Technique for reducing digit noise in very fast magnetic-film stores

Abstract

Noise is produced in the sensing system of a magnetic-film store by current pulses in the digit line, and noise due to reflected pulses continues after removal of the original digit pulse. A principal cause of these reflections is the difficulty of terminating low-impedance transmission lines with their characteristic impedances. A new, simple construction avoiding this difficulty involves making the digit lines longer than the active section of the matrix, injecting digit currents into the centre of each long digit transmission line and sensing at points spaced equal distances to either side of the centres, about 1–2ft away. Active film cells occupy the next few feet of line, after which the lines continue for 10ft or more before termination. Imperfections in r.f. matching are therefore unimportant because of r.f. attenuation in the lines. Any l.f. mismatch is attenuated by the action of the short transmission-line section between the sense terminals, and can be further reduced by filters in the sense amplifier.Preliminary tests suggest that noise excited by a digit pulse having a rise time of 7 ns and width of 20 ns need not exceed 1% of the pulse amplitude, given suitable filtering and a good differential amplifier. Use of separate digit and sense lines with the new design could probably reduce peak noise during writing to below 0.1%, i.e. to about the magnitude of a typical signal from a magnetic-film cell, so that subsequent reading need not be long delayed by noise.