integrated with a log-periodic spiral antenna) is measured by using Fourier Transform Spectrometer (FTS). The stability of the receiver is also investigated by means of Allan variance time measurements performed with an intermediate frequency (IF) bandwidth of 40 MHz.