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26 DESIGN007 MAGAZINE I JULY 2024 greater distances, the aggregate field can be approximated by the differential field between the conductor and the return conductor, which tends to cancel. When you minimize the return loop in a circuit, it helps reduce electromag- netic radiation. When a DC step voltage is applied to a trans- mission line, the electron dri velocity will increase in speed proportionally to the strength of the electric field. However, electron dri speed is very slow; dri velocity is about 4 Km/ hr. AC voltages cause no net movement. e electrons oscillate back and forth in response to the alternating electric field, over a distance of a few micrometers. e velocity of propagation of an EM wave is only limited by the dielectric properties of the surrounding materials. EM waves travel at the speed of light in a vacuum or air but are slowed down as they enter a dielectric. e relative permeability or dielectric constant (Dk), of the surrounding materials, impacts the velocity of propagation (v) at the speed of light (c). erefore, FR-4 material (with Dk = 4) in a stripline configuration slows the propaga- tion speed and decreases the wavelength of the electromagnetic wave by about half. But that all depends on the exact dielectric constant of the combined surrounding materials. It is important to note that this speed applies to all electromagnetic waves, including direct current (DC) signals. Even though DC signals are typically associated with steady cur- rents, their electromag- netic fields still propagate at the same speed as other electromagnetic waves. So, whether it's a radio wave, visible light, or a DC signal, they all travel at this universal speed. When a DC voltage is applied to a transmission line, the particles move, generating a magnetic field, which in turn creates an EM wave along the line. Signals do not travel as electrons or charge movement; it would be way too slow to be useful. Signals travel at a fraction of light speed because it's the EM wave that travels. Electromagnetic waves are generated by the acceleration of charged particles. 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. e strength of the DC current does not affect the frequency of the electromagnetic wave, but it does affect the amplitude or intensity of the wave. A stronger current will result in a stron- ger electromagnetic wave with a higher ampli- tude and thus more energy. When a DC voltage is connected to a load (via a PCB trace), a transient current flows through the conductors. is transient current is actu- ally an alternating current and behaves like a propagating electromagnetic wave. It gener- ates fluctuating electric and magnetic fields as it travels along the conductor. If the imped- ance of the transmission line is not matched to the source and the load, it will reflect, creating overshoot and undershoot as in Figure 2. Figure 2: A DC step voltage applied to a transmission line will ring from reflections.