Interference—Unwanted Signal or Light Affecting Measurements

Introduction

Interference in measurement systems refers to any unwanted electromagnetic, optical, or electrical energy that distorts or corrupts the intended measurement signal. It is a central concern in aviation, telecommunications, industrial automation, and critical infrastructure, where reliability and accuracy are essential. As the density of electronic equipment and the use of the electromagnetic spectrum increase, so does the risk of interference, making its understanding and mitigation a necessity for compliance, safety, and efficient operations.

Measurement systems are engineered to capture information with high accuracy. However, interference—whether from ambient electromagnetic fields, stray optical reflections, or other sources—can undermine data integrity, cause operational hazards, and result in regulatory non-compliance. International standards, such as those set by the International Civil Aviation Organization (ICAO), strictly prescribe allowable interference levels. Even subtle forms of interference can have major consequences, such as undetected sensor drift or false alarms in safety-critical environments.

What Is Interference in Measurements?

Interference in measurement systems is any unwanted signal—electromagnetic, optical, or electrical—that enters a measurement pathway and distorts, masks, or corrupts the desired signal. It may be continuous or transient, intentional or unintentional, and can arise from environmental sources, equipment, or operational factors.

Key Characteristics

• Intentional Interference: Such as electronic jamming or spectrum sharing, often regulated and well-characterized.
• Unintentional Interference: Arises from environmental factors or byproducts of nearby equipment, often unpredictable and difficult to manage.
• Multidomain Impact: Affects analog, digital, and hybrid systems, including circuits, sensors, and optical components.
• Measurement Impact: Causes errors, noise, or false readings, affecting safety and reliability.

Sources

• Environmental: Lightning, solar flares, cosmic radiation.
• Equipment-related: Poorly shielded cables, malfunctioning transmitters, digital devices.
• Operational: Proximity to high-power transmitters, dense building materials, weather conditions.

Core Concepts and Terminology

Interference covers a wide range of phenomena, each with specific sources, effects, and mitigation strategies.

Types of Interference

Understanding these types is essential for troubleshooting, system design, and regulatory compliance in high-reliability environments.

Interference Sources

Sources of interference can be diverse and complex:

Intentional vs. Unintentional

• Intentional: Generated for specific purposes (e.g., radar, transmitters, jamming).
• Unintentional: Byproducts of normal operations (e.g., digital devices, power supplies).

Natural vs. Man-Made

• Natural: Lightning, solar activity, precipitation static.
• Man-Made: Power lines, industrial machinery, radio transmitters, urban electronics.

Equipment and Environmental Factors

• Electronic Devices: Smartphones, Wi-Fi routers, Bluetooth devices.
• Industrial Sources: Motors, welders, variable speed drives.
• Cabling Issues: Poor shielding, corrosion, improper routing.

Regulators like ICAO demand rigorous monitoring and management, especially around critical infrastructure.

Impact of Interference on Measurement Systems

Interference can cause both subtle and severe problems:

• Signal Degradation: Lower signal-to-noise ratio (SNR), increased error rates.
• Equipment Malfunction: Resets, faults, or shutdowns due to strong interference.
• System Failures: Communication loss, navigation outages, safety-critical alarms.
• Quality Issues: Increased defects, unreliable data, costly rework.

Notable Use Cases:

• Wireless Communications: Reduced throughput and coverage.
• Industrial Automation: Disrupted process control, safety hazards.
• Scientific/Medical Devices: Masked phenomena or false alarms.

Strict standards (e.g., ICAO) enforce interference limits for safety and operational integrity.

Measurement and Detection of Interference

Tools

• Spectrum Analyzers: Visualize and quantify RF/EMI/RFI signals.
• Oscilloscopes: Detect impulse noise and transients.
• Field Strength Meters: Measure RF intensity for site surveys.
• Directional Antennas: Pinpoint interference sources.
• Optical Power Meters: Detect unwanted light in optical systems.

Spectrum Analyzer Settings

• Reference Level: Sets measurement range—avoid overload for accuracy.
• Resolution Bandwidth (RBW): Controls frequency selectivity and sensitivity.
• Max Hold: Captures transient/intermittent interference.
• Zero Span: Provides time-domain analysis at specific frequencies.

Strategies

• Baseline Measurements: Establish normal conditions for comparison.
• Span/RBW Tuning: Optimize for capturing all relevant signals.
• Antenna Selection: Use omnidirectional for scanning, directional for localization.
• Mask Monitoring: Automates anomaly detection.

Example Procedure

1. Connect analyzer to system.
2. Scan target and adjacent bands.
3. Adjust RBW and reference levels.
4. Use Max Hold to catch transients.
5. Deploy directional antenna to locate sources.

Standardized procedures are critical for effective detection and compliance.

Detailed Types of Interference

Electromagnetic Interference (EMI)

Any electromagnetic field or signal that disrupts electronic/electrical systems. Can be radiated or conducted.

Example: A switching power supply emits harmonics that couple into a nearby radio, causing audible noise.

Radio Frequency Interference (RFI)

EMI within the radio frequency spectrum (3 kHz–300 GHz). Affects wireless communications, radar, and navigation.

Example: Wi-Fi signals in the 2.4 GHz band interfere with Bluetooth devices, causing data loss.

Crosstalk

Unwanted signal transfer between adjacent channels or wires, common in dense wiring or high-frequency systems.

Example: VHF radio signals leaking into cockpit intercom circuits, distracting pilots.

Impulse Noise

Short, high-energy disturbances from sources like lightning or switching.

Example: Arc welding in a hangar disrupts nearby radio receivers.

Intermodulation

Mixing of signals in nonlinear components, generating new, unwanted frequencies.

Example: Two transmitters produce spurious signals that interfere with navigation.

Co-channel and Adjacent Channel Interference

• Co-channel: Signals from different transmitters overlap on the same frequency.
• Adjacent Channel: Leakage from nearby frequency bands.

Example: Overlapping air traffic control transmissions or VOR/ILS frequency overlap.

In-band and Out-of-band Interference

• In-band: Unwanted signals within the operating frequency.
• Out-of-band: Signals from outside the operating band entering due to poor filtering.

Example: Overlapping Wi-Fi access points (in-band) or local TV broadcasts leaking into comms (out-of-band).

Thermal Noise

Fundamental electronic noise from thermal agitation in conductors, present in all receivers.

Optical Interference

Unwanted light entering optical sensors, degrading measurement accuracy.

Example: Sunlight causing false readings in cockpit photodetectors.

Mitigation and Best Practices

• Shielding: Protect cables and devices from external fields.
• Filtering: Use high-quality filters to reject unwanted frequencies.
• Separation: Physically separate sensitive equipment from interference sources.
• Cable Management: Use twisted pairs, proper routing, and grounding.
• Regulatory Compliance: Adhere to ICAO, ITU, and other standards.
• Monitoring: Regular site surveys and real-time spectrum monitoring.

Conclusion

Interference is a pervasive challenge to measurement accuracy and system reliability. Understanding its sources, mechanisms, and impacts enables effective mitigation, ensuring compliance, safety, and operational excellence in critical sectors like aviation, telecommunications, and automation.

For more information on protecting your systems from interference, contact our experts or schedule a personalized demo.

References:

• ICAO Annex 10 – Aeronautical Telecommunications
• ICAO Doc 8071 – Manual on Testing of Radio Navigation Aids
• ICAO Circular 322 – Space Weather Effects
• ITU-R Recommendations on Electromagnetic Compatibility
• Relevant manufacturers’ technical manuals and best practice guides

Last updated: 2024