A time delay filter comprising: • a substrate comprising a first isolative layer (1, 3-7, 9), the first isolative layer comprising a first surface and a second surface, the second surface substantially parallel to the first surface; • a resonator, comprising a capacitive element coupled to an inductive element, the capacitive element located at an intermediate position within the substrate, the inductive element comprising: ∘ a first conductive region coupled to the first surface of the first isolative layer, ∘ a second conductive region coupled to the second surface of the first isolative layer, ∘ a first via that is electrically coupled to and extends between the first and second conductive region; ∘ a second via, separate from the first via, that is electrically coupled to and extends between the first conductive region and a first portion of the capacitive element; ∘ a third via, separate from the first and second vias, that is electrically coupled to and extends between the second conductive region and a second portion of the capacitive element; ∘ wherein the first conductive region, the first via, the second conductive region, the second via, the capacitive element, and the third via form a loop as a continuous conductive path interrupted by the capacitive element; • a first coupling point, electrically coupled to the first conductive region of the resonator; and • a second coupling point, electrically coupled to the second conductive region of the resonator; • wherein the capacitive element and the inductive element are coupled together into a circuit including a first element, whose reactance at radio frequencies is negative, and a further element, whose reactance at radio frequencies is positive, the first element and the further element connected in parallel between the first coupling point and the second coupling point. • a substrate comprising a first isolative layer (1, 3-7, 9), the first isolative layer comprising a first surface and a second surface, the second surface substantially parallel to the first surface; • a resonator, comprising a capacitive element coupled to an inductive element, the capacitive element located at an intermediate position within the substrate, the inductive element comprising: ∘ a first conductive region coupled to the first surface of the first isolative layer, ∘ a second conductive region coupled to the second surface of the first isolative layer, ∘ a first via that is electrically coupled to and extends between the first and second conductive region; ∘ a second via, separate from the first via, that is electrically coupled to and extends between the first conductive region and a first portion of the capacitive element; ∘ a third via, separate from the first and second vias, that is electrically coupled to and extends between the second conductive region and a second portion of the capacitive element; ∘ wherein the first conductive region, the first via, the second conductive region, the second via, the capacitive element, and the third via form a loop as a continuous conductive path interrupted by the capacitive element; ∘ a first conductive region coupled to the first surface of the first isolative layer, ∘ a second conductive region coupled to the second surface of the first isolative layer, ∘ a first via that is electrically coupled to and extends between the first and second conductive region; ∘ a second via, separate from the first via, that is electrically coupled to and extends between the first conductive region and a first portion of the capacitive element; ∘ a third via, separate from the first and second vias, that is electrically coupled to and extends between the second conductive region and a second portion of the capacitive element; ∘ wherein the first conductive region, the first via, the second conductive region, the second via, the capacitive element, and the third via form a loop as a continuous conductive path interrupted by the capacitive element; • a first coupling point, electrically coupled to the first conductive region of the resonator; and • a second coupling point, electrically coupled to the second conductive region of the resonator; • wherein the capacitive element and the inductive element are coupled together into a circuit including a first element, whose reactance at radio frequencies is negative, and a further element, whose reactance at radio frequencies is positive, the first element and the further element connected in parallel between the first coupling point and the second coupling point.