An off-chip signal is provided to a differential branch-line directional coupler implemented entirely on-chip. The coupler produces differential quadrature signals, which are then buffered and applied to a quadrature mixer. The coupler is implemented entirely on-chip using microstrip transmission lines. The coupler is made up of a plurality of rings and a plurality of underpasses connecting ports of the rings, wherein each of the plurality of rings is made up of four branch lines, and each branch line having an electrical length of one-quarter wavelength at the center design frequency. Coupling between the plurality of branch lines of the rings may be varied.