Higher order Thomson networks for generating sine squared pulses.

Abstract

A television system is concerned with the faithful transmission of transient signals in a restricted bandwidth. The sine-squared pulse has proved to be a suitable test signal for such systems. The C.C.I.R.-approved method of generating such pulses uses a 7th order network, devised by W. E. Thomson, of the British Post Office. The work described herein was aimed at the development of networks with a lower distortion than the 7th order Thomson network, so that the pulse generator could be used for testing very low distortion systems and units. Ninth and eleventh order transfer functions, calculated by the Thomson method, gave impulse responses of superior shape, but the improvement in overshoot was moderate. Alternative methods, giving transfer functions with higher order numerators, were studied - in these, the pulse shapes were not quite so perfect, but the overshoots were smaller than those produced by Thomson functions of the same order. Four transfer functions were selected, and networks were synthesised from these. The filters were built up, and tested for insertion loss and impulse response. In the course of the work, a number of computer programs were written for various phases of the project. Descriptions of these are given, illustrated by flow charts, and sample calculations.