Editor’s note: This question was asked in response to Interference Technology’s recent webinar by Keith Armstrong. To view the webinar, click here.
Question: How are DC signals perceived as EM waves?
Answer: They aren’t EM waves, because by definition a DC signal (or power) is unchanging, so there can be no wave propagation going on.
It is important to understand that fluctuating DC cannot exist – anything that fluctuates is actually AC. But we designers often use the term “DC” rather too loosely (rather like the way we use the term “ground” too loosely) – leading to design confusion, delays, and increased costs.
At the instant of connection of a DC voltage to a load, a transient current flows through the conductors (wires, PCB traces, etc.) charging up their capacitances. This transient current is AC, and is actually a propagating EM wave that generates fluctuating EM fields (magnetic and electric) as it travels along.
Afterwards, the current is DC and there is no more propagating EM wave. The EM fields are static instead of fluctuating.
If you want to think about electricity as electrons flowing along conductors, then continuous AC currents are associated with electrons jiggling back and forth and going nowhere overall. AC current (which includes transient currents) travel at the speed of light in the medium, which is about 300,000 kilometers per second for a bare wire in air, and about 210,000,000 km/s for a wire with a thick layer of PVC insulation.
Continuous DC currents, on the other hand, are associated with a continuous flow of electrons from the negative terminal to the positive terminal – travelling at a velocity of about 4 kilometers (about 3 miles) per hour – i.e. about walking speed!
As I said at the start, a problem with understanding the above is caused by our electronic designer jargon. We often talk about a DC voltage or current when what we really mean is a the few microseconds of unchanging (flat) voltage when a digital signal has settled down to a logic 1 or 0 after its transient overshoots and ripple have died away to acceptably low levels. Or we talk about the “DC current” that an IC draws from its DC voltage rail. But what we really mean is a waveform with a frequency spectrum that includes lower frequencies, maybe even all the way down to 0Hz, but not true DC.
Designers often mislead themselves when they talk about a DC voltage rail – it tends to suggest that the currents that ICs draw from the DC rail are DC currents, when in fact with modern digital electronics they are actually quite powerful RF currents plus a DC component, which is why we must be careful to decouple our DC rails properly at radio frequencies to control emissions.
– Keith Armstrong
