Electronics Guide

Magnetic Field Probes

Magnetic field probes, typically constructed as small loops of wire, detect the magnetic field component of electromagnetic radiation. According to Faraday's law, a time-varying magnetic flux through the loop induces a voltage proportional to the rate of change of flux. This makes loop probes responsive to AC magnetic fields, with sensitivity increasing with frequency. The induced voltage depends on the loop area, the number of turns, and the magnetic field strength perpendicular to the loop plane.

Small loop probes provide high spatial resolution for localizing current-carrying conductors. The magnetic field around a current-carrying wire is concentric, with intensity inversely proportional to distance from the wire. A small loop positioned near the wire detects this field, with the response maximized when the loop axis is perpendicular to the wire (and thus parallel to the field lines). By moving the probe and noting where the response peaks, the current-carrying conductor can be precisely located.

Larger loop probes provide greater sensitivity at the expense of spatial resolution. The larger loop intercepts more magnetic flux, producing a larger output signal. However, the response averages over the loop area, making it harder to distinguish closely spaced sources. A practical approach uses larger probes for initial surveys to identify high-field regions, then switches to smaller probes for precise source localization within those regions.

Loop probe construction affects performance characteristics. Shielded loops, with the loop conductor inside a conductive tube with a gap, reject electric field pickup that would otherwise contaminate the magnetic field measurement. This shielding is particularly important when probing near high-impedance circuit nodes where electric fields are strong. Unshielded loops may be adequate in low-impedance environments but can give misleading results when electric fields are significant.

Electric Field Probes

Electric field probes, typically short monopole or dipole structures, detect the electric field component of electromagnetic radiation. The probe acts as a receiving antenna that develops voltage proportional to the electric field along its axis. Short probes operating in the electrically small regime respond uniformly across a wide frequency range, while longer probes may exhibit resonant behavior at specific frequencies.

Electric field probes excel at detecting radiation from high-impedance nodes where voltage swings rapidly. In a circuit, nodes driven by high-impedance sources or connected to capacitive loads may have large voltage excursions that create strong local electric fields. These fields can couple to nearby conductors or radiate from apertures and edges. Electric field probing reveals these voltage sources that magnetic probes might miss.

The response of electric field probes depends strongly on their proximity to ground planes and other conductors. The probe forms a capacitive divider with nearby grounded structures, and this division ratio changes with position. Measurements close to ground planes may show reduced response even when the actual electric field is high. Understanding these proximity effects prevents misinterpretation of electric field probe results.

Monopole and dipole probe geometries provide different directional characteristics. A monopole probe responds to the field component along its axis, enabling directional sensing by rotating the probe. A dipole probe responds to the field component along the dipole axis with symmetric response in both directions. Dipole probes may provide more predictable calibration characteristics, while monopoles are often more convenient for hand-held scanning.

Combined and Specialized Probes

Some probe systems combine magnetic and electric field sensing in a single assembly, enabling simultaneous measurement of both field components. These combined probes simplify measurements when both components are of interest and ensure that magnetic and electric measurements are made at exactly the same location. However, the combined structure may compromise the optimization of each sensing element.

Specialized probes address specific measurement needs. Very small probes with sub-millimeter resolution enable probing of integrated circuits and high-density interconnects. High-frequency probes optimized for microwave frequencies maintain sensitivity well into the gigahertz range. Calibrated probes with known frequency response enable quantitative field measurements rather than just qualitative source location. The choice of probe depends on the specific diagnostic requirements.

Commercial probe sets typically include several sizes of magnetic loops and one or more electric field probes, covering the range of needs encountered in typical EMC troubleshooting. These sets often include preamplifiers to boost the probe output to levels adequate for spectrum analyzer measurement. The system frequency range, sensitivity, and spatial resolution should be matched to the expected measurement requirements.

Equipment Configuration

The spectrum analyzer provides the frequency-selective measurement capability needed to characterize emissions at specific frequencies. Configuration for near-field probing differs somewhat from far-field emissions measurements. Wider resolution bandwidths may be acceptable since the goal is source location rather than precise amplitude measurement. Faster sweep speeds enable more responsive real-time scanning. Peak hold mode captures maximum field levels during probe movement.

Preamplifiers may be needed to boost the probe output above the spectrum analyzer noise floor, particularly when using small probes with low sensitivity. The preamplifier should have flat frequency response across the measurement band, adequate dynamic range to handle both weak and strong signals, and low noise figure to maintain system sensitivity. Preamplifier gain should be accounted for when interpreting signal levels.

Cable quality affects measurement fidelity, particularly at higher frequencies. The cable from the probe to the preamplifier or analyzer should be high-quality coaxial type with good shielding. Cable length adds loss that reduces sensitivity; keeping cables as short as practical while maintaining measurement flexibility balances these factors. Damaged or worn cables with degraded shielding can pick up interference that appears as probe output.

Reference Plane and Positioning

Establishing a consistent reference for probe positioning enables meaningful comparison between measurements. The probe height above the circuit board or component surface affects the measured field intensity. Maintaining constant height during scanning ensures that variations in reading represent actual field variations rather than height changes. Probe stands, spacers, or reference surfaces help maintain consistent positioning.

The orientation reference must be consistent, particularly for magnetic probes whose response depends on loop orientation. Developing a standard orientation convention, such as always orienting the loop axis vertical for initial scans, enables consistent interpretation. When rotating the probe to find maximum response, documenting the orientation provides information about current direction that aids source identification.

The equipment under test should be in a consistent operating state during measurements. If emissions vary with operating mode, measurements during different modes should be clearly distinguished. The power supply, load conditions, and any input signals should be documented to enable reproduction of the measurement conditions. Changes in operating state between measurements can cause apparent field variations unrelated to probe position.

Environmental Considerations

Ambient electromagnetic fields can affect near-field measurements, particularly when the fields of interest are weak. Strong broadcast signals, nearby equipment, or interference from the measurement equipment itself may appear in probe measurements. Conducting measurements in a shielded enclosure eliminates ambient interference. When shielded facilities are not available, identifying ambient signals and distinguishing them from equipment emissions requires attention.

The measurement equipment itself can be a source of interference. Computer displays, switching power supplies, and data acquisition systems generate emissions that may couple to the probe. Positioning measurement equipment away from the equipment under test and using shielded cables reduces this contamination. Battery-powered equipment avoids power supply switching noise. Verifying that the measured fields actually originate from the equipment under test rather than the measurement system prevents misinterpretation.

Probe loading of sensitive circuits can alter their behavior, potentially changing the emissions being measured. Bringing a conductive probe close to high-impedance nodes adds capacitance that may affect circuit operation. If the circuit behavior changes noticeably during probing, the probe is affecting the measurement. Maintaining adequate clearance or using high-impedance probes minimizes loading effects.

Initial Survey

The initial survey covers the entire area of interest at modest spatial resolution to identify regions with elevated field levels. Using a larger probe for better sensitivity, scanning in a regular grid pattern at consistent height identifies hot spots deserving closer attention. The spectrum analyzer display indicates relative field intensity at each position. Recording or noting which areas show the highest responses prioritizes subsequent detailed scanning.

Survey scanning speed affects the ability to detect emission sources. Moving too quickly may pass over a localized source without capturing its peak field. Moving too slowly extends the survey time unnecessarily. A moderate pace that allows observation of the analyzer display while maintaining steady probe motion provides a good balance. For pulsed or modulated sources, using peak hold or max hold mode captures transient maxima.

Multiple frequency spans may be needed to survey the full frequency range of interest. The survey should cover all frequencies where emissions problems exist or are anticipated. If specific problem frequencies are already known from far-field testing, those frequencies deserve particular attention during the survey. Unknown sources may be discovered at frequencies not previously investigated.

Detailed Source Location

Detailed probing of hot spots identified during the survey precisely locates the emission source. Switching to a smaller probe improves spatial resolution. Moving the probe slowly while watching the analyzer identifies the position of maximum response. For magnetic probes, rotating to find the orientation of maximum response indicates the current direction and helps identify the specific conductor carrying the emission current.

The maximum field position may not correspond directly to the emission source circuit. Current can flow from its source through various paths before reaching the point of maximum field. Tracing along the direction of current flow often leads from the high-field point back toward the actual source circuit. This tracing requires understanding how current flows in the design and recognizing the physical structures that carry it.

Multiple sources may contribute to emissions at the same frequency. The detailed scan should identify all significant sources, not just the strongest one. Suppressing the dominant source may reveal secondary sources that then require attention. Complete characterization of all contributing sources supports comprehensive troubleshooting rather than iterative discovery of additional problems.

Documentation

Recording near-field measurement results creates a reference for comparing before and after conditions, communicating findings, and building troubleshooting knowledge. Documentation should include the probe position, frequency, signal level, and probe orientation for each significant measurement. Photographs showing the probe positioned over specific circuit features provide clear spatial reference.

Annotated drawings or board images showing field intensity at various locations create visual representations of the field distribution. Heat maps or contour plots generated from systematic grid measurements provide quantitative spatial documentation. These representations help identify patterns in the field distribution that suggest coupling mechanisms or current paths.

Comparison documentation tracks changes through the troubleshooting process. Recording measurements before and after each modification shows whether the modification reduced emissions and by how much. This before/after documentation validates the effectiveness of changes and guides decisions about further modifications. Unexpected results may indicate measurement errors, suggesting the need for verification.

Near-Field to Far-Field Relationship

The near-field intensity at a particular location does not directly predict the far-field radiation from that source. In the near field, field intensity drops rapidly with distance from the source, while far-field radiation follows different propagation characteristics. A source with intense near-field may radiate weakly if its geometry is electrically small. Conversely, a distributed source with moderate near-field may radiate efficiently if it forms an effective antenna structure.

The correlation between near-field hot spots and far-field emissions depends on the radiation mechanism. If the source directly radiates, near-field intensity correlates with radiation. If the source couples to a secondary radiating structure such as a cable, the cable's radiation efficiency determines the far-field emission regardless of the original source's near-field intensity. Understanding the radiation mechanism guides interpretation of near-field data.

Comparing near-field spectral signatures with far-field emission spectra helps validate source identification. If a near-field hot spot and a far-field emission show the same frequencies with similar harmonic relationships, the hot spot is likely the emission source. If the spectra differ, the hot spot may be an intermediate coupling point rather than the original source, or multiple sources may contribute differently to near and far field.

Source Identification

Correlating near-field findings with circuit function identifies the electrical source of emissions. A hot spot over a clock distribution trace indicates clock-related emissions. A hot spot over a switching regulator indicates power supply noise. Matching the observed frequencies with known circuit frequencies confirms the identification. When frequencies do not match any obvious source, reviewing circuit operation for less apparent sources may reveal unexpected contributors.

The spatial pattern of fields provides structural information. Magnetic fields concentrated along a trace indicate current flow on that trace. Fields that extend from the board edge may indicate radiation from an unshielded edge or coupling to external structures. Fields concentrated around a gap in the ground plane indicate current flowing around the gap. Interpreting these patterns requires understanding electromagnetic field structures around various conductor geometries.

Multiple scans at different orientations help characterize three-dimensional field structures. A single scan with fixed probe orientation may miss sources that create fields perpendicular to the probe's sensitive axis. Rotating the probe or scanning with multiple orientations ensures that all significant sources are detected regardless of their field orientation.

Common Interpretation Pitfalls

Several common pitfalls can lead to incorrect interpretation of near-field measurements. Assuming that the highest-field location is the emission source may be incorrect when fields propagate from a distant source to a more easily detected location. Ignoring probe orientation effects may cause apparent spatial variations that actually result from field direction changes rather than intensity changes.

Measurement artifacts can create apparent fields that do not represent actual circuit emissions. Probe cable pickup in strong ambient fields adds to the probe output. Preamplifier overload from strong signals causes distortion that may appear as spurious frequencies. Equipment under test behavior changes from probe loading may alter the emissions being measured. Recognizing these artifacts prevents incorrect conclusions.

Over-reliance on near-field measurements without correlation to far-field results may lead to addressing sources that do not actually contribute to the emission problem. The goal of troubleshooting is reducing far-field emissions, not near-field intensity. Near-field probing is a means to that end, valuable for source identification but not sufficient alone to validate emission reduction.

Automated Scanning Systems

Automated scanning systems use robotic positioners to move probes through precise patterns while recording measurements. These systems enable comprehensive field mapping with better reproducibility than manual scanning. The resulting data can be visualized as two-dimensional field maps or three-dimensional representations. Automated systems are particularly valuable for detailed characterization of complex board assemblies or for comparative measurements requiring precise positioning.

Automated scan data supports sophisticated analysis such as near-field to far-field transformation. Mathematical techniques can calculate the far-field radiation pattern from comprehensive near-field measurements, predicting antenna chamber results from bench-top measurements. While these transformations have limitations and require careful execution, they can reduce the need for expensive antenna chamber time during troubleshooting iterations.

The investment in automated scanning systems is justified when the volume of measurements warrants automation or when measurement reproducibility is critical. For routine troubleshooting of moderate complexity, manual scanning may be more practical and flexible. The choice depends on the organization's measurement needs and resources.

Phase and Timing Measurements

Measuring the phase relationship between fields at different locations reveals how sources combine and may identify the primary source when multiple sources contribute. Phase-coherent measurement requires a reference signal, typically derived from the circuit under test. Comparing the phase of the field at different locations shows whether sources are in phase (additive) or out of phase (partially canceling). This information helps predict the effect of suppressing individual sources.

Time-domain near-field measurements using oscilloscopes reveal the temporal characteristics of emission events. Correlating field waveforms with circuit signals identifies which circuit events create each field feature. Fast transients, ringing, and settling behavior that are not apparent in frequency-domain measurements become visible in time-domain observations. This temporal correlation is particularly valuable for debugging pulsed or transient emissions.

Triggered measurements synchronized to specific circuit events isolate the contribution of particular sources. Triggering the spectrum analyzer or oscilloscope on a circuit control signal and gating the measurement to specific time windows separates overlapping sources that would otherwise combine. This technique helps analyze complex systems with multiple simultaneous emission sources.

Calibrated Quantitative Measurements

While most near-field troubleshooting uses relative measurements, calibrated probes enable quantitative determination of field strength. Calibration relates the probe output voltage to the local field intensity in physical units such as A/m for magnetic fields or V/m for electric fields. Quantitative data supports engineering calculations of coupling and radiation rather than purely empirical troubleshooting.

Calibration of near-field probes is more complex than far-field antenna calibration because the near-field structure depends on the source geometry. Probes are typically calibrated in standard field configurations, such as over a microstrip line or in a TEM cell, and the calibration applies accurately only in similar geometries. Uncertainty increases when measuring fields with different spatial distributions than the calibration geometry.

Despite calibration limitations, quantitative near-field data enables comparison with analytical predictions and provides absolute references for comparing different designs or production units. If calculations predict a certain field level for compliant operation, measuring the actual field validates the prediction or reveals unexpected coupling. This quantitative approach supports more rigorous engineering analysis than purely qualitative hot-spot identification.