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Design007-Feb2021

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58 DESIGN007 MAGAZINE I FEBRUARY 2021 Crosstalk arises because of the uninten- tional coupling of electromagnetic fields. e current trend is toward smaller, faster boards with lower IC core voltages. But, as the sup- ply voltage drops from 3.3V to 1.5V, the allow- able noise margin is more than half. Also, the closer parallel trace segments are spaced, due to limited real estate, the greater the chance of coupling. e reflections created by crosstalk erode the noise margin. We cannot eliminate crosstalk, but as PCB designers it is our job to ascertain how to control and manage the pre- dicted interference. Crosstalk is three dimensional and depen- dent on the signal trace separation, the trace to plane(s) separation, parallel segment length, the transmission line load, and the technology employed. But crosstalk also varies depending on the physical stackup configuration. In this month's column, I will delve into the proper- ties of microstrip and stripline crosstalk and how to mitigate the concern. Crosstalk can be induced by the follow- ing stackup configurations as illustrated in Figure 1: 1. Microstrip edge coupled 2. Stripline edge coupled 3. Dual stripline broadside coupled 4. Dual stripline broadside-edge coupled Stackup Configurations to Mitigate Crosstalk Beyond Design by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Figure 1: Crosstalk stackup configurations.

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