Issue link: https://iconnect007.uberflip.com/i/981887
MAY 2018 I DESIGN007 MAGAZINE 41 with some common mode radiation from imbal- ances in the signals. If the two traces are sepa- rated enough to prevent coupling, then both act as single-ended signals. So a 100-ohm dif- ferential pair becomes two individual 50-ohm single-ended signals. This is fine, providing the loop area is small and the impedance does not change along the length of the signals. In contrast, the "bad guy," common-mode radiation, is the result of parasitics in the cir- cuit which emanate from the unwanted voltage drops in the conductors. As the signal is driven down the transmission line, capacitive coupling between the trace and plane conductors com- pletes the loop and displacement current flows through the capacitance which returns to the source (Figure 4). The common-mode current that flows through the ground impedance pro- duces a voltage drop in the digital logic ground system and generates magnetic radiation. To make things worse, when cables are con- nected to the PCB, they are driven by this com- mon mode ground potential forming antennae. In fact, it only takes 3uA of common mode cur- rent, on a 1m long cable at 100MHz, to fail an FCC Class B EMC test. Differential-mode radia- tion can be controlled by stackup design and routing however, common mode radiation can be difficult to understand and control because it is unintentionally designed into the system. The schematics do not show the sometimes radical, current paths taken that are vital to our understanding of signal performance, crosstalk and electromagnetic emissions. The most prevalent form of common mode radiation emanates from cables, of the system, that act like dipole or monopole antennae. So, it is important to limit the common mode cur- rent by: 1. Reducing the magnitude of the current and source voltage. 2. Reducing the rise-time, frequency and harmonic content of the current. 3. Reducing the antennae length. 4. Provide a common mode impedance choke in series with the cable and isolate the cable from the PCB with a transformer or optical coupler if possible. 5. Shield the cable and shunt the current off the cable. The PCB power planes should not be allowed to extend into the ground area of the I/O con- nectors. This is because the power plane will usually contain high frequency switching noise and if extended into the I/O area, can couple the noise to the I/O signals and ground. The key here is to have a very low impedance con- nection, at one point only, between the I/O ground and the enclosure/chassis ground. Also, on the PCB, microstrip stubs can act as antennae once their length approaches one quarter wavelength. These short stubs become very efficient transmitters and so are best avoided. In multi-drop buses as with DDR3/4 fly-by architecture, stubs can be avoided by routing the signals directly through each mem- ory device pad in succession. To control common-mode radiation, it is important to minimize the common mode ground voltage that drives the antennae at Figure 4: Common-mode signal return path.