In a resonant device, a first via electrode and a second via electrode connect a first plane electrode and a third plane electrode. A third via electrode and a fourth via electrode connect the first plane electrode and a second plane electrode. When the first plane electrode is seen in plan view in a normal direction of the first plane electrode, a first imaginary line connecting the first via electrode and the second via electrode intersects with a second imaginary line connecting the third via electrode and the fourth via electrode.

A filter circuit comprising: a signal path for carrying a signal from an input to an output; the signal path comprising a first reactive component; a first node on the signal path; a first series resonant circuit comprising at least a second reactive component in series with a third reactive component, the first series resonant circuit being connected between the first node and a ground; an active circuit; the active circuit comprising a voltage controlled current source (VCCS) arranged to change the current flow through the second reactive component in dependence on a voltage sensed (or measured) on the signal path. The first series resonant circuit forms a single harmonic trap with a notch frequency defined by the component values of its reactive components. The effectiveness of the series resonant circuit is dependent upon the strength with which it draws current from the signal path at its resonant frequency.

An adaptive radio frequency including an input, an output, at least one fixed passive inductor and at least one variable active inductor connected between the input and the output.