The dependence of the performance of Superconductor-Insulator-Superconductor (SIS) mixers on the ambient temperature is investigated both theoretically and experimentally at millimeter and submillimeter wavelength. We found that the mixers' conversion gain plays a dominant role in the temperature-induced variation of the overall receiver sensitivity. To achieve a straightforward image of how the ambient temperature influences the conversion gain, we introduce a pure shape factor that allows for determining the mixers' conversion gain from the nonlinearity of their IV curves. We measured the performance of an SIS mixer at 100GHz band with the ambient temperature ranging from 3.8K to 8.5K to verify the validity of the quantum mixing theory. The experimental results are found in good agreement with the simulation results. This agreement indicates the validity of quantum mixing theory at a relatively high temperature up to the superconducting critical temperature. The measurement results show that at 100GHz band, the Nb SIS mixer can maintain a reasonable performance at the ambient temperature as high as 80% of the superconducting critical temperature. In addition to the gain and noise, the receiver stability in terms of Allen variance was also measured at various ambient temperatures without any apparent difference found.
The dependence of the performance of Superconductor-Insulator-Superconductor (SIS) mixers on the ambient temperature is investigated both theoretically and experimentally at millimeter and submillimeter wavelength. We found that the mixers' conversion gain plays a dominant role in the temperature-induced variation of the overall receiver sensitivity. To achieve a straightforward image of how the ambient temperature influences the conversion gain, we introduce a pure shape factor that allows for determining the mixers' conversion gain from the nonlinearity of their IV curves. We measured the performance of an SIS mixer at 100GHz band with the ambient temperature ranging from 3.8K to 8.5K to verify the validity of the quantum mixing theory. The experimental results are found in good agreement with the simulation results. This agreement indicates the validity of quantum mixing theory at a relatively high temperature up to the superconducting critical temperature. The measurement results show that at 100GHz band, the Nb SIS mixer can maintain a reasonable performance at the ambient temperature as high as 80% of the superconducting critical temperature. In addition to the gain and noise, the receiver stability in terms of Allen variance was also measured at various ambient temperatures without any apparent difference found.
The dependence of the performance of Superconductor-Insulator-Superconductor (SIS) mixers on the ambient temperature is investigated both theoretically and experimentally at millimeter and submillimeter wavelength. We found that the mixers' conversion gain plays a dominant role in the temperature-induced variation of the overall receiver sensitivity. To achieve a straightforward image of how the ambient temperature influences the conversion gain, we introduce a pure shape factor that allows for determining the mixers' conversion gain from the nonlinearity of their IV curves. We measured the performance of an SIS mixer at 100GHz band with the ambient temperature ranging from 3.8K to 8.5K to verify the validity of the quantum mixing theory. The experimental results are found in good agreement with the simulation results. This agreement indicates the validity of quantum mixing theory at a relatively high temperature up to the superconducting critical temperature. The measurement results show that at 100GHz band, the Nb SIS mixer can maintain a reasonable performance at the ambient temperature as high as 80% of the superconducting critical temperature. In addition to the gain and noise, the receiver stability in terms of Allen variance was also measured at various ambient temperatures without any apparent difference found.
A 600-700GHz SIS mixer with the twin-junction tuning circuit has been designed and fabricated [1] The surface impedance of thin-film superconducting microstrip lines, based on Mattis-Bardeen theory [2], is included in the optimization of RF impedance matching. The receiver noise temperature measured in the frequency range of 630-660GHz is below 200K and the lowest receiver noise temperature of 181K is achieved at 656GHz. The FTS response of the SIS mixer shows a good RF coupling from 600GHz to 700GHz. Both the noise performance and the FTS response can be quantitatively described by numerical results using the quantum theory of mixing. Some detail considerations on the mixer model calculation, such as spreading inductance around the junction tuning structure and mixer's embedding impedance, are discussed. Introduction Nb-based superconductor-insulator-superconductor (SIS) mixers have been used very successfully in sub-mm wavelength detection. Their receiver noise temperature has reached to three times the quantum limit, —3 hfrkB, below the junction's gap frequency. However, at high frequency bands, Nb SIS mixers need to be designed carefully because the photon energy is close to or higher than the gap energy of Nb, A-1.4meV. When the frequency band covers the gap frequency of Nb, —670GHz, the RF loss of Nb superconducting transmission lines increases dramatically and the surface impedance has strong frequency dependence. Usually, the RF properties of a superconducting transmission line can be described by the Mattis-Bardeen theory [2] in good accuracy. From results reported so far [4-9], the best receiver noise temperature is close to 5 hi7kB in the band of 600-700GHz. Despite of the input loss contributed by the experimental setup, some other properties of SIS mixers should be taken into account. In this paper, we characterize the performance of a 600-700GHz mixer by comparing the experimental and simulated results.
Heterodyne mixers based on superconducting SIS (superconductor-insulator-superconductor) tunnel junctions have been demonstrated to be the most sensitive coherent detectors at millimeter and submillimeter wavelengths. In fact, conventional superconducting SIS mixers with Nb/AIO(x)/Nb junction and Nb/SiO2/Nb tuning circuit have shown good performances with the noise temperature reaching as low as three times the quantum limit below 0.7THz, which is the gap frequency of Nb-based SIS junctions. However, due to the large loss in Nb thin-film superconducting microstrip lines, the noise performance of Nb SIS mixers deteriorates significantly above 0.7THz. With a gap frequency double that of Nb-based SIS junctions, NbN-based SIS junctions are of particular interest for the development of heterodyne mixers in the terahertz region.
A computer simulation is constructed to describe the performance of superconductor-insulator-superconductor (SIS) quasiparticle heterodyne receivers in terms of photon-assisted tunneling theory. The results predict that conversion efficiencies greater than unity can be realized at millimeter wave frequencies, with noise temperatures approaching the fundamental quantum limit. The nonlinearity responsible for the mixing is the extremely rapid onset of quasiparticle tunneling current in SIS junctions near the full gap voltage 2Δ/e, and is independent of the Josephson pair currents. Substantial gain is expected under a variety of conditions and for a wide range of source and load impedances.
We have designed, fabricated, and tested superconductor-insulator-superconductor (SIS) mixers based on Nb/AlN/NbN twin tunnel junctions for waveguide receivers operating in a frequency range of 790-950 GHz. Electromagnetic simulations and measurement results of the mixer performance are presented. The junctions have a high gap voltage of 3.15 mV and a high current density of about 30 kA/cm2, providing a wide receiver band, which was confirmed by Fourier transform spectrometer (FTS) and noise temperature measurements. The corrected receiver noise temperature varies from 240 K at low frequencies to 550 K at the high end of the band. The influence of the SIS junction heating on its characteristics has been studied and compared to another similar high current density technologies. The heating does not have a critical impact on the mixer performance.
Superconductor-Insulator-Superconductor (SIS) tunnel junction mixers are now commonly used in as- tronomical receivers at (sub)millimeter wavelengths because of their superb sensitivity, high dynamic range and stability of operation. Niobium SIS mixers operating at frequencies well below the super- conducting gap (∼680 GHz) have already achieved quantum limited sensitivity. Therefore to further enhance the receiver sensitivity, increasing the Intermediate Frequency (IF) bandwidth of SIS mixers has became crucial. This thesis focuses on the theoretical modeling, design and experimental verifi- cation of Nb SIS mixers operating around 230 GHz with a wide IF bandwidth of 1–15 GHz. These mixers were designed for a single baseline heterodyne interferometer (GUBBINS), which is being built to observe the Sunyaev-Zel’dovich effect in the Cosmic Microwave Background. The combination of wide IF bandwidth SIS mixers and complex analogue correlators will allow GUBBINS to feature high surface brightness sensitivity, that helps to distinguish the weak SZ effect from the background noise. The SIS mixer detector system was assembled inside the GUBBINS cryostat together with the IF electronics and RF/LO optical systems. Low noise temperatures of around 71 K were then measured in the GUBBINS system. The Nb SIS mixer we have developed uses a unilateral finline and fully integrated planar circuits deposited on a silicon substrate, to couple the electromagnetic radiation from the waveguide into the SIS junction. The finline mixer allows a broad-band RF coupling, an easy integration of the on-chip planar circuits and an easy-to-fabricate mixer block. To achieve a wide IF bandwidth, the output impedance of the SIS mixer was well matched to the input impedance of the amplifier by a multi-stage microstrip circuit. Additionally, the planar circuit of the SIS mixer was also designed to have a small lumped inductance and capacitance. The SIS mixer chip was extensively simulated by rigorous electromagnetic software (HFSS) and the S-parameter was exported to a quantum mixing package SuperMix to produce a full-wave model of the mixer. Experimental testing yielded a best noise temperature of 50 K with an average noise temperature of 75 K over an RF bandwidth of 160 GHz– 260 GHz. We have performed thorough experimental and computational investigation of the IF system in particular the constraints on the bandwidth caused by the lumped element capacitance of the mixer chip and the matching of the output impedance of the mixer to the IF amplifier. Our conclusion is that a bandwidth of 1–15 GHz could be achieved using our mixer design, subject to the performance of the amplifier. Finally, a variable temperature load system was successfully developed and tested inside the cryostat, to avoid the losses from the room-temperature optics. We have showed that the noise temperature of the SIS detector could be reduced by as much as 15 K by testing the mixer using a variable temperature load inside the cryostat.