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Journal of Applied Physics

The thermally saturated SUPARAMP

M. J. Feldman

Published Mar 1, 1977• 24 citations
Publisher

Abstract

The superconducting unbiased parametric amplifier can be saturated by low levels of broadband thermal radiation. Saturation increases the fluctuation noise temperature and complicates the determination of the operating parameters of this device. In this paper the theory of the thermally saturated SUPARAMP is presented. It is found that a particular amplifier can be described by a characteristic temperature, its maximum possible output brightness temperature. The implications of this theory for the interpretation of past experiments and for the design of future experiments are discussed.

DOI
10.1063/1.323775

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The superconducting unbiased parametric amplifier can be saturated by low levels of broadband thermal radiation. Saturation increases the fluctuation noise temperature and complicates the determination of the operating parameters of this device. In this paper the theory of the thermally saturated SUPARAMP is presented. It is found that a particular amplifier can be described by a characteristic temperature, its maximum possible output brightness temperature. The implications of this theory for the interpretation of past experiments and for the design of future experiments are discussed.
The noise temperature and gain stability of the superconducting unbiased parametric amplifier (SUPARAMP) have been calculated. The results of the analysis suggest that extremely low-noise amplification of signals from cryogenic sources may be achieved. It seems possible to obtain an effective input noise temperature of 1 K for signal frequencies up to 10 GHz, having a stable power gain of 20–30 dB. This paper provides the fundamental design specifications necessary for the construction of a low-noise SUPARAMP.
The single junction SQUID was previously shown by analysis and simulation to be an attractive candidate for a parametric amplifier. Further calculations of the noise and saturation behavior of the nearly degenerate parametric amplifier have now been performed by numerical simulation. These simulations clearly show that the amplifier noise temperature will be approximately the device temperature T0, and that the amplifier will be completely saturated in the presence of white noise characteristic of 30T0. Signal saturation of the amplifier also occurs for an output power10^{-2}F_{o}\phi_{o}^{2}/2L, strongly limiting the dynamic range, However, a coherent array of N single junction SQUIDs is shown to have a signal saturation level increased by N relative to a single SQUID, with no increase in noise temperature, resulting in an N-fold improvement in dynamic range.
A 9-mm degenerate parametric amplifier has been constructed using a linear series array of unbiased Josephson junctions as the active nonlinear element. We used a balanced diode mixer as a synchronous detector, with a single source serving both as the pump and as the mixer local oscillator. A stable net gain of 15 dB in an instantaneous bandwidth (FWHM) of 3.4 GHz has been achieved. We measured a system noise temperature of 220±5 K (DSB) with a SUPARAMP contribution of only 20±10 K. Output saturation, however, has been observed. This complicates the interpretation of our noise-temperature measurements and may render them upper limits. Comparison is made with the results of an earlier 3-cm SUPARAMP. The data are in substantial agreement with theoretical predictions.
The signal and noise properties of two basic types of externally pumped parametric amplifiers, using Josephson junctions with low capacitance, are calculated. The first type is a single- frequency nondegenerate amplifier and second one is a two-frequency degenerate amplifier, referred to sometimes as SUPARAMP. Calculations are based on the conventional RSJ model of the Josephson junction. In contrast to the works published earlier, the most realistic open circuit approximation is used for idler combinational frequencies, as well as for the pumping frequency harmonics. A single-frequency amplifier is shown to have relatively low noise temperatures (T_{N} \approx 25T) at signal frequencies of the order of the Josephson junction characteristic frequency ωc. On the other hand, SUPARAMP can have much lower noise but at lower frequencies, and it behaves approximately as a usual parametric amplifier with a critical frequency\omega_{cr} \approx 0.2 \omega_{c}.
A 9mm degenerate parametric amplifier was constructed using a linear, series array of unbiased Josephson junctions as the active, nonlinear element. A balanced diode mixer was used as a synchronous detector, with a single source serving both as the pump and as the mixer local oscillator. A stable, net gain of 15 dB in an instantaneous bandwith (FWHM) of 3.4 GHz was achieved. A system noise temperature of 220 K + or - 5 K (DSB) was measured with a SUPARAMP contribution of only 20 K x or - 10 K. Output saturation was observed and complicates the interpretation of the noise temperature measurements and may render them upper limits. A comparison was made with the results of an earlier 3 cm suparamp. The data is in substantial agreement with theoretical predictions.
A comprehensive account of the theory of the singly quasidegenerate Josephson junction parametric amplifier is given. In this mode the signal and idler frequencies are both approximately equal to half the pump frequency, and hence the signal and idler channels have a common termination. It is shown that the performance of the amplifier characterized by properties such as gain-bandwidth product, noise temperature, and saturation depends strongly on the embedding microwave circuit. In particular any uncompensated series reactance may deteriorate the overall performance. Internal shot and Johnson noise as well as incident thermal radiation is included in the discussion. It is shown that the noise temperature is virtually independent of noise injected from external sources and that minimum noise temperature is automatically achieved if the amplifier is tuned to maximize the gain-bandwidth product. The analytical results are illustrated by self-consistent numerical solutions of the circuit equations.