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Microstrip Superconducting Quantum Interference Device Amplifiers with Submicron Josephson Junctions: Enhanced Gain at Gigahertz Frequencies

Plourde, Britton, M. P. DeFeo, P. Bhupathi, Yu, Kang, Tom Heitmann

Published Aug 5, 2010• 9 citations
OpenAlex

Abstract

We present measurements of an amplifier based on a dc superconducting quantum interference device (SQUID) with submicron Al-AlOx-Al Josephson junctions. The small junction size reduces their self-capacitance and allows for the use of relatively large resistive shunts while maintaining nonhysteretic operation. This leads to an enhancement of the SQUID transfer function compared to SQUIDs with micron-scale junctions. The device layout is modified from that of a conventional SQUID to allow for coupling signals into the amplifier with a substantial mutual inductance for a relatively short microstrip coil. Measurements at 310 mK exhibit gain of 32 dB at 1.55 GHz.

Related papers

Microstrip superconducting quantum interference device amplifiers with submicron Josephson junctions: Enhanced gain at gigahertz frequencies

M. P. DeFeo, P. Bhupathi, K. Yu +4 more

We present measurements of an amplifier based on a dc superconducting quantum interference device (SQUID) with submicron Al–AlOx–Al Josephson junctions. The small junction size reduces their self-capacitance and allows for the use of relatively large resistive shunts while maintaining nonhysteretic operation. This leads to an enhancement of the SQUID transfer function compared to SQUIDs with micron-scale junctions. The device layout is modified from that of a conventional SQUID to allow for coupling signals into the amplifier with a substantial mutual inductance for a relatively short microstrip coil. Measurements at 310 mK exhibit gain of 32 dB at 1.55 GHz.

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DC Superconducting Quantum Interference Devices (SQUIDs) are widely used to amplify low-level, radio frequency (rf) electrical signals. SQUID amplifiers offer low noise, high gain, and low power dissipation. One method of implementing a SQUID rf amplifier for frequencies from a few hundred megahertz to several gigahertz is to operate the integrated input coil on top of the SQUID washer as a microstrip resonator. This is achieved by applying the input signal between one end of the coil and the SQUID washer, which acts as a groundplane; the other end of the coil is left open. Substantial levels of gain can be achieved from the microstrip SQUID amplifier for a signal frequency at the fundamental resonant frequency of the microstrip, at which the length of the microstrip is equal to one-half wavelength, λ/2. Since the length of the microstrip has to be made shorter for higher frequencies, however, the mutual inductance between a SQUID with a given geometry and the microstrip—and thus the gain—decreases with increasing frequency. We show that a significantly enhanced gain can be achieved by operating the microstrip resonator in higher-order modes, for example, with a microstrip length of 3λ/2 or 5λ/2, provided the winding sense of the microstrip for each consecutive λ/2 section is reversed. For a 4λ/2 resonator, we demonstrate a gain of 24 dB at 2.6 GHz, an increase in gain of about 10 dB compared to a λ/2 resonator on a SQUID of the same geometry and characteristics.

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The superconducting quantum interference device microstrip amplifier: Computer models

Michael Mück, John L. Clarke

Computer models are presented for a microstrip amplifier based on a dc superconducting quantum interference device (SQUID). In this device, the signal is applied between one end of the spiral input coil and the square washer on which it is deposited. The amplifier exhibits substantial power gain when the signal frequency is such that a half wavelength is approximately equal to the length of the microstrip formed by the coil and the groundplane. The resonant frequency is lowered significantly by the inductance of the square washer transformed into the input coil; this reduction is consistent with predictions of a simple model and with analog simulations. With the washer grounded, the gain of the amplifier peaks at a frequency that is lowered from the unloaded resonant frequency by the damping of the resistance associated with the source. The position and magnitude of the peak are in good agreement with both a lumped circuit model and with a model representing the microstrip as a transmission line. When the counter electrode of the SQUID is grounded and the washer floats, feedback from the output of the SQUID to the input via the capacitance of the microstrip plays a major role and is well described by simulations using the transmission line model. Measurements of the input impedance of the microstrip amplifier show that the return loss can be positive or negative, depending on the sign of the feedback and whether the frequency is above or below the resonant frequency. This behavior is in good accord with simulations.

Journal of Applied Physics•Dec 1, 2000•20 citations
We present measurements of an amplifier based on a dc superconducting quantum interference device (SQUID) with submicron Al-AlOx-Al Josephson junctions. The small junction size reduces their self-capacitance and allows for the use of relatively large resistive shunts while maintaining nonhysteretic operation. This leads to an enhancement of the SQUID transfer function compared to SQUIDs with micron-scale junctions. The device layout is modified from that of a conventional SQUID to allow for coupling signals into the amplifier with a substantial mutual inductance for a relatively short microstrip coil. Measurements at 310 mK exhibit gain of 32 dB at 1.55 GHz.
We present measurements of an amplifier based on a dc superconducting quantum interference device (SQUID) with submicron Al–AlOx–Al Josephson junctions. The small junction size reduces their self-capacitance and allows for the use of relatively large resistive shunts while maintaining nonhysteretic operation. This leads to an enhancement of the SQUID transfer function compared to SQUIDs with micron-scale junctions. The device layout is modified from that of a conventional SQUID to allow for coupling signals into the amplifier with a substantial mutual inductance for a relatively short microstrip coil. Measurements at 310 mK exhibit gain of 32 dB at 1.55 GHz.
DC Superconducting Quantum Interference Devices (SQUIDs) are widely used to amplify low-level, radio frequency (rf) electrical signals. SQUID amplifiers offer low noise, high gain, and low power dissipation. One method of implementing a SQUID rf amplifier for frequencies from a few hundred megahertz to several gigahertz is to operate the integrated input coil on top of the SQUID washer as a microstrip resonator. This is achieved by applying the input signal between one end of the coil and the SQUID washer, which acts as a groundplane; the other end of the coil is left open. Substantial levels of gain can be achieved from the microstrip SQUID amplifier for a signal frequency at the fundamental resonant frequency of the microstrip, at which the length of the microstrip is equal to one-half wavelength, λ/2. Since the length of the microstrip has to be made shorter for higher frequencies, however, the mutual inductance between a SQUID with a given geometry and the microstrip—and thus the gain—decreases with increasing frequency. We show that a significantly enhanced gain can be achieved by operating the microstrip resonator in higher-order modes, for example, with a microstrip length of 3λ/2 or 5λ/2, provided the winding sense of the microstrip for each consecutive λ/2 section is reversed. For a 4λ/2 resonator, we demonstrate a gain of 24 dB at 2.6 GHz, an increase in gain of about 10 dB compared to a λ/2 resonator on a SQUID of the same geometry and characteristics.
A series of five dc superconducting quantum interference devices (SQUIDs) have been operated as microstrip amplifiers at frequencies ranging from 2.2 to 7.4 GHz. In these devices, the signal is connected between the SQUID washer and coil, which acts as a microstrip resonator. The gain measured at 4.2 K ranged from 12±1 to 6±1 dB. The noise temperature of three devices at 4.2 K in the frequency range 2.2–4 GHz was between 1 and 2 K, and the saturation temperature was between 150 and 250 K. Applications of these devices include readout for axion detectors, and intermediate-frequency amplifiers for superconductor–insulator–superconductor and hot-electron bolometer mixers.
The resonant frequency of an amplifier based on a low-temperature dc superconducting quantum interference device (SQUID) is tuned by varying the capacitance across the end of the input microstrip by means of a varactor diode. The resonant frequency can be reduced substantially, for example, from 190 to 110 MHz, while maintaining an on-resonance gain of 27–28 dB. Two microstrip SQUID amplifiers with the same resonant frequency are cascaded to achieve a gain of 33.5±1 dB at 386 MHz; with the SQUIDs tuned to different frequencies, the gain is 22 dB or higher from about 260 to 420 MHz.
A dc superconducting quantum interference device (SQUID) was used to amplify radio-frequency signals that were coupled to one end of the microstrip formed by the input coil and the SQUID washer. For one device, the resonant frequency of the microstrip was increased from about 200 to 620 MHz by progressively shortening the length of the coil. At an operating temperature of 4.2 K, the gain was typically 18 dB, and the system noise temperature ranged from 0.5±0.3 K at 80 MHz to 3.0±0.7 K at 500 MHz.
Computer models are presented for a microstrip amplifier based on a dc superconducting quantum interference device (SQUID). In this device, the signal is applied between one end of the spiral input coil and the square washer on which it is deposited. The amplifier exhibits substantial power gain when the signal frequency is such that a half wavelength is approximately equal to the length of the microstrip formed by the coil and the groundplane. The resonant frequency is lowered significantly by the inductance of the square washer transformed into the input coil; this reduction is consistent with predictions of a simple model and with analog simulations. With the washer grounded, the gain of the amplifier peaks at a frequency that is lowered from the unloaded resonant frequency by the damping of the resistance associated with the source. The position and magnitude of the peak are in good agreement with both a lumped circuit model and with a model representing the microstrip as a transmission line. When the counter electrode of the SQUID is grounded and the washer floats, feedback from the output of the SQUID to the input via the capacitance of the microstrip plays a major role and is well described by simulations using the transmission line model. Measurements of the input impedance of the microstrip amplifier show that the return loss can be positive or negative, depending on the sign of the feedback and whether the frequency is above or below the resonant frequency. This behavior is in good accord with simulations.