Trace the disturbance from its source to the receiver

Electromagnetic interference, or EMI, occurs when unwanted electrical or electromagnetic energy disrupts a circuit's operation. It can be random, repetitive or transient. Industrial symptoms include false input transitions, unstable measurements and communication errors; the mechanism is much broader than random fluctuations in radio-wave intensity.

Noise and interference are not identical categories. A circuit also has internal noise, such as thermal noise in resistors. Here the concern is unwanted coupling from a source into another circuit. Emissions describe the disturbance leaving the source, while immunity describes the receiver's ability to continue operating in its presence.

Follow a three-part chain

Source: Variable frequency drives, PWM switching, switch-mode supplies, relays and contactors can generate disturbances. A drive's carrier frequency does not define its entire interference spectrum: fast switching edges contain higher-frequency components.

Coupling path: Energy can travel through supply wiring, shared return conductors, electric or magnetic fields between nearby circuits, and structures acting as antennas. Receiver: A PLC input, sensor, encoder or communication interface may be affected. Finding the source is only part of the diagnosis; identify how the disturbance reaches the receiver.

Different paths need different remedies

Conducted interference follows electrical connections. Capacitive coupling involves changing electric fields; inductive coupling involves changing magnetic fields. With common-impedance coupling, a voltage developed across a shared return path enters the sensitive circuit. Radiated interference couples through electromagnetic fields. Near-field coupling inside a cabinet does not require a distant radio transmitter.

An input can count an event that never happened

A disturbance can appear on a digital input that should remain low. If its amplitude and duration satisfy the receiver's detection conditions, it may be interpreted as a transition. In this example, reducing coupling keeps the disturbance below an illustrative threshold. Real thresholds, hysteresis and pulse-width requirements depend on the device.

Press Start to reveal the disturbances; Stop restores the static comparison. The source is unchanged; the coupled effect at the receiver differs. These teaching sketches have no calibrated voltage scale and are not a filter simulation or a two-second physical response.

Reduce the coupling before adding hardware

Cable routing: Separate motor and power wiring from sensitive signal circuits. Reduce unnecessary parallel runs and current-loop area. Twisted pairs and suitable differential receivers can reduce the effect of coupled noise, but are not universal cures.

Shielding and bonding: Performance depends on frequency, field type and termination. Copper and aluminium do not suppress every magnetic field equally. Consider protective earthing, chassis bonding and high-frequency return paths together. Follow the equipment's installation guidance rather than applying a blanket rule that every shield must be connected at one end.

Filtering: Select an electromagnetic filter for the relevant connection and frequency range. Mains input filters, motor output filters and common-mode reactors serve different purposes. A harmonic filter is not a general remedy for all EMI. An unsuitable passband can also distort the wanted signal.

Isolation and source suppression: Appropriate galvanic isolation can interrupt some conductive paths, while parasitic capacitance across the barrier can still carry high-frequency disturbances. Suitable suppressors and supply design may address the source. Active cancellation requires sensing and a purpose-designed system; generating an opposite-phase wave is not a general cabinet installation fix.

Make sure the measurement is not the problem

An oscilloscope reveals timing and waveform shape; a spectrum analyzer reveals frequency content. Near-field E/H probes help locate local sources, while suitable current probes investigate high-frequency cable currents. A normal multimeter reading does not rule out fast transients.

A long probe ground lead creates its own measurement loop. It can pick up interference and introduce apparent ringing. Use appropriate probes, bandwidth and connections, then compare results under matching conditions. For voltage measurements not referenced to earth, use an appropriately rated differential or isolated measurement system; never defeat the oscilloscope's protective earth.

Related reading

References

Analog Devices: EMI radiation and signal propagation; Analog Devices: Understanding Common-Mode Signals; Analog Devices: AN-139, Power Supply Layout and EMI; Texas Instruments: Voltage probing; Rohde & Schwarz: EMI debugging at board level; Tektronix: Floating measurements and operator protection.