Digitally controlled, conductance tunable active filters

The use of digitally controlled variable conductances (DCVC's) to control the center frequency and selectivity of an active state variable filter is described theoretically and experimentally. Advantages of this approach are: zero sensitivity of selectivity with respect to the control conductances, excellent tuning linearity (1 per cent over one decade) can be achieved with 1 per cent accuracies, a common digital control can be used to drive simple three-element variable conductances, and the input signal can be very low (about 0.5 mV) without any adverse effect on the tuning accuracy or linearity.