Electronically tunable analog active filters

Electronically tunable
RC
-active ladder filter transfer functions are realized by employing the four quadrant analogue multiplier as the basic nonlinear network element. All the frequency dependent branches of a conventional
RC
-active ladder structure realizations are simulated by means of impedance scaling networks. The scaling of these impedances allows the cutoff frequency of an elliptic low-pass or high-pass transfer function, of arbitrary order, to be varied by means of a tuning voltage. The method is strictly analogue and leads to practical filters that employ presently available integrated circuits; external timing signals or reference oscillators are not required in order to accomplish tuning.