Interview Questions for SystemLevel EMI Troubleshooting

Conducted and radiated emissions are two ways electromagnetic interference (EMI) can propagate. Think of it like this: conducted emissions are like electricity flowing through wires, while radiated emissions are like radio waves traveling through the air.

Conducted emissions are currents or voltages that travel along conductors, such as power cords or signal cables. They’re typically measured at the input/output points of a device, like where a power cord connects to the wall outlet or a USB cable connects to a computer. These emissions can couple into other circuits and cause malfunctions. A common example is a noisy power supply injecting interference onto the AC power line.

Radiated emissions are electromagnetic fields that propagate through space. They are generated by rapidly changing currents and voltages within a device, acting as miniature antennas. These emissions can couple into other circuits and devices wirelessly, even at a distance, causing interference. A cell phone’s radio transmission is a classic example of intended radiated emissions, while unwanted radiated emissions might come from poorly shielded switching power supplies causing interference with nearby radio receivers.

Many components within electronic systems can generate EMI. Sources are often categorized by the type of interference they produce. Common culprits include:

• Switching Power Supplies: These create sharp voltage transitions, generating high-frequency noise that can be both conducted and radiated.
• Clock Circuits: High-speed clocks generate significant EMI, especially when unshielded or improperly routed.
• Motors and Relays: These electromechanical components generate EMI due to their switching actions and sparking.
• High-speed Digital Circuits: Fast data rates create transient currents and voltages that radiate EMI. Think about the signals zipping around a modern microprocessor.
• Connectors and Cables: These can act as antennas, both radiating and receiving EMI. Poorly designed or improperly shielded connectors are common offenders.
• RF Transmitters and Receivers: These devices by their nature generate and receive strong electromagnetic fields and must be carefully designed to prevent interference.

Identifying the specific source often involves a combination of careful observation, systematic testing, and using specialized measurement equipment.

EMI/EMC compliance is measured using specialized equipment in a controlled environment, typically a shielded anechoic chamber (a room designed to absorb electromagnetic waves). The process involves:

• Conducted Emissions Testing: Measuring EMI conducted on the power lines (AC mains) using a Line Impedance Stabilization Network (LISN) and a spectrum analyzer. This determines how much noise is injected back into the power grid.
• Radiated Emissions Testing: Measuring EMI radiated by the device using a broadband antenna and a spectrum analyzer. This assesses how much electromagnetic energy the device emits into the surrounding environment.
• Conducted Immunity Testing: Injecting various levels of noise into the power lines and signal lines to assess the device’s ability to function correctly under those conditions.
• Radiated Immunity Testing: Exposing the device to various levels of electromagnetic fields to test its resilience to external interference.

The measurements are compared against relevant standards (discussed in the next answer) to ensure the device meets regulatory requirements.

Several international and national standards govern EMI/EMC compliance. The most prominent include:

• CISPR (International Special Committee on Radio Interference): Sets international standards for limits of radio interference from electronic equipment. CISPR 22 covers information technology equipment and CISPR 24 covers industrial equipment.
• FCC (Federal Communications Commission): Enforces regulations on radio interference in the United States. Their Part 15 covers unintentional radiators.
• CE Marking (Conformité Européenne): Indicates that a product meets EU health, safety, and environmental protection requirements, including EMC directives.

These standards specify acceptable levels of emissions and susceptibility for different types of equipment, operating frequencies, and geographical locations.

Shielding effectiveness refers to the ability of a material (usually a conductive enclosure) to attenuate electromagnetic fields. It’s measured in decibels (dB) and represents the reduction in field strength on one side of the shield compared to the other. A higher dB value indicates better shielding.

Several factors affect shielding effectiveness:

• Material Conductivity: Higher conductivity materials (like copper or aluminum) provide better shielding.
• Shield Thickness: Thicker shields offer better attenuation.
• Shielding Integrity: Seams, gaps, and openings in the shield can significantly reduce its effectiveness. Proper grounding and bonding are critical.
• Frequency: Shielding effectiveness generally decreases at higher frequencies.

Imagine a shield as a barrier, blocking electromagnetic waves from passing through. The more conductive and complete the barrier, the more effective the shielding.

EMI filters are circuits designed to attenuate unwanted frequencies. Different filter types are employed depending on the application:

• LC Filters (Inductor-Capacitor): Simple, passive filters using inductors and capacitors to block specific frequencies. They are commonly used for power line filtering to attenuate conducted emissions.
• Pi Filters: A specific type of LC filter configuration providing improved attenuation. The name comes from the circuit diagram’s resemblance to the Greek letter pi.
• Common Mode Chokes: These suppress common-mode noise – noise present on both wires of a balanced signal pair (e.g., differential signaling). Crucial in reducing noise on power lines.
• Ferrite Beads: Small, high-permeability magnetic beads used to suppress high-frequency noise. Often placed directly on cables or PCB traces.
• Active Filters: More complex filters using active components like operational amplifiers to provide more precise and adaptable frequency attenuation. These are useful when high attenuation is required over a broader frequency range.

The choice of filter depends on the frequency range, the level of attenuation required, and the impedance of the circuit.

Debugging EMI problems in complex systems is a systematic process. Here’s a structured approach:

1. Identify the problem: Pinpoint the symptoms – which circuits are affected, under what conditions, and what is the nature of the interference (frequency, amplitude).
2. Initial investigation: Use diagnostic tools like oscilloscopes, spectrum analyzers, and current probes to measure signals and identify potential sources.
3. Isolation and narrowing down suspects: Systematically isolate sections of the circuit to identify the EMI source. This might involve removing components, disconnecting cables, or using shielding to isolate potentially noisy parts.
4. Targeted mitigation: Once the source is identified, implement appropriate mitigation techniques such as shielding, filtering, grounding, and signal integrity improvements (e.g., proper cable routing, termination, twisted-pair wiring).
5. Verification and retesting: After implementing mitigation, thoroughly test the system again to verify the problem is solved and to rule out unexpected consequences.

Remember, a methodical approach, good documentation, and the use of appropriate measurement equipment are vital to successfully debug EMI issues.

Effective grounding is paramount in mitigating EMI. It provides a low-impedance path for conducted noise, preventing it from radiating or coupling into sensitive circuits. Common techniques include:

• Single-Point Grounding: All ground connections converge at a single point, minimizing ground loops. Think of it like a tree – all branches connect to a single trunk. This is excellent for minimizing noise currents circulating between components.
• Star Grounding: Similar to single-point, but often used with higher-frequency signals where minimizing inductance is crucial. All ground planes radiate outwards from a central point.
• Plane Grounding: Uses a large ground plane on a PCB or chassis to distribute ground potential evenly. This acts as a shield, reducing noise coupling between signals.
• Guard Grounding: Used to shield sensitive signals from noise. A separate ground surrounds the sensitive signal lines, preventing interference from external sources. This is often used in high-precision instrumentation.

Choosing the right grounding technique depends heavily on the system’s architecture, frequency range, and noise sensitivity. For instance, a high-speed digital system might benefit from a plane ground, while a simpler analog circuit might only require single-point grounding.

PCB layout is critical for EMI reduction. A poorly designed PCB can act as an antenna, radiating noise or becoming susceptible to interference. Key aspects include:

• Ground Plane Design: A continuous ground plane provides a low-impedance return path for high-frequency currents, reducing noise coupling and radiation. Avoid discontinuities and ensure a solid connection to the system ground.
• Component Placement: Sensitive analog components should be separated from noisy digital components. High-power components should be carefully placed to minimize their radiated fields.
• Signal Routing: Keep high-speed signal traces as short as possible and use controlled impedance routing. Use differential signaling where appropriate to reduce noise susceptibility. Avoid long parallel runs of signal and ground traces, as these can act as transmission lines, picking up and radiating noise.
• Shielding: Use metal shields to isolate sensitive circuitry from noise sources. Ensure proper grounding of the shield to prevent it from becoming a radiator.
• Power Plane Decoupling: Proper placement of bypass capacitors near power pins helps to suppress high-frequency noise on the power supply lines.

Imagine a city: well-planned roads (traces) keep traffic (signals) flowing smoothly, while buildings (components) are placed strategically to prevent congestion and accidents (noise).

Reducing radiated emissions involves controlling the electromagnetic fields generated by the device. Techniques include:

• Shielding: Enclosing the device or susceptible components in a conductive enclosure significantly reduces radiation. The enclosure must be properly grounded to be effective.
• Filtering: Employing EMI filters on input/output lines prevents noise from radiating. These filters provide a low impedance path to ground for high-frequency noise.
• Cable Management: Properly shielded and routed cables minimize radiation and susceptibility. Twisting wires helps to cancel out electromagnetic fields.
• Optimized PCB Layout (as discussed above): Careful PCB design minimizes the device’s ability to radiate electromagnetic energy.
• Low-Emission Components: Selecting components with inherent low emission characteristics can reduce overall radiation.

Think of it like soundproofing a room – shielding acts as the walls, filtering as sound-absorbing materials, and cable management as minimizing noise sources.

Simulation tools, like ANSYS HFSS, CST Microwave Studio, or Keysight ADS, are invaluable for EMI/EMC analysis. These tools use computational electromagnetics (CEM) to model the device’s electromagnetic behavior.

The process typically involves:

• 3D Model Creation: Building a detailed 3D model of the device and its environment.
• Material Definition: Specifying the electrical properties of materials used in the design.
• Simulation Setup: Defining the excitation source (e.g., a current source or incident wave) and simulation parameters.
• Simulation Run: Running the simulation, which can be computationally intensive.
• Post-Processing: Analyzing the results, including radiated emissions, conducted emissions, and near-field characteristics.

These simulations help identify potential EMI problems early in the design phase, reducing the need for costly rework later. For example, a simulation might reveal that a particular PCB trace geometry is radiating excessive energy, allowing designers to optimize the layout before prototyping.

Troubleshooting EMI/EMC issues requires a combination of tools. Key tools include:

• Spectrum Analyzer: Measures the frequency and amplitude of radiated and conducted emissions, helping to identify the sources of interference.
• EMI Receiver: Similar to a spectrum analyzer, but specialized for EMI measurements, often offering more sensitive detection and features.
• Near-Field Probe: Measures electromagnetic fields close to the device, helping to pinpoint the location of radiation sources.
• Current Probe: Measures currents flowing in various parts of the circuit, allowing identification of noise sources.
• Oscilloscope: Displays voltage waveforms, useful for identifying timing issues and other transient phenomena that may contribute to EMI.
• Network Analyzer: Measures impedance and S-parameters, helping analyze signal integrity and identify impedance mismatches which can be sources of reflection and noise.

Using these tools in conjunction allows for a systematic approach to identifying and resolving EMI problems. For example, a spectrum analyzer might reveal a strong emission at a particular frequency, while a near-field probe could help pinpoint its origin on the PCB.

Impedance matching refers to the condition where the impedance of a source (e.g., a signal generator) is equal to the impedance of the load (e.g., an antenna or receiver). Proper impedance matching ensures maximum power transfer and minimizes reflections. In the context of EMI reduction, impedance mismatches can cause reflections of signals, leading to increased radiated emissions and susceptibility to interference.

When there’s a mismatch, some of the signal energy is reflected back to the source instead of being fully absorbed by the load. This reflected energy can be radiated as EMI. To improve impedance matching, techniques like using matching networks (e.g., L-networks, Pi-networks) are used to transform the source impedance to match the load impedance over the frequency range of interest. This minimizes reflections and optimizes signal transfer, thus reducing EMI.

Consider a sound system: if the amplifier (source) doesn’t match the speaker impedance (load), you get poor sound quality and potentially damage to the components. Similarly, in electronics, mismatches generate unwanted reflections that contribute to EMI.

Common-mode and differential-mode noise represent different ways noise can appear on signal lines.

• Differential-mode noise: This is the voltage difference between two signal lines. It’s the normal signal, but with added noise riding on top of it. Think of it like static on a radio – the music (signal) is still there but overlaid with noise.
• Common-mode noise: This is the voltage between the signal lines and ground. Both lines have the same noise voltage relative to ground. It’s like a hum affecting both channels of a stereo equally.

EMI filters usually address both types. For example, common-mode chokes are designed to suppress common-mode noise by providing a high impedance path to common-mode currents, while differential-mode chokes suppress differential-mode currents. Understanding the mode of noise is critical in selecting the appropriate filtering and mitigation strategy.

Near-field probes are invaluable tools for pinpointing EMI sources. Unlike far-field measurements which capture radiated emissions, near-field probes measure the electromagnetic fields very close to the source, allowing for highly localized identification. Think of it like trying to find a noisy cricket in a field – a far-field measurement might just tell you there’s noise somewhere, while a near-field probe allows you to pinpoint the exact cricket.

These probes, typically consisting of small loops or electric dipoles, are connected to a spectrum analyzer. By systematically scanning the suspected area, you can identify components or traces emitting significant electromagnetic energy. Strong signals indicate a potential culprit. For example, a high-frequency switching power supply might exhibit strong near-field emissions near its switching transistors or output filter. The peak frequency of these emissions will point toward the source’s operating frequency and help identify the mechanism.

The process involves careful scanning, referencing the analyzer’s display to identify strong emissions correlated to specific frequencies and locations. This targeted approach saves considerable time and resources compared to broad-spectrum, far-field measurements.

The electromagnetic spectrum is fundamental to understanding EMI/EMC because it defines the range of frequencies over which electromagnetic energy can propagate. EMI and EMC are all about managing electromagnetic energy; understanding the spectrum allows us to predict and mitigate interference. Different parts of the spectrum have different propagation characteristics and affect different types of electronic circuits. For instance, low-frequency noise (e.g., 50/60 Hz power line noise) may couple easily through power lines, while high-frequency noise (e.g., GHz signals from digital circuits) is more likely to radiate through space.

The spectrum covers a vast range from extremely low frequencies (ELF) to gamma rays. Within this range, various standards and regulations exist to limit emissions and susceptibility in different frequency bands to ensure compatibility between devices. Understanding the spectrum enables us to select appropriate testing methods, design effective shielding and filtering techniques, and predict potential interference issues.

Susceptibility testing determines a device’s vulnerability to external electromagnetic fields. It’s the counterpart to emission testing, which measures how much EMI a device generates. Susceptibility testing assesses how well a device maintains its intended functionality when exposed to various levels and types of electromagnetic interference. Think of it as a stress test for your device’s immunity to external electromagnetic forces.

The process involves exposing the device under test (DUT) to controlled electromagnetic fields, across a range of frequencies and intensities, while monitoring its performance. Different standards define the test levels and methods depending on the device’s intended application and environment. For example, a medical device will have significantly stricter susceptibility requirements compared to a consumer electronic device. Failure to meet these requirements can have serious consequences, leading to malfunction or even safety hazards.

Typical susceptibility tests include conducted susceptibility (injecting noise into power lines or signal lines) and radiated susceptibility (exposing the DUT to an electromagnetic field in an anechoic chamber). The results indicate the device’s immunity limits and identify frequencies or field strengths that may cause malfunctions.

Interpreting EMI/EMC test reports requires a strong understanding of the standards and testing methods used. The report usually contains graphs showing emission levels across different frequencies, which are compared against limits specified in relevant standards (e.g., CISPR, FCC). It also includes the test setup and procedures. One must scrutinize the margins between measured emissions or susceptibility and the limits.

A key aspect is identifying compliance. If the measured emission levels exceed the regulatory limits, it indicates a non-compliance situation needing immediate attention. This requires further investigation to pinpoint the source and implement corrective actions. The report also often includes details about the measurement uncertainties, which are crucial for accurate interpretation. For example, a marginally non-compliant result might be within the measurement uncertainty and might not necessitate design changes.

Furthermore, the report should include specifics on the conducted and radiated emissions and susceptibility tests, providing valuable information for troubleshooting issues. Understanding these nuances and the specific conditions of the test is key to correctly interpreting the results and implementing the necessary corrections.

EMI filters are crucial components for suppressing unwanted electromagnetic interference. They act as frequency-selective barriers, allowing desired signals to pass while attenuating undesired frequencies. Common filter topologies include LC, pi, and T filters. These filters utilize combinations of inductors (L) and capacitors (C) to achieve this selectivity.

• LC Filter: This simplest form comprises a single inductor and a single capacitor in series or parallel. It’s effective for specific frequencies but limited in its bandwidth.
• Pi Filter: A pi filter uses two capacitors and one inductor, arranged in a ‘pi’ configuration. This offers better attenuation compared to the LC filter and a wider bandwidth.
• T Filter: Similar to the pi filter, but with two inductors and one capacitor in a ‘T’ configuration. It provides improved attenuation at lower frequencies compared to the pi filter.

The choice of filter topology and component values depends on the specific frequency range of the interference to be suppressed and the impedance matching requirements of the circuit. For instance, a high-frequency switching power supply might require a more complex filter design combining several LC, pi, or T sections to effectively attenuate high-frequency switching noise.

EMI/EMC testing at high frequencies presents unique challenges. As frequencies increase, the wavelengths become shorter, making the design and construction of test setups more critical. For example, the physical dimensions of components and connectors must be considered to avoid unintended resonances and reflections that can affect measurements. In high-frequency testing, even small changes in cable length or the orientation of components can significantly impact results.

Another challenge is the increased sensitivity to parasitic capacitances and inductances. These parasitic effects, which are less significant at lower frequencies, can strongly influence the behavior of circuits at higher frequencies, complicating both emission and susceptibility testing. Moreover, achieving accurate measurements at high frequencies requires specialized equipment such as high-frequency probes, cables, and anechoic chambers. These specialized components can be more expensive and require specific expertise to use correctly.

Furthermore, the higher the frequency, the more difficult it becomes to effectively shield against electromagnetic radiation. This leads to more complex and often costlier shielding solutions. Therefore, understanding and managing these high-frequency effects are crucial for accurate testing and successful design.

Power supplies are notorious sources of EMI due to their switching nature. Addressing EMI issues in power supplies involves a multi-pronged approach.

First, careful design practices are essential. This includes selecting appropriate switching frequencies, employing proper layout techniques (keeping high-current loops small and minimizing parasitic inductance), and using shielded components where necessary. Proper grounding and shielding of the power supply are crucial steps in mitigating EMI. Incorrect grounding practices can create significant ground loops and radiate noise.

Secondly, effective filtering is vital. This typically involves incorporating a combination of LC, pi, or T filters at various points within the power supply’s design. These filters help attenuate high-frequency switching noise and harmonics. The careful selection of filter components (inductors and capacitors) is crucial for achieving the desired attenuation without impacting efficiency.

Finally, proper testing and measurement are crucial for verifying that the implemented solutions are effective. Using near-field probes allows the quick identification and localization of potential emission sources within the power supply itself. Testing ensures compliance with relevant standards and minimizes EMI issues.

Cable management is crucial for reducing EMI because unshielded or poorly managed cables act as antennas, radiating and receiving electromagnetic interference. Think of it like this: a loose, dangling wire is much more likely to pick up radio signals than a neatly organized and shielded one. Effective cable management involves several key strategies:

• Bundling and routing: Keeping cables organized and away from sensitive circuits minimizes coupling. Twisting pairs of wires further reduces radiated emissions.
• Shielding: Using shielded cables significantly reduces the electromagnetic field emanating from or entering the cable. The shield acts as a Faraday cage, preventing interference.
• Proper termination: Untended cable ends act as radiators. Proper termination ensures signals are properly absorbed, preventing reflections and emissions.
• Cable routing separation: High-speed signal cables should be separated from low-level signal cables and power cables to minimize cross-talk and interference. Consider using different cable trays or routing paths.

For instance, in a high-speed data acquisition system, poorly managed cables can lead to significant noise in the acquired signal, corrupting data. Implementing proper cable management, including shielding and twisting, is critical for obtaining reliable and accurate data.

Various shielding materials offer different levels of effectiveness against EMI, depending on the frequency and intensity of the interference. The choice of material depends on factors like weight, cost, and required attenuation. Here are some common choices:

• Copper: Excellent conductivity, widely used for its effectiveness across a broad frequency range. It’s relatively inexpensive but can be heavy.
• Aluminum: Lighter than copper, but slightly less conductive. Often used in applications where weight is a concern.
• Nickel-iron alloys (Mu-metal): High magnetic permeability, making them excellent for shielding against magnetic fields. However, they are more expensive than copper or aluminum.
• Conductive polymers: Offer flexibility and ease of application, suitable for conformal coating. Their shielding effectiveness is generally lower than metals.

The effectiveness of a shield is also influenced by its construction. A seamless shield offers better protection than a shield with gaps or seams. Proper grounding of the shield is critical to ensure that the induced currents flow to ground rather than circulating within the shield.

For example, in a medical device with sensitive electronics, a carefully designed enclosure using mu-metal and copper shielding could effectively protect the internal circuitry from external electromagnetic interference, ensuring reliable and safe operation.

Proper grounding and bonding are fundamental to EMI reduction. They create a low-impedance path for conducted interference, preventing it from circulating through the system and causing problems. Think of grounding as providing a safe ‘escape route’ for unwanted electrical energy.

• Grounding: Connects the chassis and other metallic parts of the equipment to earth ground, providing a reference point for all voltages. This minimizes potential differences that could generate electromagnetic fields.
• Bonding: Connecting different metallic parts within a system to ensure they are at the same electrical potential. This prevents circulating currents that could generate EMI.

Without proper grounding and bonding, interference currents can flow through various paths, causing voltage fluctuations, noise in sensitive circuits, and even malfunctions. Imagine a poorly grounded audio system; you might hear a persistent hum caused by interference currents flowing through the chassis.

In a power supply, a properly bonded chassis will prevent interference currents from inducing noise in sensitive control circuitry, improving reliability and performance. The quality of ground and bonding connections is therefore crucial. Using high-conductivity connectors and ensuring low-resistance paths are critical aspects of good EMI design.

Designing for EMI/EMC compliance from the outset is far more efficient and cost-effective than trying to fix problems later. This involves a proactive approach that incorporates several key strategies:

• Component selection: Choose components with inherent low EMI emissions. Look for components with certifications like CE marking and FCC ID.
• PCB layout: Careful layout of components on the printed circuit board can minimize EMI generation and coupling. This includes using ground planes, controlled impedance traces, and proper component placement.
• Shielding: Incorporate shielding in the design to isolate sensitive circuits from sources of EMI. Consider using metal enclosures, conductive coatings, or shielded cables.
• Filtering: Use filters to attenuate EMI signals entering or leaving the equipment. This can include input/output filters, power line filters, and common-mode chokes.
• Simulation and modeling: Use electromagnetic simulation software to predict EMI performance and identify potential problem areas before building prototypes.

By employing these techniques during the design phase, potential EMI issues are identified and addressed early, saving time, money, and potential regulatory setbacks. For example, in the design of a wireless communication device, careful PCB layout and component selection can minimize radiated emissions, ensuring compliance with regulatory limits.

Electromagnetic field modeling involves using computer software to simulate the behavior of electromagnetic fields in a given system. This allows engineers to predict and analyze EMI issues without building physical prototypes, significantly speeding up the design process and reducing development costs.

These models use numerical techniques like the Finite Element Method (FEM) or the Method of Moments (MoM) to solve Maxwell’s equations, which govern electromagnetic phenomena. Engineers input the geometry of the system, material properties, and excitation sources (such as antennas or circuits). The software then calculates the resulting electric and magnetic fields, allowing for the analysis of radiated emissions, susceptibility to interference, and coupling between different components.

The results of electromagnetic field modeling provide valuable insights into potential EMI issues, guiding design modifications to improve EMC compliance. For example, modeling can help optimize the placement of shielding to minimize interference or identify potential resonant frequencies that could lead to amplified EMI. Software packages like CST Studio Suite, ANSYS HFSS, and COMSOL Multiphysics are commonly used for this purpose.

Throughout my career, I’ve extensively used various EMI/EMC test equipment, including:

• EMI receivers: Used to measure radiated emissions and susceptibility. These instruments are capable of measuring across a wide frequency range and providing accurate measurements of the electric and magnetic field strength.
• Spectrum analyzers: Provide a visual representation of the frequency spectrum of signals, allowing for the identification of EMI sources and their frequencies.
• LISNs (Line Impedance Stabilization Networks): Used to ensure consistent impedance between the equipment under test and the power line during conducted emissions measurements.
• EMC chambers: Shielded rooms used for conducting radiated emissions and immunity tests, minimizing external interference and ensuring accurate measurements.
• Near-field probes: Used for detailed EMI investigation close to the source of the interference. These probes provide localized information that can help pinpoint the exact location of the problem.

My experience with this equipment extends to both manual and automated measurements, including data analysis and report generation. I’m proficient in utilizing these tools for compliance testing according to various international standards, including CISPR and FCC regulations.

One challenging EMI problem I encountered involved a high-speed data acquisition system experiencing intermittent data corruption. Initial investigations revealed significant noise spikes in the acquired signal, particularly at specific frequencies. These spikes were intermittent, making diagnosis difficult.

My troubleshooting approach involved a systematic process:

• Systematic investigation: We started by carefully examining the entire signal path, from the sensors to the data acquisition board and finally the computer. We used near-field probes to pinpoint the source of noise.
• Spectrum analysis: A spectrum analyzer helped identify the frequencies of the interference, revealing they were related to specific switching frequencies within the power supply of the system.
• Power supply modification: After isolating the problem to the power supply, we added additional filtering stages (common-mode chokes and LC filters) to attenuate the noise at the offending frequencies. This significantly reduced the interference.
• Cable shielding: Additionally, we replaced the unshielded data acquisition cables with shielded twisted-pair cables, reducing further the amount of electromagnetic interference picked up by the data cables.
• Grounding verification: Finally, we reviewed the grounding scheme, confirming good bonding between all chassis grounds to further suppress EMI pathways.

Through this combination of techniques, we successfully mitigated the EMI problem, resulting in stable and reliable data acquisition. This case highlighted the importance of a thorough and methodical approach to EMI troubleshooting, involving a combination of analysis, diagnosis, and remediation.