EMI problems? Part two: Where does EMI come from?
Bonnie Baker -March 15, 2012
Electromagnetic interference is a part of our lives. Many people think that the proliferation of electronic products is a good thing because they improve our comfort, safety, and health (Reference 1). These products also bring with them the potential for electronically harmful EMI signals. EMI signals can come from various sources, including the common electronic devices around us, as well as vehicles and heavy equipment. In automotive designs, some of these EMI generators reside in the same cabinet as the vehicles’ sensitive electronic circuits. This proximity affects the audio equipment, automatic door controls, and other equipment.
Every electronic device, including cell phones, creates both good and bad characteristics. Cell phones these days offer the convenience of talking to friends, family, and business associates from just about anywhere. However, they also have the potential to produce EMI signals—and those signals are only part of the problem. The evolution of these devices exceeds the basic phone services by including smartphone capabilities.
Neighboring equipment and circuits do not expect this type of EMI noise. Cell phones rely on high levels of RF energy to do their jobs. Although they comply with regulations, they may become sources of unintended EMI to susceptible devices.
PCBs, clock circuits, oscillators, digital circuits, and processors also can be sources of EMI in circuits. Electromechanical devices that switch currents produce EMI during make-or-break operations. These EMI signals do not necessarily have a negative effect on other electronic equipment. The spectral content and intensity of an EMI signal determine whether it has the potential for an unexpected response from a susceptible circuit.
You can simplify the spectral content of a digital signal to its frequency and rise time. The clock or system frequency establishes a time reference for the circuit, but its edge rates create interfering harmonics. Figure 1 shows the spectral content of a 10-MHz square wave. The 10-MHz signal has an edge rate of 10 nsec. The magnitude of these harmonics decreases with frequency. Generally, the potential EMI for this type of signal spans to the maximum frequency, or 1/(π×tRISE), where tRISE is the rise time, equating to approximately 31.8 MHz for a 10-nsec edge rate.
The figure shows that the last significant harmonic occurs at 30 MHz. Meanwhile, the 1-nsec edge rate in Figure 2 equates to a maximum frequency of 318 MHz. The EMI harmonics might cause interference in your circuit if it is susceptible to frequencies within the 318-MHz bandwidth.
It is better to stop the interfering signal at its source, rather than allow it to propagate through circuits. As for vehicles, carmakers are constructing more vehicle bodies with plastic, which becomes a problem when you need to find a low-impedance ground or provide shielding.
Once the signals are free and roaming about, they stand a chance of entering your sensitive systems and wreaking havoc. Next month’s column will detail how the EMI signal travels through the medium to get to your circuits.
Bonnie Baker is a senior applications engineer at Texas Instruments and author of A Baker’s Dozen: Real Analog Solutions for Digital Designers.