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Applied Physics Letters

Radio-frequency amplifier based on a niobium dc superconducting quantum interference device with microstrip input coupling

Michael Mück, Marc-Olivier André, John L. Clarke, Jost Gail, Christoph Heiden

Published Jun 1, 1998• 99 citations
Publisher

Abstract

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.

DOI
10.1063/1.121490

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Journal of Applied Physics•Dec 1, 2000•20 citations
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.
A dc superconducting quantum interference device (SQUID) with a resonant microstrip input and a cooled heterostructure field-effect transistor as a postamplifier is used as a radio-frequency amplifier in the frequency range 90–500 MHz. At liquid He3 temperatures, gains of 24 and 20 dB and intrinsic noise temperatures of 0.06±0.02 and 0.12±0.10 K were achieved at 89.6 and 438 MHz, respectively. The system noise temperature at 438 MHz was also estimated from the Nyquist noise produced by a resonant circuit coupled to the input of the microstrip SQUID.
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.
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 low noise radiofrequency amplifier (10), using a dc SQUID (superconducting quantum interference device) as the input amplifying element. The dc SQUID (11) and an input coil (12) are maintained at superconductivity temperatures in a superconducting shield (13), with the input coil (12) inductively coupled to the superconducting ring (17) of the dc SQUID (11). A radiofrequency signal from outside the shield (13) is applied to the input coil (12), and an amplified radiofrequency signal is developed across the dc SQUID ring (17) and transmitted to exteriorly of the shield (13). A power gain of 19.5.+-.0.5 dB has been achieved with a noise temperature of 1.0.+-.0.4 K. at a frequency of 100 MHz.
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.
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.