Charge-routing networks

The fundamental techniques for charge manipulation achievable with MOS charge-coupling technology include storage, transfer, splitting, combining, insertion, and extraction. We idealize and generalize these operations to define a general class of networks for discrete-time linear filtering. A p-phase charge-routing network (CRN) consists of a collection of storage cells divided into p subgroups and a routing procedure controlled by a p-phase clock. During a particular clock phase, charge is routed from a particular subgroup of storage cells into another subgroup. In this manner charge is routed successively through each subgroup of cells and a periodically time-varying linear discrete-time network is defined by specifying matrices of weight values associated with the routing procedure. Analysis of a
p
-phase CRN yields a reduced system of linear time-invariant dynamic state equations convenient for signal-processing studies. Necessary and sufficient conditions on such a system of state equations are given for realizability as a
p
-phase CRN. The varied structures attainable with CRN's can realize infinite inpulse-response filter transfer functions. However, certain fundamental restrictions exist on the class of transfer functions realizable with CRN's. In particular, we show the existence of forbidden zones within the unit circle in the z-plane, where poles (or natural modes) of a CRN cannot occur. A parallel can be drawn between classical RC networks and charge-routing networks. Both types of networks have substantial restrictions on the class of filters they can by themselves realize. For RC networks, we know the limitations can be overcome by the addition of another component (inductance or operational amplifier). A similar potential may exist for CRN's. This paper provides a foundation for a general theory that could offer realizability conditions and sythesis procedures for discrete-time filtering via charge routing networks.