On frequency transformation for nonsymmetric bandpass filters
P. Lin · IEEE Transactions on Circuits and Systems · 1987
When a bandpass filter has specifications that are not geometrically symmetric, the proper choice of the constantKin the frequency transformationS = (s + K/s)has an effect on the selectivity parameter of the transformed lowpass characteristic. In this note we prove that if the objective is to minimize the selectivity parameter, then the best choice ofKis the product of the two passband edge frequencies. On the other hand, for a bandstop filter, the best choice ofKin the transformationS = 1/(s - K/s)is the product of the two stopband edge frequencies.