State variable filter

A state variable filter (SVF) is a type of active filter in electronic circuits. In a state-variable filter architecture the signal and its successive derivatives are treated as the core state variables of the system.

In state variable filters parameters like gain, cutoff frequency and Q can be independently controlled. This makes it, for example, possible to build a tunable filter where the cutoff frequency is adjustable, while the gain and Q remain constant. A state variable filter consists of one or more integrators, connected in some feedback configuration. It is essentially used when a precise Q factor is required, as other multi-order filters are unable to provide. The most common implementation sums the input signal with its integral and its double integral.

State variable filters can also be classified as Multiple Feedback (MFB) and Multi Feedback Loop (MFL) Filters because of the typical feedback configurations. The multiple feedback architecture contributes to the reduced sensitivity to component values of state variable filters.