Electronics Guide

LISN Circuit Architecture

A typical 50-ohm LISN for each power conductor contains a series inductor connected between the mains input and the equipment output, with a network of capacitors connecting to ground and to the measurement port. The 50 microhenry inductor presents high impedance at frequencies above approximately 150 kHz, effectively isolating the measurement from mains impedance variations. A 0.1 microfarad capacitor shunts high-frequency energy from the equipment side to ground, while a series combination of a 1 microfarad blocking capacitor and a 50-ohm resistor couples the noise to the measurement output while blocking the mains voltage.

For three-phase power systems, three-phase LISNs provide stabilized impedance on each phase conductor and the neutral. The construction is more complex, but the operating principles remain the same. Some LISNs include provisions for direct current (DC) power systems used in automotive and aerospace applications, with different impedance specifications reflecting the characteristics of those power distribution networks. The CISPR 25 standard specifies LISN requirements for automotive conducted emission measurements, using lower inductance values appropriate for the lower source impedances in vehicle electrical systems.

LISN Selection and Specifications

Selecting an appropriate LISN requires matching its specifications to the applicable test standard and the equipment under test. Key specifications include the voltage and current ratings, which must exceed the equipment's power requirements with appropriate margin. The impedance specification defines the RF impedance presented to the equipment and must comply with the applicable standard's tolerance. The isolation specification indicates how well the LISN blocks external mains noise from affecting measurements.

Additional specifications include the voltage division factor, which relates the noise voltage at the measurement port to the noise voltage at the equipment terminals, and the capacitive discharge time, which affects safety when testing equipment that could leave hazardous charge on the coupling capacitors. High-quality LISNs include features such as transient protection, ground fault interrupters, and calibration verification ports. The mechanical construction must provide appropriate safety isolation between mains-connected terminals and measurement ports accessible during operation.

LISN Installation and Grounding

Proper LISN installation is critical for accurate measurements. The LISN ground terminal must be bonded to the reference ground plane with a short, wide, low-impedance connection. A ground plane, typically a metal sheet at least 2 meters by 2 meters, provides the reference for the measurement. The LISN should be positioned at the edge of the ground plane, with the equipment under test positioned at a specified distance from the LISN and the ground plane edges.

Multiple LISNs are required for multi-conductor power inputs. Single-phase equipment requires two LISNs (line and neutral), while three-phase equipment requires four (three phases and neutral). Some test standards allow a single LISN to be switched between conductors rather than using simultaneous measurement on all conductors, but this approach is slower and may miss transient emissions. The unused LISN measurement ports must be terminated in 50 ohms to maintain proper impedance on all conductors during measurement.

AMN Impedance Characteristics

The impedance presented by an AMN varies with frequency according to the component values and circuit topology. At frequencies below the measurement range, the impedance is dominated by the mains connection and appears low. As frequency increases through the measurement range, the series inductor and shunt capacitors establish the designed impedance, typically 50 ohms. At very high frequencies, parasitic effects in the inductor windings and capacitor leads can cause impedance variations.

CISPR 16-1-2 specifies tolerance limits for AMN impedance magnitude and phase across the measurement frequency range. A compliant AMN must present an impedance magnitude between 43 and 57 ohms (50 ohms plus or minus 14 percent) at frequencies from 150 kHz to 30 MHz. The phase angle tolerance is plus or minus 11.5 degrees relative to a purely resistive reference. Calibration verifies that the AMN meets these specifications, and periodic recalibration ensures continued compliance as components age or are stressed by fault events.

Calibration and Verification

AMN calibration verifies that the network meets its specified impedance and voltage division requirements. Calibration requires a network analyzer or impedance analyzer capable of measuring complex impedance at the frequencies of interest. The measurement is typically performed by injecting a signal at the equipment port and measuring the voltage at the measurement port, with the mains port terminated appropriately. The voltage division factor relates these measurements.

Verification can also be performed using a substitution method where a known signal is applied and the AMN output compared to the expected level. Regular verification between full calibrations catches problems before they affect test results. Many laboratories perform verification weekly or before each test session, with full calibration annually or after any event that might affect performance. Calibration certificates document the AMN's performance and traceability to national standards.

Current Probe Types

Clamp-on current probes feature a split core that can be opened and closed around a conductor without disconnecting it. This non-invasive characteristic makes them valuable for troubleshooting and diagnostic measurements where modifying the circuit is impractical. The split in the core introduces a small gap that reduces sensitivity and can pick up external magnetic fields if not properly designed. High-quality clamp-on probes use precision machined mating surfaces and shielding to minimize these effects.

Fixed current probes have solid cores through which conductors must be threaded during installation. The continuous core provides better sensitivity and rejection of external fields but requires planning the installation during assembly. Fixed probes are often used in permanent test fixtures and in applications where maximum sensitivity is required. Some probes combine both features with removable core sections for installation flexibility while maintaining the performance of a solid core when assembled.

Common-Mode Current Measurement

When all conductors of a cable or cord pass through a current probe aperture, the probe measures only the common-mode current. Differential-mode currents flow in opposite directions on different conductors and cancel in the transformer core, while common-mode currents flow in the same direction on all conductors and add. This makes current probes ideal for identifying common-mode emission problems, which are often the dominant source of both conducted and radiated emissions.

The relationship between common-mode current and emission levels can be calculated knowing the cable length and configuration. A cable carrying common-mode current acts as a monopole antenna, and the emission intensity is proportional to both the current and the electrical length of the cable. Reducing common-mode current directly reduces both conducted and radiated emissions. Current probes enable engineers to monitor the effectiveness of filtering and grounding changes in real time during troubleshooting.

Probe Calibration and Usage

Current probe calibration establishes the transfer impedance versus frequency characteristic required to convert voltage measurements to current values. Calibration is typically performed by passing a known current through the probe and measuring the output voltage at multiple frequencies across the operating range. The calibration data may be provided as a table, a graph, or correction factors to be applied to measurements.

Proper usage requires attention to probe positioning and cable routing. The conductor should pass through the center of the aperture perpendicular to the core face for maximum sensitivity and minimum position sensitivity. Multiple turns of the conductor through the aperture increase sensitivity proportionally but also increase the probe's self-capacitance and cable loading. The probe output must be terminated in the specified load impedance, typically 50 ohms, for accurate measurements.

Capacitive Voltage Probes

Capacitive voltage probes use a series capacitor to couple high-frequency signals while blocking the DC and low-frequency mains voltage. A resistive termination at the measurement end provides the load for the capacitive divider and matches the cable impedance. The coupling capacitor value determines both the low-frequency cutoff and the division ratio. Larger capacitors provide lower cutoff frequencies but higher capacitive loading on the source.

The voltage division ratio of a capacitive probe depends on the ratio of the coupling capacitor reactance to the termination resistance at each frequency. At frequencies where the capacitive reactance is much smaller than the termination resistance, the division ratio approaches unity (zero attenuation). At lower frequencies where the capacitive reactance becomes significant, attenuation increases. The frequency response must be characterized through calibration and applied as correction factors to measurements.

Active Voltage Probes

Active voltage probes incorporate amplifier circuits to provide high input impedance, controlled gain, and signal conditioning for optimal measurement performance. The active circuitry can provide frequency response shaping to extend the usable bandwidth and compensate for cable losses. Active probes require power, either from batteries or from the measurement equipment, and introduce their own noise floor that limits sensitivity to very low-level emissions.

Differential active probes measure the voltage difference between two points, rejecting common-mode voltages that might otherwise overload the measurement system or corrupt results. This capability is valuable for measuring differential-mode emissions in the presence of large common-mode voltages. The common-mode rejection ratio (CMRR) specification indicates how well the probe rejects common-mode signals, with higher values indicating better rejection.

Frequency Range and Span

The frequency range for conducted emission measurements typically spans from 150 kHz to 30 MHz for power line measurements, though some standards require measurements starting at 9 kHz or extending to higher frequencies. The analyzer's start and stop frequencies should be set to cover the required range with some margin to capture emissions near the band edges. Multiple sweeps at different span settings may be used to provide both overview and detailed examination of problem frequencies.

Narrow span settings provide higher frequency resolution and can reveal closely spaced spectral components that might merge together in a wide span view. However, narrow spans require longer sweep times to maintain measurement accuracy. For initial characterization, a full span sweep identifies the major emission peaks, followed by narrower spans centered on problem frequencies for detailed investigation. Zoom features in modern analyzers allow rapid changes between overview and detail views.

Reference Level and Dynamic Range

The reference level sets the amplitude at the top of the analyzer display and determines the input attenuator setting. Setting the reference level too high reduces sensitivity and may cause small emissions to fall below the noise floor. Setting it too low risks overloading the analyzer with large signals, causing distortion and erroneous measurements. The optimal reference level places the largest expected signal near the top of the display while maintaining adequate headroom to prevent overload.

The dynamic range of the measurement is the ratio between the largest signal that can be measured without distortion and the smallest signal that can be distinguished from the noise floor. Increasing the reference level increases the maximum measurable signal but raises the noise floor proportionally, maintaining approximately constant dynamic range. Preamplifiers can improve sensitivity for low-level measurements, while additional attenuation prevents overload when measuring high-level emissions.

Sweep Time and Triggering

Sweep time determines how quickly the analyzer moves through the frequency range. Faster sweeps complete measurements more quickly but may miss short-duration or intermittent emissions. The minimum sweep time depends on the resolution bandwidth and the span; narrower resolution bandwidths require longer sweep times for accurate measurements. Most analyzers indicate when the sweep time is too fast for accurate results.

Triggering options control when the analyzer begins its sweep. Free-run mode sweeps continuously, appropriate for steady-state emissions. Video triggering starts the sweep when the signal exceeds a threshold, useful for capturing transient emissions. External triggering synchronizes the sweep to events in the equipment under test, enabling correlation between equipment operation and emission signatures. Gated sweeps measure only during specific time windows, useful for isolating emissions from particular operational states.

Quasi-Peak Detection

The quasi-peak (QP) detector was developed specifically for EMC measurements to weight emissions according to their perceived annoyance factor in analog radio and television reception. The quasi-peak detector incorporates specific charge and discharge time constants that cause its output to depend on both the amplitude and repetition rate of pulsed signals. Continuous signals read the same on peak and quasi-peak detectors, but pulsed signals read lower on the quasi-peak detector, with greater reduction for lower repetition rates.

CISPR standards define the quasi-peak detector characteristics for different frequency bands. In Band A (9 kHz to 150 kHz), the charge time constant is 45 milliseconds and the discharge time constant is 500 milliseconds. In Band B (150 kHz to 30 MHz), these values are 1 millisecond and 160 milliseconds respectively. These parameters determine how the detector responds to different pulse patterns and are critical for obtaining correct quasi-peak measurements. Only calibrated EMI receivers with certified quasi-peak detectors should be used for compliance measurements.

Quasi-peak measurements are time-consuming because the detector must dwell at each frequency long enough for the time constants to settle. A full quasi-peak scan of the conducted emission range can take many minutes, compared to seconds for a peak scan. Common practice uses peak detection for initial scans and applies quasi-peak measurement only to frequencies where the peak value approaches or exceeds the limit. Since quasi-peak values are always equal to or less than peak values, this approach identifies all potential problem frequencies while minimizing measurement time.

Average Detection

The average detector computes the mathematical average of the signal envelope over the measurement time. For continuous wave signals, peak, quasi-peak, and average detectors all give the same reading. For pulsed signals, the average value depends on the duty cycle and can be substantially lower than the peak value. Some emission standards specify average limits in addition to or instead of quasi-peak limits, particularly for equipment that produces modulated or pulsed emissions.

Average measurements are faster than quasi-peak measurements while still providing weighting for pulsed signals. The video bandwidth setting affects average measurements by filtering the detected signal before display; narrower video bandwidths provide more averaging but require longer sweep times. Some standards specify video bandwidth settings for average measurements to ensure consistent results. Modern analyzers often provide simultaneous peak and average traces, enabling rapid identification of signals where the average value might provide compliance margin.

RMS and Other Detectors

Root-mean-square (RMS) detectors measure the power content of the signal rather than its envelope. For sinusoidal signals, the RMS value is 3 dB below the peak value. For complex signals with varying crest factors, RMS measurements provide information about the actual power that relates to heating effects and interference potential. Some specialized standards and certain customer requirements specify RMS measurements.

EMI receivers may also include specialized detectors such as the CISPR average detector, which has specific response characteristics for amplitude-modulated signals, and the root mean impulse (RMI) detector for particular aerospace and military applications. Understanding which detector is appropriate for each measurement scenario and how to interpret the results ensures accurate characterization of emission levels and correct compliance determinations.

Bandwidth and Noise Measurement

The measured level of broadband noise depends directly on the measurement bandwidth. Noise power increases by 3 dB for each doubling of bandwidth, or equivalently by 10 dB per decade of bandwidth increase. This relationship allows conversion of noise measurements between different bandwidths when necessary for comparison or diagnostic purposes. The formula for bandwidth correction is: level at new bandwidth equals level at old bandwidth plus 10 times the logarithm of the ratio of new bandwidth to old bandwidth.

Narrowband signals such as clock harmonics and switching frequency harmonics are not affected by bandwidth changes (within reasonable limits where the signal fits within the filter passband). This distinction between narrowband and broadband behavior helps identify the nature of emissions. Narrowband emissions appear as constant-level peaks that do not change with bandwidth, while broadband emissions rise and fall with bandwidth changes. Mixed emissions from equipment with both clock circuits and switching supplies display both characteristics.

CISPR Bandwidth Requirements

CISPR 16-1-1 specifies the measurement bandwidths and filter characteristics for each frequency band. Band A (9 kHz to 150 kHz) uses a 200 Hz bandwidth with specific pulse response characteristics. Band B (150 kHz to 30 MHz) uses the 9 kHz bandwidth. Bands C and D for higher frequencies use different bandwidths appropriate to those frequency ranges. The filter must meet specifications for shape factor and impulse response in addition to the nominal bandwidth.

Commercial spectrum analyzers often provide configurable resolution bandwidths in a standard sequence (1, 3, 10, 30, 100 Hz, etc.). The CISPR bandwidths of 200 Hz and 9 kHz are not in this standard sequence, so EMC-specific analyzers or EMI receivers include these specific filter options. Using a close approximation (such as 10 kHz instead of 9 kHz) introduces measurement error that can cause incorrect compliance determinations. For compliance testing, only equipment with the exact CISPR bandwidths should be used.

Shielded Room Measurements

Shielded rooms provide the ultimate solution for ambient noise control by enclosing the measurement in a conductive enclosure that blocks external electromagnetic fields. A properly constructed shielded room can provide 80 to 120 dB of shielding effectiveness, reducing ambient signals to negligible levels. Power entering the room passes through filtered feedthroughs that block conducted noise on the mains while allowing power delivery.

Shielded rooms require proper design and construction to achieve their rated performance. Doors, ventilation penetrations, and cable entries must maintain shield integrity through the use of finger stock contacts, waveguide-below-cutoff tubes, and filtered connectors. Periodic verification of shielding effectiveness ensures that degradation from mechanical wear, corrosion, or modifications is detected and corrected. The investment in a shielded room is justified for laboratories performing frequent compliance testing.

Test Site Selection and Preparation

When shielded rooms are not available, careful site selection and preparation can reduce ambient noise to manageable levels. Industrial environments with heavy electrical equipment, variable frequency drives, and fluorescent lighting tend to have high ambient noise. Residential and commercial office environments are generally quieter. Testing during off-hours when nearby equipment is not operating can further reduce ambient levels.

Additional filtering on the mains supply feeding the measurement area can reduce conducted ambient noise. A line filter or isolation transformer upstream of the LISN attenuates noise from the facility power system. Ferrite clamps on cables entering the measurement area reduce conducted noise on signal connections. Positioning the measurement setup away from obvious noise sources such as computer monitors, switching supplies, and communications equipment minimizes direct coupling.

Ambient Correction Procedures

When ambient noise levels approach or exceed the equipment's emissions at certain frequencies, correction procedures can estimate the equipment's true emission level. If the combined level (equipment plus ambient) and the ambient-only level are both measured, the equipment contribution can be calculated by logarithmic subtraction. However, this procedure becomes increasingly uncertain as the equipment level approaches the ambient level, and most standards consider measurements invalid if the equipment level is not at least 6 dB above ambient.

Documentation of ambient conditions is required for compliance test reports. The report should identify the measurement environment, describe any ambient noise reduction measures employed, and include ambient measurement data. Frequencies where ambient levels might have affected results should be identified. If compliance cannot be determined at certain frequencies due to ambient noise, the report should state this limitation and recommend retesting in a lower-noise environment if necessary.

Hardware Components

The EMI receiver or spectrum analyzer forms the core of the measurement system and must meet the specifications required by applicable standards. Modern receivers include computer interfaces (GPIB, USB, or Ethernet) for automated control and data transfer. Multiple measurement modes and detectors enable both fast scanning and compliant measurements. Memory depth and sweep speed determine how quickly full measurements can be completed.

LISN switching networks allow a single receiver to measure conducted emissions on multiple power conductors without manual cable changes. Electromechanical switches or solid-state relays under computer control select which LISN output connects to the receiver input. The switching network must maintain 50-ohm impedance and provide adequate isolation between channels to prevent leakage from degrading measurements. Unused LISN ports must be terminated in 50 ohms, often through termination ports built into the switch matrix.

Additional hardware may include programmable power sources that can vary voltage and frequency to test equipment under different power conditions, environmental chambers for temperature-varied testing, and antenna positioning systems for radiated emission measurements. Integration of all these elements under unified software control enables complex test sequences that would be impractical to perform manually.

Software Features

Test automation software provides functions for defining test configurations, executing measurement sequences, processing data, and generating reports. Configuration tools allow users to specify frequency ranges, bandwidths, detectors, limits, and other parameters for each measurement type. Test sequence editors enable creation of automated procedures that perform multiple measurements and comparisons without operator intervention.

Data processing features include calibration correction, limit comparison, margin calculation, and statistical analysis. Graphical displays present results as spectra, tables, and pass/fail indicators. Historical data storage and retrieval support trend analysis and test-to-test comparisons. Limit lines from various standards are included or can be entered by users. Automatic determination of worst-case frequencies and optimization of measurement time through peak-to-quasi-peak correlation are sophisticated features in advanced systems.

Report generation produces formatted documentation meeting the requirements of test standards and accreditation bodies. Reports include equipment identification, test configuration details, calibration status, ambient conditions, measurement data, limit comparisons, and pass/fail determinations. Export functions enable data transfer to other analysis tools, databases, and customer formats. Secure electronic signatures and audit trails support regulatory requirements for test record integrity.

Calibration and Uncertainty

Automated systems must incorporate calibration data for all elements in the measurement chain. LISN voltage division factors, cable losses, switching network losses, and receiver calibration all contribute to the overall measurement uncertainty. The software applies these corrections automatically so that displayed results represent the actual emission levels at the equipment terminals. Periodic recalibration ensures continued accuracy as components age.

Measurement uncertainty analysis quantifies the confidence in measurement results. Contributions from each element in the measurement chain combine according to statistical rules to give an overall uncertainty figure. Standards such as CISPR 16-4-2 provide guidance on uncertainty calculation for EMC measurements. Compliance determinations must consider measurement uncertainty; a measurement that falls within the uncertainty range of the limit may not clearly demonstrate compliance or non-compliance.

Equipment Configuration

The EUT must be configured to represent typical operating conditions likely to produce maximum emissions. For equipment with multiple operating modes, the mode producing highest emissions must be identified and tested. Peripheral devices and accessories normally used with the equipment should be connected during testing. Loading conditions for power supplies and outputs should reflect worst-case scenarios, as load current often affects emission levels.

Documentation of the EUT configuration is essential for report completeness and test reproducibility. Photographs showing the physical arrangement of equipment, cables, and test apparatus supplement written descriptions. Software versions, option settings, and any non-standard configurations should be recorded. Any deviations from standard setup requirements must be documented and justified, as they may affect the validity of results for compliance purposes.

Cable and Interconnect Handling

Cables associated with the EUT significantly affect conducted emission measurements. Power cables should be of specified length (typically 1 meter) or the standard length supplied with the equipment, bundled in a defined configuration. Excess cable length is arranged in a flat bundle at a specified height above the ground plane. Signal cables connecting to auxiliary equipment should be of minimum necessary length to reduce their influence on measurements.

The routing of cables relative to the LISN and ground plane affects measurements through capacitive and inductive coupling. Standards specify cable positions to minimize these effects and ensure reproducibility. Cables should not drape over the edge of the ground plane or approach the LISN too closely except at the power connection. Where multiple cables exit the EUT, they should be separated according to function and positioned consistently for each test.

Source Identification

Emissions at the switching frequency and its harmonics point to the power converter as the source. The severity of these emissions depends on switch edge rates, power level, and input filter effectiveness. Emissions at clock frequencies and their harmonics originate in digital circuits, with strength depending on the number of gates switching simultaneously and the power distribution network impedance. Identifying the fundamental frequency of harmonic-related emissions reveals the source.

Systematically disabling circuit sections while monitoring emissions isolates the source more directly. Removing clock signals, stopping processors, or disconnecting loads eliminates their contribution to emissions. When isolation is not possible, shielding test sections with grounded metal foil helps identify which areas contribute most to emissions. Near-field probes can locate hot spots on circuit boards where high-frequency currents concentrate.

Mitigation Verification

After implementing modifications intended to reduce emissions, measurements verify effectiveness. Compare spectra before and after the change at the same test conditions to isolate the effect of the modification. Improvements at the target frequencies should not come at the cost of degradation at other frequencies. Multiple modifications may interact, so systematic change management with measurements after each step builds understanding of the system's behavior.

When modifications prove effective in reducing emissions, document the changes thoroughly for incorporation into production units. Component values, placement, and assembly procedures all affect EMC performance. Validation testing on multiple units confirms that the fix is robust and reproducible. Margin testing that intentionally stresses the system verifies that adequate performance remains under worst-case conditions likely to be encountered in production variation and customer use.

Pre-Measurement Checklist

Before beginning measurements, verify that all equipment is properly connected and configured. Confirm that LISNs are bonded to the ground plane with low-impedance connections. Verify that unused LISN measurement ports are terminated in 50 ohms. Check that the spectrum analyzer or receiver is configured for the correct frequency range, bandwidth, and detector. Ensure that calibration is current and corrections are applied. Power the EUT and allow it to reach thermal equilibrium in its normal operating mode.

Perform an ambient noise measurement with the EUT powered off to establish the measurement floor. Compare ambient levels to the applicable limits and determine whether ambient noise might affect results at any frequencies. Document ambient conditions including temperature, humidity, and any unusual sources of electromagnetic noise present in the test environment. Take photographs of the test setup showing equipment arrangement, cable routing, and LISN positioning.

Data Quality Verification

After measurements, review the data for anomalies that might indicate measurement errors. Sudden jumps in the spectrum, unexplained peaks, or unusually high noise floors warrant investigation. Compare results to previous measurements of similar equipment to identify any unexpected differences. Check that calibration corrections were properly applied and that the frequency and amplitude scales are correct.

Repeated measurements verify consistency. Significant variation between measurements suggests setup instability, intermittent equipment behavior, or changing ambient conditions. Statistical analysis of multiple measurements provides confidence intervals for emission levels and helps distinguish true emissions from measurement artifacts. For compliance testing, multiple scans with different detectors and at multiple operating conditions build a complete picture of the equipment's emission characteristics.