Another step towards an ideal data acquisition channel

In a paper by R. Pintelon et al. (see ibid., vol.39, p.116-120, Feb. 1990) a method has been presented for the design of amplitude- and phase-compensating IIR (infinite impulse response) filters. The procedure is based on inversion of the transfer function, identified in the z-domain. Unstable poles are reflected into the unit circle, and the spoiled phase response is equalized again using all-pass sections. The authors point to the fact that this method is not optimal in the sense that it does not reach the minimum error for a given order (or provide the desired error compensation by using a too high order). By refitting the compensating filter, a lower order filter is obtained with the same performance. In the investigated case the order is reduced from 34/34 to 26/26