Coupling Mechanism
The Four EMI Coupling Paths
Interference does not travel by magic; it follows physics. In capacitive coupling, a time-varying voltage on the aggressor drives a displacement current through the parasitic mutual capacitance Cm between the two conductors. Because the coupled current is Cm×dV/dt, this mechanism worsens at higher frequencies and higher source impedances, and it preferentially injects noise into high-impedance victim nodes. In inductive coupling, the aggressor current sets up a magnetic field that links the victim loop; the induced voltage is M×dI/dt, where M is the mutual inductance set largely by the loop area enclosed between source and victim. High-current, low-impedance circuits (switching regulators, motor drives) are classic inductive aggressors.
Conducted coupling occurs when two circuits share a finite-impedance return path, typically a ground trace or a power rail. Current from one circuit develops a voltage I×Zcommon across the shared impedance, and that voltage appears as noise to the other circuit. This is the basis of ground bounce and is why single-point or star grounding and rail decoupling matter so much below a few megahertz. Radiated coupling, by contrast, transfers energy as a propagating electromagnetic wave once the conductors are long enough relative to a wavelength to act as antennas, becoming the dominant path above roughly 30 MHz where cables and board edges radiate efficiently.
The same physical structure can support several mechanisms at once, and the dominant one shifts with frequency and geometry. A shielded cable, for example, addresses capacitive coupling through its grounded screen, while the twist of its inner pair attacks inductive coupling, and the screen termination quality governs how much radiated energy leaks. Correctly attributing measured noise to its mechanism is what separates an effective fix from a costly guess.
Governing Equations for Each Mechanism
Icoupled = Cm × dV/dt ⇒ Vnoise ≈ Icoupled × Zvictim
Inductive (magnetic-field) coupling:
Vcoupled = M × dI/dt, M = μ0 × (Aloop / 2πd) (parallel-wire approx.)
Conducted (common-impedance) coupling:
Vnoise = Iaggressor × Zcommon
Near-field to far-field boundary:
rb = λ / (2π) ≈ 48 mm @ 1 GHz, ≈ 4.8 mm @ 10 GHz
Where Cm = mutual capacitance, M = mutual inductance, Aloop = enclosed loop area, d = conductor separation, Zvictim = victim node impedance, Zcommon = shared return impedance, λ = wavelength.
Mechanism Comparison and Mitigation
| Mechanism | Field / Path | Dominant Range | Frequency Trend | Primary Fix |
|---|---|---|---|---|
| Capacitive | Electric (E), Cm | Near field | ∝ f (rises with f) | Grounded electrostatic shield; lower Zvictim |
| Inductive | Magnetic (H), M | Near field | ∝ f (rises with f) | Reduce loop area; twisted pair; mu-metal |
| Conducted | Common Zcommon | < ~1 MHz | Broadband | Star ground; rail decoupling; filtering |
| Radiated (near) | E and H, r < λ/2π | 1 to 30 MHz | ∝ f2 to f3 | Conductive enclosure; gasketing |
| Radiated (far) | Plane wave, 377 Ω | > 30 MHz | ∝ f | Shielded room; cable termination |
Frequently Asked Questions
How do I tell whether interference is coupling capacitively or inductively?
Change the victim impedance and watch the noise. Capacitive coupling injects a current (Cm×dV/dt) that develops more voltage across a high-impedance node, so shorting the victim load makes the noise nearly vanish. Inductive coupling induces a series voltage (M×dI/dt) set by loop area, so it persists when the load is shorted but scales with cable geometry. Twisting the pair attacks inductive coupling; a grounded shield attacks capacitive coupling.
At what distance does coupling change from near-field to far-field behavior?
The boundary is near r = λ/(2π), about 48 mm at 1 GHz and 4.8 mm at 10 GHz. Inside it the wave impedance is set by the source: a high-voltage source looks high-impedance (E-field, capacitive), a high-current loop looks low-impedance (H-field, inductive). Beyond it the wave impedance settles to 377 Ω and energy propagates, so radiated coupling governs and a thin conductive enclosure is usually sufficient.
Which coupling mechanism dominates above 30 MHz?
Below ~1 MHz, conducted coupling through shared ground and power impedances dominates. From ~1 to 30 MHz, near-field capacitive and inductive coupling between adjacent traces and cables prevails. Above 30 MHz, cables and PCB structures are an appreciable fraction of a wavelength and radiate efficiently, so radiated coupling dominates. CISPR 32 reflects this by using conducted-emission tests from 150 kHz to 30 MHz and radiated tests from 30 MHz to 6 GHz.