Passive variable fractional delay filter design using distributed maximal flatness method

In the paper a novel method for passive fractional delay filter design based on the distributed maximal flatness method is presented. The proposed approach is based on the design method of the one of the most commonly used fractional delay filters - the maximally flat filter. This filter demonstrates the excellent performance only in a relatively narrow frequency range around zero frequency but it is passive, its magnitude response is no greater than one. This passiveness is the reason why despite of its narrow band of accurate approximation, the maximally flat filter is widely used in applications in which the adjustable delay is required in feedback loop. In the proposed method the maximal flatness conditions forced in standard approach at zero frequency are spread over the desired band of interest. In the result passive FD filters are designed but the width of the approximation band can be adjusted to suit the designers needs. Moreover the weighting function can be applied to the error function allowing for designs differing in error characteristic. The proposed method can also be readily adapted for variable fractional delay filter design.