Effects of superconducting return currents on RSFQ circuit performance

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.