Practical Filter Design Part 10

42 PRACTICAL FILTER DESIGN PART 10 by H. Baggott This final part of the series discusses all-pass filters. Strictly speaking, these networks are not filters since (ideally) they have zero attenuation at all frequencies. However, they introduce a specific phase shift or time delay that is very useful in many applications. Although all-pass networks have zero attenuation at all frequencies, they intro- duce a certain phase shift and act, there- fore, as a sort of delay line. They may be used, for instance, to delay a signal in time or to modify the phase behaviour of an other filter. A look at the complex field of these fil- ters shows that their zeros of network function are mirror images of their poles. Since the poles are always located to the left of the y-axis (because of the required stability of the filter), the zeros must always be to the right of the ordinate. Thus, a first-order network is always a real pole-zero combination. It is interesting to note that owing to the uni...