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IEEE Transactions on Applied Superconductivity

Noise analysis of the quantum flux parametron

H.L. Ko, G.S. Lee

Published Jan 1, 1992• 28 citations
Publisher

Abstract

An analysis of the limitations imposed by thermal noise on the application of the quantum flux parametrons (QFPs) as current comparators and timing discriminators is presented. At low operational frequencies, a thermal activation approach is used, while at high frequencies a circuit simulation method is required. At frequencies greater than the Josephson junction plasma frequency, an approximate expression for the noise is given. The calculations are in good agreement with the approximations and the experimental results.>

DOI
10.1109/77.160155

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An analysis of the limitations imposed by thermal noise on the application of the quantum flux parametrons (QFPs) as current comparators and timing discriminators is presented. At low operational frequencies, a thermal activation approach is used, while at high frequencies a circuit simulation method is required. At frequencies greater than the Josephson junction plasma frequency, an approximate expression for the noise is given. The calculations are in good agreement with the approximations and the experimental results.>
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The current noise (inferred from switching probabilities) of quantum flux parametron (QFP) comparators has been measured as a function of exciter slew rates. It was found that at high slew rates the switching probability follows an error-function distribution quite closely. However, at low slew rates, the distribution resembles a Fermi-Dirac distribution. Good agreement with Monte-Carlo simulations for high slew-rates is seen with a current noise proportional to the Johnson noise of the damping resistors. The current noise decreases with decreasing slew rate and is in good agreement with thermal activation calculations in the limit of slow slew rates.>
Comparators are a critical component in sigma-delta A/D converters. We have studied experimentally the sensitivity of a quantum flux parametron (QFP) comparator operated over a range of sampling frequencies F/sub S/ from 40 Hz to 40 MHz. In one experimental method, following Ko and Lee, we measure the firing probability of the QFP as a function of applied flux. The sensitivity can be derived from the slope of this curve. In a second method, we measure the sensitivity directly by observing the spectrum of the QFP output while the amplitude of a small sinusoidal applied flux is adjusted to exceed the noise floor by 3 dB. The two methods were found to be in good, but not perfect agreement. The sensitivity was measured as we varied both F/sub S/ and the dock rise time. The spectrum of the quantization noise exhibits, as expected, a flat floor whose level is inversely proportional to F/sub S/. The best energy sensitivity that we observed was for a clock frequency of /spl sim/20 MHz with a 10 ns rise time. The measured sensitivity was about 1500 h (Planck's constant). The readout circuit prevented us from clocking the comparator into the GHz range for even greater sensitivity. We also believe that the comparator could be optimized to improve sensitivity further. The noise floor was low enough that we could observe excess low-frequency noise below 5 Hz. We have not yet determined whether it is intrinsic to the comparator or originates from our test electronics. We hypothesize that the noise floor will continue to fall as F/sub S/ increases until we reach the speed limit of the comparator, at which point successive output samples will no longer be uncorrelated, or until we reach the uncertainty-principle limit (h) in the 100s of GHz range.
Abstract Current sensitivity of a quantum flux parametron (QFP) was evaluated by measuring gray zone width on the basis of both circuit simulation and measurements, for superconducting sensing systems composed of a superconducting sensor array and superconducting read-out and signal processing circuits. Simulation results indicate a narrow gray zone width can be obtained by decreasing inductances comprising the QFP. Moreover, both high-sensitivity and low-power operation of the QFP can be realized by using optimized circuit parameters with an excitation current that has long rise time. A gray zone width of approximately 0.5 μ A, which is smaller than that of the Josephson current comparator based on a single flux quantum circuit, was obtained experimentally, using a trapezoidal excitation current with rise time of 50 μ s. These results indicate the QFP is promising for the read-out circuit in superconducting sensing systems, due to its high sensitivity and low power consumption.
A root-mean-square (RMS) time jitter of 380 fs has been demonstrated in a single quantum flux parametron (QFP) race arbiter circuit with approximately 60-ps-risetime signal inputs. Because of the simple relationship between the QFP current comparator and the race arbiter, it is possible to make direct comparison of the time jitter and current noise results. The measured current noise in a QFP current comparator implies that, for a signal risetime of 10 ps, a time jitter of approximately 200 fs is expected for this race arbiter circuit. Circuit modeling suggests that an optimized design can achieve approximately 100-fs time jitter.>