Correction coils for wire delay lines

The storage capacity of a wire delay line can be increased substantially by reshaping the output waveform to reduce the interference between adjacent pulses. Scarrot and Naylor (1956) pointed out that, for a delay line equipped with magnetostrictive transducers, this could be accomplished by using a detection coil with an appropriately chosen winding density function. Such a correction coil is a linear filter whose transfer function is equal to the Fourier transform of the winding density function and is, in fact, the continuous analog of the transversal filter commonly employed to equalize communication channels. Although the design of a correction coil is straightforward in principle, a number of difficulties arise in practice. These include the presence of discontinuities in the winding density function and a tendency for the coil length to be unmanageably large. This paper describes the design techniques that have been developed to overcome these difficulties. An important feature is that they can be easily implemented with a digital computer program which does not require manual intervention. They include the division of the coil into two distinct windings, one with substantial high-frequency content and the other without, in such a way that neither winding has any discontinuities except at its edges; and a procedure for designing a correction coil of a specified length which minimizes the mean-square intersymbol interfence. In addition, experimental results obtained with a typical correction coil are presented.