Microwave Filters for Communication Systems: Fundamentals, Design and Applications

CHAPTER 17

ALLPASS PHASE AND GROUP DELAY EQUALIZER NETWORKS

The ideal characteristics for a filter network call for a brick-wall amplitude response and a linear phase across the band of interest. Such a response is not feasible, even with idealized components. However, it can be approximated with tradeoffs between the amplitude and the phase responses as a function of the order and thus complexity of the filter network.

From a communication system perspective, filters are required to channelize a given frequency band into a number of RF Channels. This must be accomplished with a minimum of guard band between the Channels in order to achieve the highest efficiency in the usage of the available bandwidth. As a result, most filter designs are based on the minimum phase networks since they provide the highest rate of attenuation. The filter response is optimized on the basis of amplitude response, providing a specified level of attenuation outside its passband. The phase response (and group delay) of such a filter is uniquely related to the filter's amplitude response. This implies that once the amplitude response has been optimized, the resulting phase and the group delay response must be accepted as is. For many applications, this tradeoff is acceptable.

However, it is possible to improve the phase and group delay response of a filter without sacrificing the amplitude Performance. This can be accomplished by either incorporating internal equalization (linear phase filters) or using ...