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IEEE Transactions on Applied Superconductivity

Switching probability of QFP comparators as a function of exciter slew-rate

Rich Ruby, G. Lee, H.L. Ko, Andrew T. Barfknecht

Published Mar 1, 1993• 8 citations
Publisher

Abstract

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.>

DOI
10.1109/77.233982

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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.>
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.>
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.
The quantum flux parametron (QFP) is a flux-transfer, flux-activated Josephson logic device which realizes much lower power dissipation than other Josephson logic devices. Being a two-terminal device, its correct operation may be affected by coupling to other QFPs. The problems include backcoupling form active QFPs through inactive QFPs (relay noise), coupling between QFPs activated at different times because of clock skew (homophase noise), and interaction between active QFPs (reaction hazard). Previous QFP circuits worked by wired-majority, which is a linear input logic and has low input margin. A logic gate (D-gate) using a QFP to perform logic operations has been analyzed and tested by computer simulation. Relay noise, homophase noise, and reaction hazard are substantially reduced. Moreover, the inputs have little interaction, and hence input margin is greatly improved. Using only D-gates it is possible to realize any combinational logic function. Important logic functions can be realized using one gate, such as a two-input multiplexer, majority, and parity. Since the QFP has a latching effect, D-gates can realize fully pipelined circuits without additional registers.
The Quantum Flux Parametron (QFP) is a new cryogenic switching device based on the parametron principle and Josephson technology. Due to the magnetic flux quantization in superconducting devices, the switching mechanism of the QFP is quite different from that of semiconductors. This presentation describes the device characteristics. QFPs have been fabricated using the Nb/Pb-alloy Josephson junction process, and their basic operation is evaluated. Over 20dB current gain is attained. One-half frequency-divider operation up to 1.8GHz has been successfully observed.
The Quantum Flux Parametron (QFP) is a new cryogenic switching device based on the parametron principle and Josephson technology. Due to the magnetic flux quantization in superconducting devices, the switching mechanism of the QFP is quite different from that of semiconductors. This presentation describes the device characteristics. QFPs have been fabricated using the Nb/Pb-alloy Josephson junction process, and their basic operation is evaluated. Over 20dB current gain is attained. One-half frequency-divider operation up to 1.8GHz has been successfully observed.