In modern industrial automation, signal integrity determines system reliability. Unwanted electrical noise degrades sensor data, distorts 4-20 mA current loops, and causes intermittent fieldbus communication drops. Engineers designing Programmable Logic Controller (PLC) systems, Distributed Control Systems (DCS), and Turbine Supervisory Instrumentation (TSI) must identify and mitigate these disturbances effectively.

This technical guide analyzes the primary internal and external noise sources affecting control systems and electronic hardware.

Understanding Signal Noise in Control Systems

Signal noise represents any unwanted electrical variation that distorts an information-bearing signal. In a ideal control circuit, analog voltage or current maintains a clean waveform. However, internal physical phenomena and external electromagnetic interference (EMI) shift signal amplitudes unpredictably.

In industrial facilities, unmitigated noise leads to false trips, erratic valve positioning, and corrupted analog-to-digital conversions.

Internal Noise Sources in Electronic Hardware

Internal noise originates from microscopic physical actions occurring directly inside semiconductor junctions, passive components, and conductors.

Thermal Noise (Johnson-Nyquist Noise)

Thermal agitation of charge carriers inside conductive media generates thermal noise. This phenomenon occurs in all resistive elements regardless of applied external voltage.

• Physical Mechanism: Thermal energy causes random movement of free electrons within conductors and semiconductor materials.
• Frequency Behavior: It acts as "white noise" with uniform power spectral density across the entire frequency spectrum.
• System Impact: Thermal noise sets the absolute minimum noise floor for high-precision analog input modules in PLCs and DCS systems.

Shot Noise

Shot noise arises due to the discrete nature of electric charge. Current flow is not a continuous fluid, but rather a stream of individual electrons crossing potential barriers.

• Physical Mechanism: Random fluctuations occur when charge carriers cross PN junctions in diodes and transistors.
• Frequency Behavior: Like thermal noise, shot noise exhibits a flat frequency response across operational bandwidths.
• System Impact: It predominantly affects sensitive optical receivers, photodiode amplifiers, and low-level sensor interfaces.

Flicker Noise ($1/f$ Noise)

Flicker noise is a low-frequency phenomenon associated with material impurities and direct current (DC) flow across semiconductor channels.

• Physical Mechanism: Trapping and releasing of charge carriers at semiconductor interfaces create slow current fluctuations.
• Frequency Behavior: Power spectral density decreases inversely with frequency ($1/f$). It dominates at frequencies below 500 Hz.
• System Impact: Flicker noise severely distorts slow-moving DC measurement signals, such as thermocouple and strain gauge readings.

Transit Time Noise

Transit time noise becomes significant when the frequency of an AC signal approaches the travel time of electrons across a semiconductor junction.

• Physical Mechanism: High-frequency operating signals force charge carriers to linger in the active region during state transitions.
• Frequency Behavior: Its magnitude increases rapidly at ultra-high and microwave frequencies.
• System Impact: While negligible in low-speed industrial control networks, it impacts high-speed RF communications and radar level transmitters.

External Noise Sources in Industrial Environments

External noise introduces unwanted energy into control circuits through magnetic fields, electric fields, or environmental events.

Crosstalk and Electromagnetic Coupling

Crosstalk occurs when signals from one conductor couple into adjacent wiring inside control panels or cable trays.

• Physical Mechanism: Stray capacitive coupling (electric field) and mutual inductance (magnetic field) transfer energy between parallel wires.
• System Impact: Parallel runs of 230 V AC power cables and 4-20 mA sensor lines often induce voltage spikes into sensitive instrumentation.

Industrial and Man-Made Noise

Factory automation environments feature heavy electrical machinery that continuously generates wideband electromagnetic noise.

• Physical Mechanism: Switching transients from Variable Frequency Drives (VFDs), arc welders, and high-voltage switchgear produce steep voltage spikes ($dv/dt$).
• System Impact: Unshielded communication buses (such as RS-485, PROFIBUS, or CAN-bus) experience high bit error rates and packet loss.

Environmental and Atmospheric Noise

Atmospheric disturbances generate severe, high-energy voltage transients that propagate across long signal lines.

• Physical Mechanism: Natural lightning strikes and electrostatic discharge (ESD) emit high-power electromagnetic radiation.
• System Impact: Outdoor field instrumentation, remote terminal units (RTUs), and substation automation modules require robust surge protection devices (SPDs).

Summary Matrix of Electronic Noise Types

Practical Application: Mitigating VFD Noise in a DCS Architecture

In a thermal power plant application, a Distributed Control System (DCS) monitored vibration signals from a feed-water pump using 4-20 mA transmitters. High-frequency electrical noise from a nearby 6 kV Variable Frequency Drive (VFD) coupled into the analog input channels, causing false high-vibration alarms.

Solution Implementation

1. Cable Separation: Signal cables were relocated to dedicated, grounded metallic conduits separated from VFD power cables by at least 30 cm.
2. Shielded Twisted-Pair (STP) Wiring: Field technicians replaced standard wiring with STP cables, grounding the drain wire at the control panel end only to eliminate ground loops.
3. Low-Pass Active Filtering: Engineers enabled digital low-pass filtering on the DCS analog input cards to attenuate high-frequency switching noise.

These corrective actions restored signal integrity and eliminated false process shutdowns.

Technical Insights and Industry Trends

As industrial automation transitions toward Industry 4.0, noise immunity becomes increasingly critical. Higher clock speeds in industrial Ethernet (PROFINET, EtherCAT) and thinner semiconductor nodes in compact PLCs lower margin thresholds for signal distortion.

Engineers should adopt a multi-layered shielding and isolation strategy early in system design. Implementing galvanically isolated I/O modules, maintaining single-point grounding architectures, and strictly adhering to IEC 61000-4 EMC standards ensure long-term operational resilience.

About the Author

Zhang Wei is a Senior Industrial Automation Architect with over 15 years of field experience in power generation, petrochemical automation, and plant safety systems. He specializes in PLC/DCS system design, signal conditioning, and fault diagnostics for large-scale industrial infrastructure.