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Tunable Matching Circuit for Millimeter Wave Superconductor-Insulator-Superconductor Mixers

IP.com Disclosure Number: IPCOM000038400D
Original Publication Date: 1987-Jan-01
Included in the Prior Art Database: 2005-Jan-31
Document File: 3 page(s) / 45K

Publishing Venue

IBM

Related People

Kaplan, SB: AUTHOR [+3]

Abstract

The use of the kinetic inductance of an array of superconductor-type Josephson junctions to adjust the operating frequency will allow precise in situ tuning of a resistance, inductance, capacitance (RLC)-type circuit, which is important for mixer applications. In the RLC circuit, the inductance is varied by adjusting the direct current (DC) through a single Josephson junction or an array of Josephson junctions. This allows control over the frequency at which the attached superconductor-insulator-superconductor (SIS) mixer circuit can operate. (Image Omitted) SIS tunnel junctions have found significant applications as quantum-limited mixers in the millimeter (mm) wave region. The junctions are biased near the energy gap on the quasiparticle characteristic, and mixing occurs due to its highly nonlinear nature.

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Tunable Matching Circuit for Millimeter Wave Superconductor-Insulator- Superconductor Mixers

The use of the kinetic inductance of an array of superconductor-type Josephson junctions to adjust the operating frequency will allow precise in situ tuning of a resistance, inductance, capacitance (RLC)-type circuit, which is important for mixer applications. In the RLC circuit, the inductance is varied by adjusting the direct current (DC) through a single Josephson junction or an array of Josephson junctions. This allows control over the frequency at which the attached superconductor-insulator-superconductor (SIS) mixer circuit can operate.

(Image Omitted)

SIS tunnel junctions have found significant applications as quantum-limited mixers in the millimeter (mm) wave region. The junctions are biased near the energy gap on the quasiparticle characteristic, and mixing occurs due to its highly nonlinear nature. It is desirable for the junction or array of junctions to appear as a purely resistive element. For this purpose, passive resonant circuits are employed to tune out the junction capacitance at the operating frequency. As an example, a 2-junction array and its matching circuit are shown in Fig. 1. The impedance of this circuit in magnitude and phase, intended to operate at 115 GHz, is shown in Fig. 2. It is seen that, within a narrow region around the operating point, the impedance of the total mixer circuit is real and equal to the sum of the junction resistances.

(Image Omitted)

The operating range of the circuit, however, is narrow, making it very difficult to assure that actual fabricated circuits will be optimized at the desired operating frequency. Even with good knowledge of the material parameters involved and excellent process control, variations in the characteristic frequency on the order of 10% would be expected, but generally this is too large an error to be tolerated. The performance is improved by using a tunable RLC circuit in which a major part of the total inductance is provided by the kinetic inductance of a Josephson junction array. The behavior of a Josephson junction is described by the equations I = Icsind and V = (D0/2f)dd/dt, where Ic is the junction critical current, d is the phase difference of the order parameter across the junction, and D0 is the flux quantum
(2.07x10-15 Weber). Using the usual definition of inductance (V=LdI/ dt), there is associated with the Josephon junction a kinetic inductance Lj = (D0/2fIc)[1 - (I/Ic)]-1/2 . The dependence of junction kinetic inductance on junction current is shown in Fig. 3A, for the limit of zero current. In Fig. 3B, the change in kinetic inductance with current is shown. Th...