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International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz)

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

Ric Zannoni, K. S. Yngvesson

Published Sep 1, 2008• 1 citation
Publisher

Abstract

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.

DOI
10.1109/icimw.2008.4665750

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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.
In this paper we discuss the stability of heterodyne terahertz receivers based on small volume NbN phonon cooled hot electron bolometers (HEBs). The stability of these receivers can be broken down in two parts: the intrinsic stability of the HEB mixer and the stability of the local oscillator (LO) signal injection scheme. Measurements show that the HEB mixer stability is limited by gain fluctuations with a 1∕f spectral distribution. In a 60MHz noise bandwidth this results in an Allan variance stability time of ∼0.3s. Measurement of the spectroscopic Allan variance between two intermediate frequency (IF) channels results in a much longer Allan variance stability time, i.e., 3s between a 2.5 and a 4.7GHz channel, and even longer for more closely spaced channels. This implies that the HEB mixer 1∕f noise is strongly correlated across the IF band and that the correlation gets stronger the closer the IF channels are spaced. In the second part of the paper we discuss atmospheric and mechanical system stability requirements on the LO-mixer cavity path length. We calculate the mixer output noise fluctuations as a result of small perturbations of the LO-mixer standing wave, and find very stringent mechanical and atmospheric tolerance requirements for receivers operating at terahertz frequencies.
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To put space-based and airborne heterodyne instruments into operation at frequencies above 1 THz the superconducting NbN hot-electron bolometer (HEB) will be incorporated into heterodyne receiver as a mixer. At frequencies above 1.3 THz the sensitivity of the NbN HEB mixers outperform the one of the Schottky diodes and SIS-mixers, and the receiver noise temperature of the NbN HEB mixers increase with frequency. In this paper we present the results of the noise temperature measurements within one batch of NbN HEB mixers based on 3.5 mn thick superconducting NbN film grown on Si substrate with MgO buffer layer at the LO frequencies 2.5 THz and 3.8 THz.
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.
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Low noise receivers based on superconducting niobium nitride (NbN) hot electron bolometer (HEB) mixers have been designed, fabricated and measured in the terahertz (THz) frequency range for applications in astronomy and cosmology. The NbN HEB mixer consists of a planar antenna and an NbN bridge connecting across the antenna's inner terminals on high-resistivity Si substrates. Double sideband (DSB) receiver noise temperatures of 920 K at 1.6 THz and 1630 K at 2.5 THz have been obtained at 4.2 K without corrections. Effects of Parylene C AR coating on the lens have been studied; with such a coating a lowest DSB receiver noise temperature of 1135 K at 2.5 THz has been obtained at 4.2 K.