Maximal Flatness and Filters Transitional Between Butterworth and Inverse Chebyshev Ones

Abstract

The paper describes the family of filters which are transitional between Butterworth and inverse Chebyshev ones. Introducing a polynomial of squared frequency in the numerator of squared modulus of the Butterworth filter requires that a similar polynomial is added to the denominator of this squared modulus function if one wants to preserve the flatness property. After these two modifications are made the restoration of transfer function gives a transitional filter. If the polynomial introduced in the numerator includes all zeros of inverse Chebyshev filter then the restored filter will be the inverse Chebyshev filter. In case of partially restored flatness one obtain a transitional filter. An example of transitional filter for the fifth order Butterworth and inverse Chebyshev filters is calculated.