A note on some early work in the field of switched-capacitor networks

Some early work in the field of switched-capacitor filters, which has been done in Europe since about 1960, is discussed. In particular, reasons are given as to why the early (and some later) approaches in Europe went in quite a different direction from switched-capacitor filter design in the United States. In contrast to the later approaches, which were dominated by the idea of substituting switched capacitors instead of resistances, mostly in order to realize unidirectional signal-flow graph filters, the early European concepts concentrated on the simulation of pure reactances by means of capacitances and special switches, which could achieve lossless charge transfer in order to simulate doubly terminated reactance filters. Modern descendants of this type of filters are the switched-capacitor filters using voltage inverter switches. Since these filters are operated bidirectionally, very useful equivalences to time-invariant circuits and network theorems concerning stability and passivity can be derived. An outline of the fundamental ideas that have led to such equivalences is presented.