Electrical Characteristics of a Superconducting High-Speed Computer Package

The interconnection of chips using Josephson circuitry for building a superconducting computer requires a special package. The electrical characteristics of this package will determine the speed and quality of chip-to-chip signals and therefore have a significant effect on overall computer Performance. The package consists of different parts which are wired together with special connectors. The wiring paths are formed with superconducting lossless transmission lines which can propagate extremely fast wavefronts. When a transmission line must be connected to another transmission line on another adjoining package part, the special connector ties the two together. These connectors have self and mutual inductances which create an electrical discontinuity. Some of these connectors are also used to tie together the ground planes of adjoining package parts. A useful parameter which affects the self and mutual inductances of these special connectors is the ratio of the number of signal connectors to the number of ground connectors, called SGCR. For most special connectors, making SGCR small reduces the size of the inductances. Examples of the affect of SGCR are shown. In addition, special techniques have to be used to calculate the inductances and some examples of this are given. Another important Parameter is G, and the size of G determines the distortions and reflection a signal incurs passing through a special connector. The ratio G = (2
\tau
Z
o
)/LS where
\tau
is the rise time of the wavefront, Z
0
is the characteristic impedance of the transmission line, and LS is the self inductance of the special connector. Values of G
geq
6 lead to acceptable distortion and reflections. The crosstalk between adjacent special connectors is directly due to mutual inductance and also the parameter G. Larger values of G reduce crosstalk. Allowable chip-to-chip signals in a package must constrain the cross-talk between signal paths to acceptable limits. We can calculate plots that show cross-talk between connectors as a function of signal wavefront speed (T), using the parameter SGCR. For a given receiver design which can safely accomodate some maximum cross-talk, the allowable wavefront speed versus SGCR can be determined. From this overall signal delay can be calculated and critical timing paths in the computer can be determined.