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Circuit designers choose transmission-line technologies based on a number of factors, including expected high-frequency performance and ease of implementation. Although transmission-line choices include some more exotic varieties, such as coplanar-waveguide (CPW) technology, the decision often boils down to either microstrip or stripline. Recognizing the differences between the two technologies can help nail down the best option.
The two high-frequency transmission-line technologies date back to the early 1950s. Stripline was first developed by Robert Barrett of the Air Force Cambridge Research Center in the '50s as a circuit-based transmission-line alternative for waveguide and coaxial cables. It is essentially a center conductor surrounded by dielectric material with top and bottom metal ground planes (Fig. 1). Stripline is often described as a coaxial cable that was run over by a truck, with its outer ground planes, inner conductor, and isolating dielectric material between the three metal layers.
Microstrip is a somewhat simpler structure, with a single ground plane, a conductive trace, and a dielectric layer separating the signal conductor and ground plane (Fig. 2). Stripline conductors are well isolated by the surrounding dielectric material. As a result, they do not radiate and are described as being nondispersive. Because of this, transmission lines in stripline circuits can be closely spaced and densely packed, lending themselves to miniaturization at microwave frequencies. However, with the additional layers, they are more difficult to assemble and manufacture than microstrip circuits with their top-layer conductors.
Manufacturing Methodology
Typical approaches to manufacturing microstrip circuits start with a commercial circuit laminate with metallized top and bottom layers using a highly conductive metal, such as copper. The top layer is etched...