November 20, 2023
Poor attention to detail in design has killed AM radio.
Emissions, in this case, refers to the electromagnetic noise pollution caused when oscillators oscillate, and digital circuits, loads, and PWM/PDM/PFM loads switch on and off. Radiated are signals that travel via the Ether to other locations where they are either desired or unwanted. Conducted signals travel via the wiring, PCB, and/or metal on the die or bond wires.
How Can Radiated Emissions be Mitigated?
Through shielding via metal, conductive plastic, or another conductor for a faraday cage. EMI gaskets can block cracks. Indium tin oxide coatings are one way to provide shielding on transparent surfaces. Gold vapor deposition is another. Silver coatings increase effectiveness of shields at surfaces. Banded overbraid or similar cables can reduce EMI on cable bundles, as well as reduce susceptibility. A narrow band antenna can be filtered via quarter wave stub resonant filters and lighting arrestors. Shielded connectors like USB, FireWire, Lightning, thunderbolt, dsub, or circular mil. And shielded cabling can help. Spread spectrum oscillators can help in some cases, but affect jitter on Phy’s. Slowing edges and paying attention to slew rates, also can help. Impedance control and matching are also vital as is proper signal termination.
How Can Conducted EMI be Reduced?
Filtering via a variety of ways is commonly used on signal and power conductors. Capacitors must be selected for Low Effective Series Resistance. Capacitors that cover each frequency band should be used. Paralleling capacitors lowers ESR. Expensive porcelain capacitors are the only thing effective vs some microwave frequencies, others require capacitors and inductors done in circuit board etch. Inductors must be selected for low inter winding capacitance. Ferrite beads are effective at low microwave frequencies. Metal can EMI filters require metal fences or shield walls. SMD EMI filters are more effective with a I/O dividing shield(fence) wall. Lead and Pad inductance are important parameters as well. Termination and slew rate limits can also help conducted emissions. These can be pursued via things like gate drive resistance on MOSFETs. I/O like RS-485 or CAN bus can be selected for slew limited lower EMI parts. AC snubbers can help with busses and loads.
William is an Applied Scientist with over 45 years experience in the workforce. He graduated with a BSEE from SD MINES, and had graduate work at UTD, UTA, The Borden Institute, USAF Flight Test Engineering, and others.