INTERPOLATION IN DIGITAL MODEMS .2. IMPLEMENTATION AND PERFORMANCE

An earlier paper introduced the theoretical background for digital interpolation in a data receiver and described an interpolator control method. Here we report properties of a specific class of interpolators that are based upon polynomials. Several implementations are described, one of which is particularly convenient in practical hardware. Simulations demonstrate that simple interpolators give excellent performance. In many cases, two-point, linear interpolation is adequate. If better performance is needed, classical four-point, third-order polynomials could be used. Better yet, a novel four-point interpolating filter with piecewise-parabolic impulse response can have performance superior to that of the standard cubic interpolator and still be implemented much more simply. The NCO-based control method presented in Part I is shown to be equivalent to a conventional phase locked loop and its operation is verified by simulation.