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FEBRUARY 2019 I DESIGN007 MAGAZINE 15 electric fields (blue) of a stripline are totally contained between the two solid planes, so the speed of propagation for signals guided by the trace is entirely determined by the dielectric constant of the surrounding materials. On the other hand, a microstrip is any trace fabricated on the outer layers of a PCB. A mi- crostrip has dielectric material, a plane on one side, and air on the other. An embedded mi- crostrip is similar but covered in a conformal coating such as solder mask or another di- electric material. In this case, the effective dielectric constant should be calculated by a field solver and represents a combi- nation of the surrounding mate- rials. There are also other vari- ants of microstrip and stripline such as build-up microstrip, coplanar waveguides, and dual (a)symmetric stripline. The electromagnetic fields surrounding the microstrip ex- ist partially within the dielec- tric material(s) and partially within the surrounding air. Since air has a dielectric con- stant of one, which is always lower than that of FR-4 and solder mask, mixing a little air into the equation will speed up the signal propaga- tion. Even if the trace widths are adjusted on each layer, so that the impedance is identical, the propagation speed of microstrip is always faster than stripline—typically, by 13–17%. At frequencies above 1 GHz the dissipation factor (Df), another selection criteria for high- speed PCB substrate material, comes into play. Df is a parameter of a dielectric material that quantifies its inherent dissipation (loss) of elec - tromagnetic energy. It refers to the tangent of the angle in a complex plane between the resistive (lossy) component of an electromagnetic field and its reactive (lossless) component. Standard FR-4 has a Df of ~0.02 whereas an ultra-low- loss dielectric may have <0.005 at 10 GHz. Dielectric constant and dissipation factor contribute to the frequency dependent loss and to degrade the bandwidth and speed of the signal. The signal quality transmitted through the medium and picked up at the receiver will be affected by any impedance discontinuities and the losses of the dielectric materials. For- tunately, high-frequency dielectric materials generally have low Er and Df, enabling the signals to propagate faster, have less loss, and therefore, higher bandwidth. Figure 3 shows the loss profile of selected dielectric materials. Figure 2: Microstrip and stripline electromagnetic fields (simulated in HyperLynx). Figure 3: Loss profile for ultra-low-loss dielectric materials. (Source: iCD Materials Planner)