The bias II feedback system: Understanding and improving stability in NbN HEB Terahertz receivers

Terahertz NbN heterodyne hot electron bolometer (HEB) mixer receivers currently provide the lowest receiver noise temperatures from about 1.2 THz to 5 THz. However, it has been well documented that these receivers are less stable, as quantified by the Allan variance, than other THz receivers (SIS and Schottky barrier diode mixers). In this paper we directly address the intermediate frequency (IF) stability of (HEB) receivers by designing and implementing a feedback system to stabilize the bias of the HEB device. We report a continuum Allan time of at least 20 seconds for a THz NbN HEB receiver.