A method for realizing time-varying circuit elements
Robert W. Donaldson, Kenneth Freeman Wickwire · Proceedings of the IEEE · 1969
A practical method is presented for realizing time-varying circuit components whose instantaneous element values depend on externally generated digital control signals. These signals control the value of a discrete-state resistor in an operational amplifier circuit. The method was used to realize linear time-varying inductors and capacitors which were built and tested.