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30 DESIGN007 MAGAZINE I JULY 2019 100 MHz, planar capacitance replaces individ- ual decoupling capacitors. These plane pairs are best positioned directly under a BGA near the (top or bottom) surface rather than in the center of the symmetric stack to reduce via in- ductance. A via that provides the connection between signal traces, referenced to planes of different DC potential, creates return path discontinui- ties. In other words, the return current has to jump between the planes to close the current loop, which increases the inductance, affecting the signal quality. This return current can also excite the parallel plate resonance mode, caus- ing significant electromagnetic radiation from the fringing fields. When the return current flows through the impedance of a cavity between two planes, it generates voltage. Although quite small (typ- ically in the order of 5 mV), the accumulat- ed noise from simultaneous switching devices can become significant. This voltage, emanat- ing from the vicinity of the signal via, injects a propagating wave into the cavity, which can excite the cavity resonances or any other par- allel structure (for instance, between copper pours over planes). Other signal vias also pass- ing through this cavity can pick up this tran- sient voltage as crosstalk. Plane pairs should be coupled closely together to dampen this cavity resonance and to provide high planar capacitance. To design the perfect stackup, one needs to understand how and where the electromagnet- ic fields propagate and where the current re- turn paths flow through the substrate. Placing power supplies as copper pours allows the de- signer to make full use of the planar capaci- tance and to isolate critical signals within the substrate. Key Points • The speed of a digital signal does not de- pend intrinsically on the speed of elec- trons but rather on the speed of energy transfer between electronic components • The energy propagates between the signal trace and the return path(s) as an electro- magnetic wave • Microstrip (outer) layer EM fields tend to radiate outward as there is only one solid plane beneath, which blocks the emissions • The electric fields of a stripline are total- ly contained between the two solid planes whereas the magnetic fields are also limit- ed vertically by the planes • Radiation is reduced dramatically and is limited to just the edge fringing fields due to the shielding effects of the planes • Stripline crosstalk can be reduced by rout- ing the signal traces orthogonally on adja- cent layers, reducing the couple to just a small area • For synchronous buses, crosstalk only oc- curs when the signals are being switched and this crosstalk only has an impact within a small window around the mo- ment of the clocking • Each signal layer should be adjacent to— and closely coupled to—an uninterrupted reference plane, which creates a clear return path and eliminates broadside crosstalk • Rather than allocating one or two supplies per plane, it is best to use the dual strip- line layers to provide mixed-signal/power pours Figure 6: Mixed-signal/power pours on the inner stripline layers.