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Design007-May2018

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24 DESIGN007 MAGAZINE I MAY 2018 Essential pre-layout effort from signal integrity perspective According to guidelines, a PAM-4 chan- nel with trace length up to 8 inches on a PCB shall have insertion loss less than 10dB at 14GHz (i.e., Nyquist frequency of 28GBaud) and 20dB at 28GHz (i.e., 2 nd harmonic of 28GBaud) respectively [2] to achieve seamless data communication between the transceiv- ers. Eight essential pre-layout efforts from sig- nal integrity perspective to be discussed in the following: A. Substrate material selection There are three types of PCB substrate dielec- tric materials categorized based on the dielec- tric property (e.g., loss tangent and dielectric constant). As seen in Table 1, high-loss materi- als (e.g., Nelco N4000-6) have a loss tangent above 0.02 and dielectric constant above 4; medium-loss material (e.g., Isola FR408) has a loss tangent of about 0.01 and dielectric con- stant between 3 and 4. Meanwhile, low-loss materials (e.g., Isola I-Tera MT40) have a loss tangent of about 0.003 and dielectric constant 3.45 [5] . Dielectric attenuation is directly pro- portional to the loss tangent and square root of dielectric constant, as shown in Equation 1 [6] . (Equation 1) e r = dielectric constant f = frequency in GHz Simulated plot of differential insertion loss for dielectric loss effect of 8-inch differential microstrip signals in Figure 2 quantifies the dielectric attenuation for high, medium and low loss materials as 10dB, 5.5dB and 3.5dB respectively at 14GHz. Referring to the guide- line mentioned earlier (i.e., insertion loss less than 10dB at 14GHz for trace length up to 8 inches), low loss material shall be selected to allow more headroom contributed by other channel losses. B. Fiber weaving PCB dielectric substrate is composed of woven fiber-glass bound together with epoxy resin. The microscopic top view of PCB sub- strates of fiber weave styles 1080 and 3313 are illustrated in Figure 3 [7] . A higher number of fiber glass style (e.g., 3313), refers to denser fiber glass weaving. Fiber glass material has dielectric properties that differ very much from the properties of epoxy resin. For instance, NE-glass fiber has dielectric constant (Dk) and loss tangent (Df) 4.4 and 0.0006 respectively [8] . Meanwhile epoxy resin has Dk 3.2 [8] , which is lower than Table 1: Categories of dielectric materials. Figure 3: Fiber weave with glass style 1080 versus 3313. Figure 2: Simulated plot of differential insertion loss for dielectric loss effect with Hyperlynx.

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