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IEEE Transactions on Applied Superconductivity

A single QFP timing discriminator

H.L. Ho, G.S. Lee, Rich Ruby, Andrew T. Barfknecht

Published Mar 1, 1993• 1 citation
Publisher

Abstract

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.>

DOI
10.1109/77.233510

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IEEE Transactions on Applied Superconductivity•Jun 1, 1999•4 citations
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.>
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.>
We compare the SQUID wheel/Quantum Flux Parametron (QFP) comparator to a new Rapid Single Flux Quantum (RSFQ) compatible design. Both have been simulated to demonstrate /spl sim/0.5 ps threshold accuracy which would permit the construction of a flash analog-to-digital converter with six effective bits of resolution at 10 GHz input bandwidth, over three times better than the best performance demonstrated with any technology, At lower input frequencies, both designs have demonstrated that a 10-bit flash ADC is possible. Although simulations of the QFP-based design are more accurate at high signal slew rates due to its symmetry, the RSFQ-based design has a better signal-to-noise ratio and a faster and more flexible clocking scheme which ultimately prove to be more important.
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 single-flux quantum Josephson device with a high gain of over 20 dB and a low power dissipation of 1-nW order at 10 GHz. Logic and analog circuits using the device have been fabricated and tested. The device's margin may be reduced by a number of problems due to fabrication and design variations such as imbalance of Josephson-junction critical currents, flux loss in transformer self-inductances, and transformer coupling variations. The authors review these problems and describe three auxiliary circuits-booster, puller, and regulator-for alleviating them. A booster reduces the effect of critical current variations and doubles the output. A puller increases the activation current, thus allowing smaller clock signals. A regulator ensures full activation of the QFP over a wide range of activation levels. With these currents, the device has a much improved margin against variations originating from within itself.>
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.