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22 DESIGN007 MAGAZINE I AUGUST 2020 should get about 60 mm between the melted globs. I got 58 mm after 45 seconds. However, there is plenty of leeway in the dubious accu- racy of my plastic ruler and failing eyesight. Doubling this (120 mm) gives you the wave- length related to a frequency of 2.45 GHz. The following equation is used to calculate the velocity (v), where f is frequency and lambda (l)is the wavelength. Equation 1: For an extremely rough measurement, this is very close to the actual velocity of light (299,792,458 m/s). Note that light will travel a little slower in the air than a perfect vacuum. Now, let's look at how this relates to the speed of electromagnetic energy in multilayer PCBs. If you have a digital signal running at a clock rate of 2.45 GHz, then one would expect the wavelength to be 120 mm. Wrong! Unfor- tunately, the relative permeability or dielec- tric constant (Dk) of the surrounding materi- als impacts the velocity of propagation at the speed of light (c). Equation 2: A vacuum has a Dk=1, air=1.0006 and typ- ical FR-4=4. Then, solve Equation 2 for the wavelength, including the Dk of the dielectric material: Equation 3: Therefore, the FR-4 material in a stripline configuration slows the propagation speed and decreases the wavelength of the electromag- Figure 1: Half wavelength between hot spots. Figure 2: The wavelength of a 2.45-GHz signal on a PCB inner stripline layer.

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