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Design007-May2018

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38 DESIGN007 MAGAZINE I MAY 2018 Common Symptoms of Common-Mode Radiation Beyond Design by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Electromagnetic radiation from digital circuits, can occur as either differential mode or common mode. Differential mode is typically equal and opposite and therefore any radi- ating fields will cancel. Con- versely, common-mode radia- tion from two coupled con- ductors is identical. It does not cancel but r ather rein- forces. Unfortunately, differential- mode propagation can be converted to common mode by parasitic capacitance or any imbalance caused by signal skew, rise/fall time mismatch or asymmetry in the channel. Also, r eturn path discontinuities can create large common mode loop areas that increase series inductance and electromagnetic radiation. In this month's column, I will explore the common symptoms of, and present some cures for, common mode radiation. Differential mode radiation accompanies normal circuit operation and is the result of current flowing in the return path loop formed by the PCB conductors (traces and reference planes, as in Figure 1). Microstrip (outer layer) loops can act as small antennae that predomi- nately radiate magnetic fields, whereas stripline (inner layer) loops only emit radiation from the fringing fields at the edge of the PCB. Although these signal loops are necessary for circuit operation, their size and loop area must be controlled during the design process to mini- mize radiation. Fortunately, it is not neces- sary to evaluate each loop individually. However, the most critical loops should be analyzed. The other loops can be controlled by good stackup design practices. Please refer to the structural guidelines in my Beyond Design: Stackup Planning Parts 1-4 columns [1] for fur- ther information. Generally, the most critical loops are the high- est frequency where the signal is periodic. In a synchronous circuit, the clock is a sequence of repetitive pulses that generates the most emis- Figure 1: Normal (differential mode) signal return current.

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