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Japanese Journal of Applied Physics

Optimum Bias Conditions for K-Band Microwave Mixing Using YBaCuO Step-Edge Microbridge Josephson Junction

Yoshito Fukumoto, Rikuo Ogawa, Yoshio Kawate

Published Nov 1, 1993• 6 citations
Publisher

Abstract

Characteristics of the K-band microwave mixing using a YBaCuO step-edge microbridge Josephson junction were systematically investigated, and the optimum bias conditions were experimentally determined. A large intermediate frequency (IF) output signal was observed at every midway between the Shapiro steps, and the maximum IF output was obtained between the 0th and 1st steps (0-1) for the local oscillation (LO) power level of I 0/I c=0.2-0.3 (absolute LO power of -49.9 dBm). The maximum conversion efficiency was estimated to be -26.6 dB. Although the IF output (0-1) was observed for a wide range of LO power, it vanished or decreased radically under some peculiar conditions (I 0/I c?0 and ?(I 0/I c)/?I LO?0). These characteristics were qualitatively accounted for by the basic Josephson-mixing theory.

DOI
10.1143/jjap.32.4966

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Characteristics of the K-band microwave mixing using a YBaCuO step-edge microbridge Josephson junction were systematically investigated, and the optimum bias conditions were experimentally determined. A large intermediate frequency (IF) output signal was observed at every midway between the Shapiro steps, and the maximum IF output was obtained between the 0th and 1st steps (0-1) for the local oscillation (LO) power level of I 0/I c=0.2-0.3 (absolute LO power of -49.9 dBm). The maximum conversion efficiency was estimated to be -26.6 dB. Although the IF output (0-1) was observed for a wide range of LO power, it vanished or decreased radically under some peculiar conditions (I 0/I c?0 and ?(I 0/I c)/?I LO?0). These characteristics were qualitatively accounted for by the basic Josephson-mixing theory.
Current-voltage characteristics and microwave mixing properties of step-edge microbridge Josephson junctions (SEJJs) made of YBaCuO thin films were systematically investigated. The step-edge was formed on the surface of a MgO(100) substrate employing the common photolithograph and wet-etching techniques. YBaCuO thin film was deposited by the off-axis sputtering method, and the microbridge junctions were fabricated so as to cross the step-edge. The SEJJs showed typical DC and AC characteristics which were explained by the resistively shunted junction (RSJ) model. Mixing experiments were carried out in the K-band microwave region. An intermediate frequency (IF) output with the frequency range of 20 MHz~2.0 GHz was successfully obtained.
Millimeter-wave detection was investigated using a step-edge microbridge Josephson junction fabricated by a YBaCuO thin film for both the video detection and harmonic mixing detection. Current–voltage (I–V) characteristics, which could be described by the resistively shunted junction (RSJ) model, were obtained up to 70 K with the IcR product of 150 μV at 70 K. The responsivity and noise equivalent input power were calculated as 2400 W/V and 4.4×10−14 W/√Hz, respectively, in the video detection assuming the RSJ model approximation. Clear dependence of the intermediate frequency output on the applied dc bias voltage was observed in the harmonic mixing detection with the harmonic numbers of 5–16, reflecting the change of dynamic resistance at the constant voltage steps induced by the local oscillation signal on the I–V curve. A minimum detection power of −59.4 dB m and a conversion efficiency (η) of −49.6 dB were obtained for the harmonic number (N) of 5 at 60 K. The relationship between the η and N was expressed as η∝N−2.2.
Current-voltage characteristics and microwave mixing properties of step-edge microbridge Josephson junctions (SEJJs) made of YBaCuO thin films were systematically investigated. The step-edge was formed on the surface of a MgO(100) substrate employing the common photolithograph and wet-etching techniques. YBaCuO thin film was deposited by the off-axis sputtering method, and the microbridge junctions were fabricated so as to cross the step-edge. The SEJJs showed typical DC and AC characteristics which were explained by the resistively shunted junction (RSJ) model. Mixing experiments were carried out in the K-band microwave region. An intermediate frequency (IF) output with the frequency range of 20 MHz∼2.0 GHz was successfully obtained.
The conversion efficiency and minimum detection power of a step-edge microbridge Josephson junction mixer (SEJJ mixer) were evaluated for both fundamental mixing and harmonic mixing. The SEJJ was fabricated by a YBaCuO thin film and all of the circuits inside the mixer were made from microstrip line circuits in order to reduce the microwave propagation losses. The conversion efficiencies (η) were -31 dB, -46 dB and -53 dB for fundamental mixing, second harmonic mixing and third harmonic mixing, respectively. The dependence of η on the harmonic number (N) was expressed as η\proptN -4.6 for N≦3. The minimum detection power (MDP) of -60 dBm (1.0 nW) was obtained in fundamental mixing, with a linear dynamic range of more than 40 dB.
Using a superconducting weak link (bridge) made of YBCO, the authors have successfully observed Josephson harmonic mixing between a local oscillation (LO) at X band and a signal at Ka band. Specifically, fourth-order harmonic mixing between 35.47 GHz and 8.843 GHz was observed. For a given signal level, the IF (intermediate frequency) responses depended strongly on both LO level and DC voltage bias across the junction. For zero bias, there is still IF output which is characteristic of even-order harmonic mixing. When the LO power level is such that the zero voltage current is suppressed to the second zero, the IF responses are maximum if the junction is biased between the zeroth and first steps induced by the LO. Also studied is the internal oscillation-mixing. An incoming signal at either X or Ka band gives rise to IF signal peaks if the junction is biased at voltages such that the corresponding internal Josephson frequencies are equal to, or harmonics of, the incoming signal frequency.>
Microwave superconducting mixers operating at 10–13 GHz utilizing Josephson junctions have been investigated analytically and experimentally. Measurements on mixer sensitivity were carried out at 13 GHz using waveguide-mounted experimental point-contact Josephson junctions at 4.2 K. Results showed sensitivities of −90 dBm/MHz (10−18 W/Hz) which included mismatch losses encountered in the waveguide structure. The Josephson junction mixing mechanism was analyzed on the basis of small-signal modulation of the staircase like dc I-V characteristics. It was shown that a small-signal microwave excitation results in an ac lateral shift of the regions between steps, such that: (a) Optimum Josephson mixing occurs halfway between steps. (b) Mixing occurs between all steps with maximum conversion efficiency between the zero and first step. (c) Predicted low conversion loss and possibly internal gain for a matched mixer. The analytical model was simulated by a digital computer and the behavior of the Josephson mixer predicted for various conditions of excitation. The model chosen for the analysis and the computer simulation is based on the simplest assumptions concerning bulk-formed Josephson junctions. In the laboratory an external local oscillator set 60 MHz below the signal frequency was used to induce the well-known step structure in the dc I-V characteristic of the point-contact Josephson junction. Mixing of the Lo and signal was found to occur between constant voltage steps in agreement with both analysis and computer simulation. The correctness of the model is demonstrated by the agreement between the theory and experiment.