A controlled impedance interconnection assembly (50), for connecting an x-band coaxial connector (66) to a microstrip (60) on a circuit board (52), comprises a circuit board having a core (54) of dielectric material terminated by first and second surfaces (56, 58). An electrically conductive microstrip (60) and electrically conductive material (62) are adhered respectively to the first and second surfaces, and a hole (64), substantially at right angles to the surfaces, extends through the core and the conductive material on the second surface, and to the conductive microstrip on the first surface. The x-band coaxial connector, including an inner conductor (68) and a conductive shell (70) coaxially surrounding the inner conductor, is secured at right angles to the circuit board. The conductive shell is electrically connected to the conductive material, and the inner conductor is extended through the hole and has a mechanically smooth, right-angled transition (74, 76) with and electrical connection to the conductive microstrip. The smooth, right-angled transition essentially elminates otherwise high mismatches and VSWR (voltage standing wave ratio) in the connection and enables the circuit board to operate in excess of 14 GHz and in x-band frequencies with an augmented bandwidth. A tuning element (80) may be added to provide capacitive and inductive match for the microstrip, to focus the signal back into the microstrip, to provide a shielding from external or spurious signals and to minimize crosstalk between adjacent traces.