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28 DESIGN007 MAGAZINE I JULY 2024 Designers oen mislead themselves when they talk about a DC voltage rail. It tends to sug- gest that the currents that ICs draw from the DC rail are DC currents, when in fact, with modern digital electronics, they are actually quite pow- erful high-frequency currents plus a DC compo- nent, which is why we must be careful to decou- ple our power supply rails properly to control emissions. icker traces reduce the skin effect and minimize losses at AC, while a thicker trace at DC allows for freer movement of charged par- ticles and thus reduces emitted heat. Key Points • One must appreciate the difference between the lumped element view and that of distributed circuits. • At high frequencies, the diameter of a con- ductor affects how efficiently it absorbs energy from electromagnetic waves. A larger conductor diameter allows for more efficient energy absorption. • If the conductor has a small diameter (compared to the wavelength of the inci- dent wave), the induced currents are lim- ited, and the energy absorption is reduced. • e energy carried by an electromagnetic wave is directly proportional to the square of its amplitude. • Energy/signal traveling down a conductor is actually the speed of the electromagnetic wave, not the movement of electrons. • e EM wave traveling along (guided by) the copper trace forms a type of wave- guide. • e electric field of a conductor extends to the return conductor, which is usually closely coupled to the signal trace. • Even though DC signals are typically asso- ciated with steady currents, their electro- magnetic fields still propagate at the same speed as other electromagnetic waves. • When a DC voltage is applied to a trans- mission line, it generates a magnetic field, which in turn creates an EM wave along the line. • In the case of DC, a constant electric field creates the flow of electrons through a conductor pair, creating a constant magnetic field. is magnetic field then generates an electromagnetic wave. • Electric and magnetic fields are present around any electrical circuit, whether it carries AC or DC electricity. • It is not current that delivers the energy to the load, but rather electrons moving through the copper create EM fields, facilitating the energy transfer. DESIGN007 Resources • Beyond Design: "The Wavelength of Electro- magnetic Energy" by Barry Olney • Fast Circuit Boards, by Ralph Morrison • "Physics Stack Exchange," Physics Forum • Wikipedia: Speed of electricity • "How are DC signals perceived as EM waves?" by Keith Armstrong • University Physics Vol 2, Samuel J. Ling, et al Rick Hartley is the principal engineer at R Hartley Enter- prises and has been in the industry for over 50 years. He is one of the primary consul- tants for PCB manufacturing and design companies. Rick has also conducted classes worldwide on EMI, signal integrity, and various other electrical topics for the last 30 years. Barry Olney is managing director of In-Circuit Design Pty Ltd (iCD), Australia, a PCB design service bureau that specializes in board-level sim- ulation. The company devel- oped the iCD Design Integrity software incorporating the iCD Stackup, PDN, and CPW Planner. The software can be downloaded at www.icd.com.au. To read past columns, click here.