November 1970 | NTIA Technical Memo OT ITS TM 7

Simulation of ionospheric dispersion

Clark C. Watterson

Abstract: An ideal dispersion filter for simulating ionospheric dispersion is defined as one with constant dispersion in a channel frequency band and zero dispersion outside the band. An optimum lumped-element all-pass filter that approximates this ideal has a constant amplitude characteristic and a dispersion characteristic that has an equal-ripple variation about a constant nominal dispersion in the filter band. The pole-zero values required by optimum narrowband dispersion filters are presented with the corresponding nominal dispersions for 70 combinations of filter orders n = 1, 2, ... , 10 and ripple factors = .001, .002, .005, .01, . 02, . OS, and . 1. These results are applicable for narrowband filters. The maximum fractional bandwidths for which the narrowband solutions apply and the dispersion efficiencies of the filters are presented; both increase with increasing filter order, n, and ripple, O. A limited set of wideband solutions shows that while larger fractional bandwidths are possible with wideband designs, there is a limiting bandwidth beyond which the wideband filters are not physically realizable. A description is given of an active all-pass network that avoids several undesirable features of lattice networks, and an easy method of using the tabulated solutions to synthesize filters of this type is described.

Disclaimer: Certain commercial equipment, components, and software may be identified in this report to specify adequately the technical aspects of the reported results. In no case does such identification imply recommendation or endorsement by the National Telecommunications and Information Administration, nor does it imply that the equipment or software identified is necessarily the best available for the particular application or uses.

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