Multiplexed filtering with charge transfer devices

A multiplexed filter structure using a single charge-transfer device (CTD) with 2N stages of delay, switched weight values, and two adders can be used to implement a bank of N second-order recursive filters. Structures of this type can be combined to implement a bank of higher order recursive filters, offering a potentially economical realization for multichannel filtering applications. An important limitation of the CTD delay line is due to the effect of charge transfer inefficiency (CTI), which introduces dispersion to the frequency response of the delay stages. By viewing the multiplexed structure as a multichannel filter with N inputs and N outputs, general expressions for the N²transfer functions can be derived which exhibit the dependence on the coefficient ε of CTI. For small ε, the asymptotic deviation of the transfer functions and the poles of the transfer functions from their desired behaviors (when ε = 0) is examined. In particular, for the case where the N desired second-order filters have distinct poles, e.g., filters tuned to different resonant frequencies, it is shown that the pole positions are perturbed by a term of the order of ε^N, indicating that the effect of CTI is negligibly small. Included in the paper are results on the asymptotic dependence on ε of the interchannel crosstalk due to CTI.