A stripline to microstrip transition has been incorporated into a multilayer circuit board that supports a distributed solid-state microwave power amplifier, for the purpose of coupling the microwave signal from a buried-layer stripline to a top-layer microstrip. The design of the transition could be adapted to multilayer circuit boards in such products as cellular telephones (for connecting between circuit-board signal lines and antennas), transmitters for Earth/satellite communication systems, and computer mother boards (if processor speeds increase into the range of tens of gigahertz).
The transition is designed to satisfy the following requirements in addition to the basic coupling requirement described above:
- The transition must traverse multiple layers, including intermediate layers that contain DC circuitry.
- The transition must work at a frequency of 32 GHz with low loss and low reflection.
- The power delivered by the transition to top-layer microstrip must be split equally in opposite directions along the microstrip. Referring to the figure, this amounts to a requirement that when power is supplied to input port 1, equal amounts of power flow through output ports 2 and 3.
- The signal-line via that is necessarily a part of such a transition must not be what is known in the art as a blind via; that is, it must span the entire thickness of the circuit board.
The lower end of the via is connected to a circular pad on the bottom (ground) layer. Electrically, this pad is a dead-end or no-connection point. The pad is surrounded by a cutout in the ground layer; the cutout includes a rectangular main portion that ends in a triangular taper at the input end.
The cutout lies above a standard rectangular cavity. The combination of the triangular-taper portion of the cutout and the rectangular cavity serves to focus the electromagnetic field to propagate up the signal-line via. The cavity also prevents coupling of the signal to neighboring circuits. The rectangular cavity can be fabricated easily by conventional machining techniques; the triangular-taper portion of the cutout is fabricated easily by printed-circuit techniques. To compensate for reflections from the transition, step-matching sections are included in the vicinity of the triangular taper.
Mode-strapping vias are also included. These vias are blind; that is, they terminate at, and are connected to, an intermediate layer. These vias can be blind because they do not carry the signal. These can be closely spaced. The closeness of the spacing compensates somewhat for the unreliability of connections formed in the process of fabrication of blind vias.
This work was done by Larry Epp and Abdur Khan of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Semiconductors & ICs category. NPO-41061