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Yang ZQ, Hajenius M, Baselmans JJA, Gao JR, Klapwijk TM, Voronov B, et al. Improved sensitivity of NbN hot electron bolometer mixers by vacuum baking. In: Proc. 16th Int. Symp. Space Terahertz Technol.; 2005. p. 222–5.
Abstract: We find that the sensitivity of heterodyne receivers based on superconducting hot-electron bolometer (HEB) in- creases by 25 − 30% after baking at 85 o C and in a high vacuum. The devices studied are twin-slot antenna coupled HEB mixers with a small NbN bridge of 1×0.15 μm 2 . The mixer noise temperature, gain, and resistance versus temperature curve of a HEB before and after baking are compared and analyzed. We show that baking reduces the intrinsic noise of the mixer by 37 % and makes the superconducting transition of the bridge and the contacts sharper. We argue that the reduction of the noise is due to the improvement of the transparency of the contact/film interface. The lowest receiver noise temperature of 700 K is measured at a local oscillator frequency of 1.63 THz and a bath temperature of 4.3 K.
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Loudkov D, Tong C-YE, Blundell R, Kaurova N, Grishina E, Voronov B, et al. An investigation of the performance of the waveguide superconducting HEB mixer at different RF embedding impedances. In: Proc. 16th Int. Symp. Space Terahertz Technol.; 2005. p. 226–9.
Abstract: We have conducted an investigation of the performance of superconducting hot-electron bolometric (HEB) mixer at 800 GHz as a function of the embedding impedance of the waveguide embedding circuit. Using a single half-height mixer block, we have developed three different mixer chip configurations, offering nominal embedding resistances of 70, 35, and 15 Ohms. Both the High Frequency Structure Simulator (HFSS) software and scaled model impedance measurements were employed in the design process. Two batches of HEB mixers were fabricated to these designs using 3-4 nm thick NbN thin film. The mixers were characterized through receiver noise temperature measurements and Fourier Transform Spectrometer (FTS) scans. Briefly, a minimum receiver noise temperature of 440 K was measured at a local oscillator frequency 850 GHz for a mixer of normal state resistance 62 Ohms incorporated into a circuit offering a nominal embedding impedance of 70 Ohms. We conclude from our data that, for low noise operation, the normal state resistance of the HEB mixer element should be close to that of the embedding impedance of the mixer mount.
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Edward Tong C-Y, Loudkov DN, Paine SN, Marrone DP, Blundell R. Vector measurement of the beam pattern of a 1.5 THz superconducting HEB receiver. In: Proc. 16th Int. Symp. Space Terahertz Technol.; 2005. p. 453–6.
Abstract: Near-field vector beam pattern of the 1.5 THz superconducting Hot Electron Bolometer (HEB) receiver currently in operation in Northern Chile has been performed in our laboratory. Using an open waveguide probe, we have mapped both the amplitude and phase of the beam emerging from our 1.5 THz HEB receiver package, across a number of planes along the line of propagation of the radio-beam. With an integration time of about 100 ms per point, a signal-to-noise ratio of about 25 dB was achieved for a beam waist of 3.5 mm. These measurements have proved to be invaluable in achieving good alignment between the cryostat housing the HEB mixer and the remainder of the receiver and telescope optics. The accuracy of our beam measurement is estimated to be ±0.2 mm in position and ±5 arc minutes in angular displacement.
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Gao JR, Hajenius M, Baselmans JJA, Yang ZQ, Baryshev AM, Barends R, et al. Twin-slot antenna coupled NbN hot electron bolometer mixers for space applications. In: Proc. 9-th WMSCI. Vol 9. International Institute of Informatics and Systemics; 2005. p. 148–53.
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Jiang L, Zhang W, Yao QJ, Lin ZH, Li J, Shi SC, et al. Characterization of a quasi-optical NbN superconducting hot-electron bolometer mixer. In: Proc. PIERS. Vol 1.; 2005. p. 587–90.
Abstract: In this paper, we report the performance of a quasi-optical NbN superconducting HEB (hot electron bolome-ter) mixer measured at 500 GHz. The quasi-optical NbN superconducting HEB mixer is cryogenically cooled bya 4-K close-cycled refrigerator. Its receiver noise temperature and conversion gain are thoroughly investigatedfor different LO pumping levels and dc biases. The lowest receiver noise temperature is found to be approxi-mately 1200 K, and reduced to about 445 K after correcting theloss of the measurement system. The stabilityof the mixer’s IF output power is also demonstrated.
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