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Physica C Superconductivity

Effects of magnetic fields induced by bias currents on operation of RSFQ circuits

Motohiro Suzuki, Masaaki Maezawa, Fuminori Hirayama

Published Jun 9, 2004• 18 citations
Publisher
DOI
10.1016/j.physc.2003.12.094

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The magnetic fields induced by the bias currents and the return currents flowing on a ground plane have a great influence on the performance of complex RSFQ circuits. Conventional approach applies equal negative bias currents to the ground contacts adjacent to the bias supply contacts to alleviate this problem, but this strategy may not be optimal when the power supply network is complex. We propose a new test approach to adjusting the negative bias currents independently. By changing the current distribution and thereby altering the spatial distribution of the magnetic field across the chip, this approach can make the circuit work, or make the circuit work more stably. Experimental validation on two 8-bit RSFQ CPUs demonstrated the approach's efficacy: redistributing negative currents restored functionality in a non-operational circuit and eliminated the intermittent failure in another. These results highlight the critical role of adjusting current distribution in overcoming magnetic interference, offering a practical solution for testing and optimizing the power supply of complex RSFQ circuits where conventional approach falls short.

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•May 1, 2010•1 citation
The magnetic fields induced by the bias currents and the return currents flowing on a ground plane have a great influence on the performance of complex RSFQ circuits. Conventional approach applies equal negative bias currents to the ground contacts adjacent to the bias supply contacts to alleviate this problem, but this strategy may not be optimal when the power supply network is complex. We propose a new test approach to adjusting the negative bias currents independently. By changing the current distribution and thereby altering the spatial distribution of the magnetic field across the chip, this approach can make the circuit work, or make the circuit work more stably. Experimental validation on two 8-bit RSFQ CPUs demonstrated the approach's efficacy: redistributing negative currents restored functionality in a non-operational circuit and eliminated the intermittent failure in another. These results highlight the critical role of adjusting current distribution in overcoming magnetic interference, offering a practical solution for testing and optimizing the power supply of complex RSFQ circuits where conventional approach falls short.
Complex RSFQ circuits are typically dc-biased with one or more current bias trees, with current returning through a superconducting ground plane. As the bias currents become larger in more complex circuits, it is increasingly critical to pay attention to the distribution of these return currents, and the effects of the resulting magnetic fields on the performance of the RSFQ circuits. We have modeled the current and field distributions, and found that magnetic field and flux are indeed significant. This has been confirmed by direct measurement using a distribution of SQUIDs. Furthermore, we have measured the performance of several RSFQ circuits, and have found that currents in the ground plane can significantly affect performance margins. Approaches to circuit and system designs that can reduce these problems are discussed.
Superconducting digital logic technology, Rapid Single Flux Quantum (RSFQ), is established as the fastest technology in microelectronics based on Josephson junction devices. Technological challenges in large scale integration of superconductor circuits (RSFQ) namely: flux trapping, low power dissipation and multi-chip modules are addressed here. One of the main limitations of the integration of the RSFQ circuits is flux trapping, which arises due to the presence of residual magnetic field during transition of the circuit into superconducting state. Flux trapping is an unusual problem specific in only superconducting circuits. The effect of flux trapping in RSFQ circuits is studied, with a controlled 3-D magnetic field setup, quantitatively. Layout configuration with different moat patterns making RSFQ circuits tolerant to external magnetic field up-to 20mG was also developed. Low power issue was addressed by developing a new logic family. Power independent logic, an improved form of RSFQ logic, enables the circuits to operate only when needed and circuits can be switched off the remaining time; thereby eliminating static power consumption by retaining the logic state of the circuit. Flux trapping has also beeninvestigated in power independent cells. Current recycling is a technique to reduce the bias current applied for the operation of the RSFQ circuits. In this technique the cells are biased by serial connections. The reduction in supply current is proportional to number of blocks connected in series The effect of magnetic field on the operating margins of the current recycling circuits has also been studied. Operation of Multi-chip modules, one of the most viable solutions to develop large scale circuits, was demonstrated. Multi-chip modules with lithographically designed bumps is presented where data transmission rate between two superconducting chips exceed 100GHz. Finally, the development of the multi-modulator ADC for low noisefront end under-development is discussed.