Suggestion for a new formula to calculate Group-delay from frequency domain measurements

This paper presents a simple case study of a low Q band reject filter. Calculation based on phase derivative produces negative group-delay for this circuit. In the paper, a modified formula D g = [(partA/A)/partomega) 2 + (partPhi/partomega) 2 )] frac12 is suggested, based on a normalized derivative of the full complex transmission function T (omega) = A (omega) )e -jPhi(omega) . This formula produces positive delay for all frequencies, having a maximum value at resonance. For flat amplitude response, the formula converges to the classic formula D g = partPhi/partomega