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40 DESIGN007 MAGAZINE I MAY 2018 sions. Clock signals should always be routed first, and every effort should be made to route them in the absolute minimum loop area pos- sible. The length of the clock trace should be minimized as well as the number of layer tran- sition vias. On a multilayer PCB, clocks should be routed on a stripline (inner layer) adjacent to a solid reference plane to reduce radiation. The spacing between the clock trace and the return plane should be as small as possible to increase coupling and reduce loop area. Also, to prevent the clocks from coupling to cables, that leave the PCB assembly, the clock circuitry should be located well away from I/O connec- tors and cables. Data and address buses, together with their associated command and control signals, are second on the critical list. These buses are nor- mally terminated and can carry large peak cur- rents that radiate proportionally to the current transferred. Transient power supply currents can be another significant source of differential radiation. Although these loops can be quite small, they can carry large currents during switching. Differential mode radiation is proportional to the frequency squared and can be controlled by reducing the power distribution network (PDN) impedance to below the required target impedance, minimizing the loop area, cancel- ling out the fields by using differential signals and by dithering the clocks. If the amplitude of the emission is spread out, in the frequency band, then the radiation is also reduced. Spread spectrum clocking can reduce the radiation by up to 15dB. If a differential pair is well balanced, then tight coupling will achieve an effective degree of field cancellation. However, if they are not perfectly balanced (Figure 2), then the degree of cancellation is not determined by the spac- ing, but rather by the common mode balance of the differential pair. Most digital drivers have poor common mode balance and therefore dif- ferential pairs often radiate far more power in the common mode than in the differential mode. In such a case, one gains no radiation benefit from coupling the differential traces more closely together. Figure 3 highlights the simulated common-mode return current in the reference plane of the cross-section of an imbalanced differential pair. Differential signals that are closely coupled will operate mainly in the differential mode Figure 3: Coupled microstrip differential pair (courtesy Ansoft). Figure 2: Differential-mode signals can be converted to common mode.

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