What are common sources of signal noise?

• A. VFDs, high-current AC cables, motors, transformers, RF interference, long cable runs
• B. Shielded cables
• C. Low impedance connections
• D. Short cable runs

Answer: (A)

Signal noise in industrial signaling often comes from equipment that rapidly switches or carries large currents, plus the effect of long cable runs acting as noise antennas. Variable Frequency Drives switch power at high frequencies to control motors, creating EMI and harmonics that can couple into nearby signal lines. High-current AC cables carry strong currents, producing magnetic fields and voltage fluctuations that induce noise in adjacent conductors. Motors and transformers can radiate EMI and inject disturbances into the loop through magnetic coupling. RF interference from wireless devices or nearby transmitters can also couple into any exposed conductors. Long cable runs increase the loop area and the amount of ambient EMI picked up, making the system more susceptible to noise. Shielded cables, low impedance connections, and shorter runs are protective design choices that reduce noise pickup and susceptibility, not sources of noise themselves. In the context of 4–20 mA and HART, recognizing these sources helps you understand why noise can distort measurements and how shielding and proper cabling mitigate it.