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IEEE Transactions on Applied Superconductivity

Stabilization Scheme for Hot-Electron Bolometer Receivers Using Microwave Radiation

Рябчун Сергей Александрович, Edward Tong, R. Blundell, Gregory N Gol'tsman

Published Dec 18, 2008• 12 citations
Publisher

Abstract

We present the results of a stabilization scheme for terahertz receivers based on NbN hot-electron bolometer (HEB) mixers that uses microwave radiation with a frequency much lower than the gap frequency of NbN to compensate for mixer current fluctuations. A feedback control loop, which actively controls the power level of the injected microwave radiation, has successfully been implemented to stabilize the operating point of the HEB mixer. This allows us to increase the receiver Allan time to 10 s and also improve the temperature resolution of the receiver by about 30% in the total power mode of operation.

DOI
10.1109/tasc.2008.2008398

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We report our studies of the effect of microwave radiation, with a frequency much lower than that corresponding to the energy gap of the superconductor, on the performance of the NbN hot-electron bolometer (HEB) mixer incorporated into a THz heterodyne receiver. It is shown that exposing the HEB mixer to microwave radiation does not result in a significant rise of the receiver noise temperature and degradation of the mixer conversion gain so long as the level of microwave power is small compared to the local oscillator drive. Hence the injection of a small, but controlled amount of microwave radiation enables active compensation of local oscillator power and coupling fluctuations which can significantly degrade the stability of HEB mixer receivers.

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•Jan 1, 2007•3 citations
We present the results of a stabilization scheme for terahertz receivers based on NbN hot-electron bolometer (HEB) mixers that uses microwave radiation with a frequency much lower than the gap frequency of NbN to compensate for mixer current fluctuations. A feedback control loop, which actively controls the power level of the injected microwave radiation, has successfully been implemented to stabilize the operating point of the HEB mixer. This allows us to increase the receiver Allan time to 10 s and also improve the temperature resolution of the receiver by about 30% in the total power mode of operation.
We report our studies of the effect of microwave radiation, with a frequency much lower than that corresponding to the energy gap of the superconductor, on the performance of the NbN hot-electron bolometer (HEB) mixer incorporated into a THz heterodyne receiver. It is shown that exposing the HEB mixer to microwave radiation does not result in a significant rise of the receiver noise temperature and degradation of the mixer conversion gain so long as the level of microwave power is small compared to the local oscillator drive. Hence the injection of a small, but controlled amount of microwave radiation enables active compensation of local oscillator power and coupling fluctuations which can significantly degrade the stability of HEB mixer receivers.
We have investigated the effect of injecting microwave radiation, with a frequency much lower than that corresponding to the energy gap of the superconductor, on the performance of the hot-electron bolometer mixer incorporated into a THz heterodyne receiver. More specifically, we show that exposing the mixer to microwave radiation does not cause a significant rise of the receiver noise temperature and fall of the mixer conversion gain so long as the microwave power is a small fraction of local oscillator power. The injection of a small, but controlled amount of microwave power therefore enables active compensation of local oscillator power and coupling fluctuations which can significantly degrade the gain stability of hot electron bolometer mixer receivers.
We report the results of our study of the stability of an 800 GHz hot electron bolometer (HEB) mixer cooled with a pulse-tube cryocooler. Pulse-tube cryocoolers introduce temperature fluctuations as well as mechanical vibrations at a frequency of$\sim1Hz,bothofwhichcancausereceivergainfluctuationsatthatfrequency.Inoursystem,themotorofthecryocoolerwasseparatedfromthecryostattominimizemechanicalvibrations,leavingthermaleffectsasthedominantsourceofthereceivergainfluctuations.Wemeasuredrootmeansquaretemperaturevariationsofthe4Kstageof1 Hz, both of which can cause receiver gain fluctuations at that frequency. In our system, the motor of the cryocooler was separated from the cryostat to minimize mechanical vibrations, leaving thermal effects as the dominant source of the receiver gain fluctuations. We measured root mean square temperature variations of the 4 K stage of1Hz,bothofwhichcancausereceivergainfluctuationsatthatfrequency.Inoursystem,themotorofthecryocoolerwasseparatedfromthecryostattominimizemechanicalvibrations,leavingthermaleffectsasthedominantsourceofthereceivergainfluctuations.Wemeasuredrootmeansquaretemperaturevariationsofthe4Kstageof\sim7mK.TheHEBmixerwaspumpedbyasolidstatelocaloscillatorat810GHz.Therootmeansquarecurrentfluctuationsatthelownoiseoperatingpoint(1.50mV,56.57 mK. The HEB mixer was pumped by a solid state local oscillator at 810 GHz. The root mean square current fluctuations at the low noise operating point (1.50 mV, 56.57mK.TheHEBmixerwaspumpedbyasolidstatelocaloscillatorat810GHz.Therootmeansquarecurrentfluctuationsatthelownoiseoperatingpoint(1.50mV,56.5\mu\hbox{A})were) were)were\sim 0.12\ \mu\hbox{A}$, and were predominantly due to thermal fluctuations. To stabilize the bias current, microwave radiation was injected to the HEB mixer. The injected power level was set by a proportional-integral-derivative controller, which completely compensates for the bias current oscillations induced by the pulse-tube cryocooler. Significant improvement in the Allan variance of the receiver output power was obtained, and an Allan time of 5 s was measured.
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.