This filter is current driven and has the formulas given by the two diagram options. The circuit is from an American documentation from the 90s applying to any operational amplifier.
Found in a 1965 Radio Shack documentation this oscillator can be used with modern general purpose...
In the figure we have the way to use a single control voltage to act on two varicaps...
Found in a 90s documentation, this switch that is also sensitive enough to operate capacitively uses...