There is no generic solution for all problems associated with I/O interfaces
ORVILLE HAAS, WILLIAM E. KUNZ
SOURCES OF RF INTERFERENCE WITH CLOCK FREQUENCIES of a few gigahertz, today’s electronic systems are using pulse edges in the subnanosecond range. Networking interfaces deliver data rates in excess of 100 Mbits/s (Fast Ethernet and FDDI—fiber distributed data interface) and 155 and 622 Mbits/s (ATM—Asynchronous Transfer Mode). High quality video circuits also use pixel rates at sub-nanosecond rates. These higher processing speeds present neverending engineering challenges. One such challenge is RF interference, which originates from a fast change of electromagnetic energy. The faster the slew rate (rise/fall times) and the higher the voltage/current amplitude, the more roblematic a circuit becomes. As a result, electromagnetic compatibility (EMC) is harder to achieve today than ever before. The fast changing pulses of current between two nodes of a circuit represent the so-called differential noise source, and the field surrounding such a circuit can couple into other components and can etch connections. The noise induced via inductive or capacitive coupling represents commonmode interference. The RF interference currents are in phase with each other, and the system can be modeled as one which connects the source, “victim circuits” or “recipients,” and the return path, which in many cases is represented by a chassis. Several factors are critical in defining the amount of the interference:• Strength of the source• Size of the area encircled by the culprit current• Slew rate of the change Thus, despite many possible causes of unwanted interference in a circuit, the noise is almost always the common-mode type. Once there is some RF voltage present between a cable plugged into an I/O (input/output) connector and the enclosure or the ground plane, the resulting RF current of a few mA can be enough to exceed the allowable emission levels.Click the pdf below to read the full article.
