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IEEE Transactions on Applied Superconductivity

Measurement of the energy sensitivity of a superconductive comparator

Derrick Feld, P. Sage, Karl K. Berggren, Aleem M. Siddiqui

Published Jun 1, 1999• 4 citations
Publisher

Abstract

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.

DOI
10.1109/77.783991

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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.
The authors report on the high-speed operation of a superconducting comparator circuit, based on coupling the quantum flux parametron (QFP) to an RF SQUID, which can be used to build a flash-type analog-to-digital converter (ADC). Simulations of this circuit show that it is expected to achieve operation with input signal bandwidths greater than 4 GHz and with a dynamic range equal to at least 4 b of resolution. A QFP-based comparator fabricated with a process using NbN/Pb-alloy Josephson junctions of 5 mu m by 5 mu m and a current density of 100 A/cm/sup 2/ has been examined to evaluate the properties of the QFP-ADC. Analog-to-digital conversion of the comparator has been observed with a QFP activation frequency up to 18.2 GHz. By employing a sampling method, input signals with frequencies up to 5.4 GHz have also been digitized.>
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.
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.>
Adiabatic quantum-flux-parametron (AQFP) circuits can operate with extremely small energy dissipation (∼1 zJ per junction at 5 GHz) and high sensitivity (∼1 μA) owing to adiabatic switching. Thus, AQFP logic is suitable to use as readout interfaces for cryogenic detectors, such as superconducting nanowire single-photon detectors (SSPDs). In order to extend the application of AQFP logic to various detectors, it is crucial to achieve even better sensitivity because some detectors, such as WSi SSPDs and transition edge sensors (TESs), require sub-μA sensitivity. In the present study, we propose a high-sensitivity AQFP comparator with a current transformer (CT), which increases the equivalent input current and thereby improves the sensitivity. Numerical simulation shows that the proposed comparator achieves a sensitivity approximately six times better than that of a conventional AQFP comparator. Furthermore, we demonstrate a sensitivity of 46 nA at 4.2 K for a sampling frequency of 500 Hz in the experiment.
The quantum flux parametron QFP is a very fast superconducting logic device. Previously, the high frequency operation of the QFP was assessed by analysing the output of the device in the frequency domain. This paper shows for the first time that time domain measurements are possible in spite of the low voltage amplitude of the output signal and relatively large crosstalk with the clock frequency. The time-domain waveform of a single-bit shift register with logic inversion working at a clock frequency of 5 GHz is recorded.
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.