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.
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.
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.
It has been shown that low-noise receivers can be constructed at millimeter wavelengths by using mixers containing superconducting tunnel junctions as the nonlinear elements. This is possible because of both the low intrinsic noise of these devices and their potential for high conversion gain. Here, the quantum theory of mixing is used to derive the full-noise parameters and small-signal parameters of sinusoidally pumped SIS (superconductor-insulator-superconductor) junctions. These are then put into a form that allows the theory of two-port linear networks to be brought to bear, allowing calculation of such useful parameters as minimum noise temperature, optimum source impedance, available (or exchangeable) gain at minimum noise, and stability factor. These quantities are properties of the pumped junction that do not depend on the source or load impedance, but do depend on the terminations at the image and harmonic sideband frequencies. The harmonic sidebands are taken to be shorted, and the image termination dependence is studied. Numerical results are presented for both ideal (BSC theory) and practical (measured current-voltage characteristic) junctions. The noise parameters of the cascade connection of an SIS mixer and a (noisy) IF amplifier are considered, leading to a specification of the optimum coupling network between the two. It is noted that the SIS mixer is usually not unconditionally stable, but that oscillation can be avoided by careful design of the IF coupling network.< >
We have investigated high-frequency limits of superconducting quasiparticle mixers from experiments on Al/Al-oxide-In/Al (or Ag) tunnel junctions, and from calculations based on Tucker’s quantum theory of mixing (QTM). The small superconducting energy gap of Al enabled us to study mixer performance at around the junction gap frequency, fg =(Δ1+Δ2)/h≊87 GHz, of superconductor-insulator-superconductor (SIS) mixers, and at twice the junction gap frequency for S-I-normal metal (SIN) mixers ( fg =Δ1/h) with available millimeter wave sources. Because of the low transition temperature of Al (Tc ≊1.3 K), a dilution refrigerator was used for providing temperatures below 1 K. The best mixer conversion efficiency was obtained for a low-resistance (∼34 Ω) six-junction array SIS mixer operated at f/fg ≊1 (in this case 73 GHz). The coupled single sideband conversion gain (with no image rejection) was −13.8 dB and the available conversion gain was estimated to −8.4 dB. Complicating nonequilibrium phenomena made it difficult to compare the performance of these relatively high-current-density mixers to the QTM. However, good agreement between calculated and corresponding measured conversion efficiencies and dynamic resistance values was obtained for the low-current-density mixers. Noise due to switching between rf-induced Josephson steps and interference from Josephson mixing could be quenched by a magnetic field. Several high-frequency limits are considered in this paper (such as Josephson switching noise, nonequilibrium phenomena, pair breaking, and energy gap limitations). Our results indicate that submillimeter wave SIS mixer receivers are likely to perform well, at least up to their gap frequencies, i.e., the THz region for large gap superconductors. Our QTM calculations showed that conversion gain larger than unity should be possible up to twice the gap frequency. Above this frequency the conversion loss will increase rapidly.