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JUNE 2022 I DESIGN007 MAGAZINE 37 form stripline transmis- sion lines. Such tech- niques are recommended for signals above a few hundred megahertz. G r o u n d e d ( g u a r d ) t r a c e s a r e s o m e t i m e s used to surround digital clock signals, which tend to have fast edge rates. In addition to having a ground plane below the signal trace, copper traces are placed on either side of the trace and are stitched to the ground plane in multiple locations. For digital systems, this technique is only useful for traces on outer layers. On inner layers, it makes more sense to just leave some empty space around the trace. However, if microstrip is very closely coupled to the reference plane (<3 mil) then radiation is minimal and provid- ing ground shielding on outer layers has little benefit. When copper pours come into proxim- ity with critical signal traces (Figure 3), the impedance is reduced by 2–3 ohms. So, on the copper side of the differential pair, the imped- ance will be lower than the other side free of copper. is will convert differential mode signals into common mode signals at these points, impacting signal integrity. If a differen- tial pair is well balanced, then tight coupling will achieve an effective degree of field can- cellation. However, if they are not perfectly balanced, then the degree of cancellation is not determined by the spacing but rather by the common mode balance of the differential pair. is copper pour needs to be bordered by ground stitching vias and pulled back from the signal traces by at least 20 mils. Better still, it's le out altogether. Electromagnetic emissions from digital cir- cuits can occur as either differential mode or common mode radiation. Differential mode is typically equal and opposite and therefore any radiating fields will cancel. Conversely, common mode radiation from two coupled conductors is identical. It does not cancel but rather reinforces. Unfortunately, differ- ential mode propagation can be converted to common mode by parasitic capacitance, any imbalance caused by signal skew, rise/fall time mismatch, or asymmetry in the channel. My recommendations for pours: • All copper pours should be connected by stitching vias to the distributed ground net • Do not use isolated copper pours • Critical signals should be routed in the inner stripline layers with no pours • Closely coupled differential signals can be routed on the outer microstrip layers without GND pours • Critical single-ended signals can be routed on the outer microstrip layers if converted to CPW • However, if microstrip signals are very closely coupled to the reference plane (<3 mil) then radiation is minimal and providing ground shielding on outer layers has little benefit Figure 3: Copper pours lower the impedance of nearby traces.