For a filter to be physically realizable its power loss ratio must be of the form in (8.52).
Notice that specifying the power loss ratio simultaneously constrains the magnitude of the reflection coefficient, |(ω)|.
We now discuss some practical filter responses.
Maximally flat: This characteristic is also called the binomial or Butterworth response, and is optimum in the sense that it provides the flattest possible passband response for a given filter complexity, or order. For a low-pass filter, it is specified by