The same wire has too much inductance at high speed... from Howard Johnson (co-author of High-Speed Digital Design: A Handbook of Black Magi...
The same wire has too much inductance at high speed...
from Howard Johnson (co-author of High-Speed Digital Design: A Handbook of Black Magic), where he uses “black magic” humorously to describe counterintuitive high-speed effects.
What he means by “the same wire has too much inductance at high speed”
Inductance is a fundamental electrical property of any conductor that describes its tendency to resist changes in current. Whenever current flows through a wire, it creates a magnetic field around it. If that current changes, the magnetic field changes as well, inducing a voltage that opposes the change in current. This relationship is captured by the equation ( V = L*dI/dt, where (L) is inductance. Even a straight piece of wire has inductance, though at low speeds its effects are usually so small that they can be ignored.
In High-Speed Digital Design: A Handbook of Black Magic, Howard Johnson emphasizes that this assumption breaks down at high speed. A wire that behaves like a simple connection at low frequencies can become a source of significant problems as signal edges get faster. The physical wire does not change, but the rate at which current changes increases dramatically, making inductance a dominant factor in circuit behavior.
As signals switch more quickly, the rate of current change (dI/dt) becomes large, and even small inductance produces noticeable voltage. These induced voltages can distort signals, causing ringing, overshoot, and noise. What once appeared to be an ideal connection now behaves like an active component interfering with signal integrity.
At the same time, the path that current takes becomes more constrained. High-speed currents return along the path of lowest impedance, which lies close to the outgoing signal path. If the return path is long, indirect, or interrupted, the loop area increases, raising the effective inductance and worsening these unwanted effects. This makes the physical layout of conductors—especially the proximity of signal and return paths—critical to performance.
As edge rates continue to increase, wires must also be understood as transmission lines rather than simple connections. This introduces additional effects such as reflections, impedance mismatches, and propagation delay. These behaviors are largely invisible at low speeds but become central concerns in high-speed design.
This is why high-speed circuits can seem unintuitive. Small physical details, like trace length or grounding strategy, suddenly have a large impact on system behavior. The “black magic” described by Johnson is not magic at all, but the natural result of electromagnetic effects becoming dominant as speeds increase.
Ultimately, the wire itself has not changed—the signal has. At high speeds, every conductor participates in a dynamic electromagnetic system where inductance, current loops, and transmission line effects must all be considered to ensure reliable operation.
