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AIP conference proceedings

Biasing for precise frequency in a frequency-agile Josephson junction

Michael R. Daniel, Martin Ashkin, M. A. Janocko

Published Jan 1, 1978• 0 citations
Publisher

Abstract

Analog computer studies of a Josephson junction show that the microwave‐induced steps in the dc current‐voltage curve may be used as ideal (zero impedance) voltage sources with which to bias a second junction. Biased on one of these steps, the second junction is a microwave oscillator of precisely determined frequency. Because the biasing junction impedance →0 on a step, it contributes no noise to this second junction. A conceptual design using Dayem bridges fabricated as an integral part of a coplanar waveguide structure is proposed.

DOI
10.1063/1.31341

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IEEE Transactions on Applied Superconductivity•Jun 1, 1999•51 citations
Analog computer studies of a Josephson junction show that the microwave‐induced steps in the dc current‐voltage curve may be used as ideal (zero impedance) voltage sources with which to bias a second junction. Biased on one of these steps, the second junction is a microwave oscillator of precisely determined frequency. Because the biasing junction impedance →0 on a step, it contributes no noise to this second junction. A conceptual design using Dayem bridges fabricated as an integral part of a coplanar waveguide structure is proposed.
This final report of ONR Contract No. N00014-76-C-0609 describes some initial experimental work and then calculations on an analogue computer of properties of Josephson junctions used as a frequency agile source. The results of the calculations show that a junction may be used as a nearly ideal (zero impedance) dc voltage source with which to bias a second junction. The junctions must be isolated rf-wise from each other and the inductance level between them must not exceed about 10 to the minus 8 power H for operation at liquid helium temperatures. The second junction will then oscillate at a frequency determined by this bias voltage. Additionally the same junction bias source was shown to be capable of biasing a series array of junctions. The series array was shown to oscillate phase coherently at a frequency determined by the divided bias voltage across each junction. (Author)
Work with large two-dimensional arrays of Josephson junctions for submillimeter power generation is discussed. The basic design of the quasioptical Josephson oscillator (QJO) is presented. The reasons for each design decision are discussed. Superconducting devices have not yet been fabricated, but scale models and computer simulations have been done. A novel method for characterizing array RF coupling structures is described, and initial results with this model are presented. Microwave scale models of the radiation structure were built, and a series of measurements was made with a network analyzer. These measurements were summed in a computer to find properties of the structure when all elements in it are operating in phase. The goal of these measurements was to develop structures for oscillators which are tunable over a very broad band. Computer programs simulating a Josephson junction in any frequency-dependent coupling structure have been developed. An iterative harmonic balance technique finds the time-domain current and voltage waveforms across the junction for an arbitrary frequency-dependent RF load impedance. DC bias, and therefore oscillator frequency, can be held fixed in this technique, in contrast to time-domain techniques where the oscillation frequency is found after the waveform is found. With these programs design goals for maximum oscillator power and design limits to achieve tolerable harmonic distortion were found.
Josephson voltage standards utilize microwave-induced constant voltage steps occurring due to the ac Josephson effect. Existing standards can be considerably simplified and their accuracy improved by using a large number of series-connected Josephson tunnel junctions which are operated in the zero current step mode. For this purpose superconducting millimeter wave integrated circuits have been designed, fabricated, and tested. The circuits consist of a broadband taper between the rectangular waveguide and the planar structure, the Josephson junction series, a well-matched load, and dc pads. Circuits with various numbers of junctions have been fabricated by photo-lithographic techniques and tested at 4.2 K in liquid helium. The version with 1474 junctions produced voltages up to 1.2 V when operated at 90 GHz.
We present a microwave circuit for Josephson voltage standards. Here, the Josephson junctions are integrated in a microwave transmission line designed as coplanar strips (CPS). The new layout offers the possibility of achieving a higher scale of integration and to considerably simplify the fabrication technology. The characteristic impedance of the CPS is about 50 Ω, and this should be of interest for programmable Josephson voltage standard circuits with SNS or SINIS junctions. To demonstrate the function of the microwave circuit design, conventional 10 V Josephson voltage standard circuits with 17 000 Nb/AlOx/Nb junctions were prepared and tested. Stable Shapiro steps at the 10 V level were generated. Furthermore, arrays of 1400 SINIS junctions in this microwave layout exhibited first-order Shapiro steps.
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.
Nonhysteretic Josephson junctions with a superconductor-insulator-normal metal-insulator-superconductor (SINIS) structure fabricated using the reliable Nb-AlO/sub x/ technology have been used for the design and fabrication of a 1-V inherently stable Josephson voltage standard. The superconducting circuit can be driven at the same microwave frequencies as conventional SIS circuits. The microwave-induced 1-V step is suitable for calibration purposes. The successful operation of the 1-V circuit is due to an internal, phase-locked oscillation of all SINIS junctions which are embedded in a low-impedance strip-line. The microwave power required for exciting the coherent oscillation is very low and allows the integration of low power sources. Even operation without any external microwave source appears to be possible.