1. Conducted Noise Coupling
Conducted coupling occurs when noise travels through a shared electrical conductor between a noise source and a victim circuit. This is the most direct form of electromagnetic interference and is often the easiest to identify and mitigate. The coupling path is a physical wire, trace, or plane that both circuits share.
In a typical electronic system, conducted coupling occurs through power supply rails, ground connections, and signal buses. When a noisy circuit draws transient current through a shared impedance, it develops a noise voltage that appears at the terminals of other circuits connected to the same conductor.
V_noise = I_noise × Z_shared. The noise voltage seen by the victim is the product of the noise current and the shared impedance in the coupling path.
Common Sources of Conducted Coupling
- Switching power supplies — High dI/dt transients propagate back onto supply rails
- Digital IC switching — Simultaneous output switching creates current spikes on VDD/VSS
- Motor drivers — PWM switching injects broadband noise onto power lines
- Relay contacts — Contact bounce creates conducted transients
Cause → Effect → Mitigation
Switching regulator shares power rail with sensitive ADC. Fast current transients from the regulator develop noise voltage across rail impedance, corrupting ADC readings.
Add LC filter between regulator output and ADC supply. Use separate power plane pour for ADC. Add bulk + local decoupling. Star-ground the ADC section.
2. Capacitive Coupling
Capacitive coupling occurs when a changing voltage on one conductor induces a displacement current through the parasitic capacitance to an adjacent conductor. This is also called electric field coupling. The mutual capacitance between two parallel traces on a PCB is the primary mechanism for high-frequency crosstalk.
The coupled noise voltage depends on the rate of voltage change (dV/dt), the mutual capacitance (Cm), and the impedance of the victim circuit. Faster edge rates produce more capacitive coupling because the displacement current i = Cm × dV/dt is proportional to the rate of change.
Cm ≈ ε₀ × εr × L × h / (π × d²) (approximate for parallel traces)
Reducing Capacitive Coupling
- Increase trace separation — Cm decreases as 1/d² for parallel traces
- Reduce parallel run length — Cm is proportional to coupling length
- Use ground guard traces — Grounded traces between aggressor and victim shield the E-field
- Route on adjacent layers orthogonally — Minimizes overlap capacitance
- Reduce rise time only if needed — Slower edges reduce dV/dt but may impact timing
Keep trace center-to-center spacing at least 3× the trace width to reduce crosstalk to approximately 10% of the aggressor signal.
3. Inductive Coupling
Inductive coupling occurs when a changing current in one conductor creates a time-varying magnetic field that induces a voltage in a nearby loop. This is governed by Faraday's law of electromagnetic induction. The mutual inductance (M) between two loops determines the coupling strength.
Unlike capacitive coupling which depends on dV/dt, inductive coupling depends on dI/dt. High-current, fast-switching circuits (power converters, motor drivers, bus drivers) are the primary sources of inductive coupling. The key to reducing inductive coupling is minimizing the loop area of both source and victim circuits.
Reducing Inductive Coupling
- Minimize loop area — Route signal and return path adjacent; use ground planes
- Increase separation — M decreases with distance
- Orient loops perpendicular — Orthogonal loops have zero mutual inductance
- Use twisted pairs — Alternating twist cancels net flux linkage
- Shield with ground plane — Eddy currents in ground plane oppose the changing field
4. Common Impedance Coupling
Common impedance coupling occurs when two or more circuits share a common current return path that has non-zero impedance. The noise current from one circuit flowing through the shared impedance creates a noise voltage that appears in the other circuit. This is one of the most prevalent coupling mechanisms in poorly designed PCBs.
The classic example is two circuits sharing a ground trace. When Circuit A draws a transient current pulse, the voltage drop across the shared ground trace impedance (V = I × Z) shifts the local ground reference for Circuit B, injecting noise directly into its signal path.
where Z_common = R + jωL of the shared conductor
Common impedance coupling is the #1 cause of noise in mixed-signal systems. A digital circuit's ground bounce directly corrupts analog measurements if they share a return path. Always use star grounding or separate ground planes with a single-point connection for mixed-signal designs.
5. Crosstalk
Crosstalk is the combination of capacitive and inductive coupling between parallel transmission lines on a PCB. When a signal transitions on an aggressor trace, both electric field (capacitive) and magnetic field (inductive) coupling induce noise on adjacent victim traces. The two components add at the near end (NEXT) and partially cancel at the far end (FEXT).
NEXT vs FEXT
| Parameter | NEXT (Near-End) | FEXT (Far-End) |
|---|---|---|
| Direction | Backward (toward driver) | Forward (toward receiver) |
| Coupling | Cm + Lm (always adds) | Cm - Lm (may cancel) |
| Duration | 2× propagation delay | Proportional to rise time |
| Amplitude | Saturates with length | Increases with length |
| Polarity | Same as aggressor edge | Opposite for stripline |
FEXT: Kf = (1/2)(Cm/C₀ - Lm/L₀) × coupled_length / rise_time_distance
A grounded guard trace between aggressor and victim can reduce crosstalk by 10-20 dB, but only if the guard trace is grounded with vias at intervals less than λ/10 at the highest frequency of interest. Without adequate grounding, the guard trace can actually increase coupling.
6. Radiated Coupling
Radiated coupling occurs when electromagnetic energy propagates through free space from a source to a victim. Unlike conducted and near-field coupling, radiated coupling operates in the far-field region (distance > λ/2π) where electric and magnetic fields are orthogonal and propagate as a plane wave. At frequencies above approximately 100 MHz, most PCB structures can act as unintentional antennas.
PCB traces, cables, connectors, and enclosure slots can all act as radiating structures. A trace of length L becomes an efficient radiator when L ≥ λ/10. For a 1 GHz signal in FR4 (εr ≈ 4.3), λ ≈ 144 mm, so traces as short as 14.4 mm can radiate significantly.
where f = frequency (MHz), A = loop area (m²), I = current (A), r = distance (m)
Reducing Radiated Coupling
- Shielding — Metallic enclosure attenuates fields (SE = 20 log10(Et/Ei) dB)
- Minimize loop area — Reduces both radiation and susceptibility
- Filter at I/O — Prevent RF on cables that act as antennas
- Cable management — Shielded cables with proper grounding, ferrite cores
- Board layout — Continuous ground planes, short trace lengths for high-speed signals