Miniaturized balun transformer

Abstract

A balun circuit includes a dielectric substrate having planar opposing surfaces: a groundplane conductor layer disposed on a first opposing surface; an interlayer conductor layer disposed on a second opposing surface and including first and second electrically isolated conducting strips, with a balance point gap between first ends thereof, and second ends thereof being short-circuited; an interlayer dielectric layer having substantially planar opposing surfaces, with a first opposing surface thereof being disposed over the interlayer conductor layer; and a top conductor layer disposed over a second opposing surface of the interlayer dielectric layer and including a third conducting strip overlying the first and second conducting strips, one end of the third conducting strip providing an unbalanced port terminal and another end of the third conducting strip being open-circuited. The third conducting strip includes a first and a second set of series-connected line sections each having diverse impedances which are a mirror opposite of each other relative to a center plane of the balun circuit. The first and second conducting strip have impedances which are a mirror opposite of each other relative to the center plane of the balun circuit. The impedances of the first and second conducting strips can be diverse impedances. Phase and amplitude balance at the balance point gap is achieved by the mirror opposite relationship of the impedances of the first and second set of line sections and the mirror opposite relationship of the impedances of the first and second conducting strips.