Issue link: https://iconnect007.uberflip.com/i/981887
42 DESIGN007 MAGAZINE I MAY 2018 the source. PDN noise is the main driver of radiated emissions. Minimizing PDN noise is accomplished by blocking the escape of noise from the processor into the power and ground planes, and by designing a PDN that has AC impedance below the required target impedance up to the maximum bandwidth. The goal of a low impedance PDN is real- ized by minimizing the spacing between the power and ground planes and by using low impedance decoupling capacitors with low inductance mounting to the power/ground planes. The use of a PDN Planning tool is rec- ommended to effectively analyze the complex requirements. Also, good grounding minimizes noise sources by presenting common mode currents with a low impedance path to ground poten- tial. The use of multiple ground planes, in the stackup, is a very effective way to do this. The importance of avoiding slots or splits, in the solid planes, cannot be overemphasized. If the return path of a common mode current is far from the signal path, then the common mode current will radiate. However, if you engineer the return path to be in proximity to the source current, then the loop area will be small and therefore the common mode current will not radiate. So not all microstrip traces will radiate–just the poorly designed paths. Key Points: • Differential-mode radiation is equal and opposite and therefore any radiating fields will cancel. • Common mode radiation, from two cou- pled conductors, is identical. It does not cancel but rather reinforces. • Microstrip loops can act as small antennae that predominately radiate magnetic fields. • Stripline loops only emit radiation from the fringing fields at the edge of the PCB. • The most critical loops are the high-fre- quency clocks where the signal is periodic. • Clocks should be routed on a stripline adjacent to a solid reference plane to reduce radiation. • The spacing between the clock trace and the return plane should be as small as pos- sible to increase coupling and reduce loop area. • Differential mode radiation can be con- trolled by reducing the PDN impedance, minimizing the loop area, cancelling out the fields by using differential signals and by dithering the clocks. • If a differential pair is not perfectly bal- anced, then the degree of cancellation is not determined by the spacing, but rather by the common mode balance of the dif- ferential pair. • Differential signals, that are closely cou- pled, will operate mainly in the differential mode with some common mode radiation from imbalances in the signals. • If the two differential signals are separated enough, to prevent coupling, then both act as single ended signals. • Common mode radiation is the result of parasitics in the circuit which emanate from the undesired voltage drops in the conductors. • When cables are connected to the PCB, they are driven by the common mode ground potential forming antennae. • Power planes should not be allowed to extend into the ground area of the I/O connectors as noise can be coupled into the I/O signals and ground. • Microstrip stubs can act as antennae once their length approaches one quarter wave- length. • Stubs can be avoided by routing the sig- nals directly through each memory device pad in succession. • To control common mode radiation, it is important to minimize the common mode The importance of avoiding slots or splits, in the solid planes, cannot be overemphasized.