Electronics Guide

Chamber Design Considerations

Designing a chamber for combined vibration-EMC testing presents several challenges:

Shielding integrity: The shielded enclosure must maintain its shielding effectiveness while accommodating the mechanical motion of the shake table. This typically requires flexible shielding connections between the moving platform and the fixed chamber walls.

Shaker electromagnetic compatibility: Electrodynamic shakers generate significant electromagnetic fields from their armature coils. These fields can interfere with EMC measurements if not properly shielded or if the shaker is not sufficiently distant from the equipment under test.

Cable management: Test cables connecting the EUT to external measurement equipment must accommodate platform motion while maintaining signal integrity. Cables that are too stiff can affect vibration characteristics; cables that are too loose can introduce microphonic noise.

Absorber placement: RF absorbing materials used to create anechoic conditions must be secured against vibration-induced motion and positioned to avoid interference with mechanical fixtures.

Fixture resonances: Test fixtures must support the EUT securely while avoiding resonances in the frequency range of interest. Fixture design becomes more complex when RF performance and mechanical performance must both be addressed.

Integration Approaches

Several approaches exist for integrating vibration and EMC testing:

Shaker inside chamber: Placing a small shaker inside a shielded room provides the closest integration but limits shaker size and force capability. This approach is suitable for small EUTs and moderate vibration levels.

Shaker external with through-floor coupling: A large shaker is mounted below the chamber floor, with the armature extending through a flexible seal into the chamber. This provides high force capability while maintaining shielding integrity.

Isolated shaker pit: The shaker is located in a separate shielded pit to reduce electromagnetic interference with measurements. A rigid shaft connects to the test platform inside the chamber.

Separate but synchronized testing: When full integration is not practical, vibration and EMC tests can be performed separately but with carefully controlled conditions that allow correlation of results.

Measurement Considerations

Performing EMC measurements during vibration requires attention to several factors:

Background noise: Shaker operation generates electromagnetic noise that can mask EUT emissions or appear as false failures in susceptibility testing. Background measurements with the shaker running but the EUT off establish the noise floor.

Microphonic effects: Test equipment and cables can exhibit microphonic behavior, generating noise that corrupts measurements. Low-noise cables and secure mounting of measurement equipment are essential.

Time-varying phenomena: Vibration-induced EMC effects may occur only at specific vibration frequencies or amplitudes. Swept-frequency vibration with continuous EMC monitoring captures these transient conditions.

Statistical considerations: Random vibration creates continuously varying mechanical conditions. EMC measurements during random vibration require statistical analysis to characterize the probability distribution of emissions or susceptibility.

Testing for Acoustic Emissions

Measuring acoustic noise from electromagnetic sources requires isolation from other noise sources and appropriate measurement techniques:

Anechoic or semi-anechoic chambers: These rooms provide controlled acoustic environments that minimize reflections and background noise. The chamber must be quiet enough to measure low-level product noise.

Correlation with electrical operation: Acoustic measurements should be synchronized with electrical operating conditions. Load variations, switching activity, and other electrical parameters that affect acoustic output should be controlled and recorded.

Frequency analysis: Spectral analysis of acoustic emissions reveals the relationship to electrical operating frequencies. Harmonics of switching frequencies, power line frequencies, and mechanical resonances can be identified and traced to their sources.

Operating condition sweeps: Acoustic emissions often vary with operating conditions. Testing across the full range of input voltage, load, temperature, and other parameters reveals worst-case noise conditions.

Testing for Acoustic Susceptibility

Exposing equipment to controlled acoustic fields while monitoring electrical performance tests susceptibility to acoustic interference:

Acoustic sources: Speakers, acoustic drivers, or compressed air jets can generate controlled acoustic fields. The source must be capable of producing the required sound pressure level across the frequency range of interest.

Reverberant acoustic chambers: For high-intensity testing, reverberant chambers efficiently generate diffuse acoustic fields at high levels. These chambers are commonly used for aerospace acoustic qualification.

Electrical monitoring: Sensitive electrical parameters must be monitored during acoustic exposure. This includes power supply voltages, signal integrity, sensor outputs, and any parameters susceptible to mechanical disturbance.

Correlation with vibration response: Acoustic excitation causes mechanical vibration. Simultaneously measuring vibration and electrical response helps identify the coupling mechanism.

Microphone and Audio System Testing

Audio input devices require testing that addresses both acoustic performance and electromagnetic susceptibility:

EMI susceptibility of microphones: Testing for sensitivity to electromagnetic fields while the microphone receives a known acoustic signal reveals interference effects on audio performance.

Acoustic isolation during EMI testing: When testing EMI susceptibility, the acoustic environment must be controlled to distinguish electromagnetic effects from acoustic pickup.

Combined stresses: Testing with simultaneous acoustic input and electromagnetic exposure represents real-world conditions where both types of signals are present.

Shock Generation Methods

Several methods generate controlled mechanical shocks for testing:

Drop shock: The EUT is raised to a controlled height and dropped onto a controlled surface. The shock pulse shape and amplitude are determined by drop height and the impact surface characteristics.

Shock machines: Mechanical or pneumatic shock machines apply controlled pulses. These machines can produce classical shock pulses (half-sine, sawtooth, trapezoidal) with specified amplitude and duration.

Pyrotechnic shock: For simulating explosive separation events in aerospace applications, pyrotechnic devices generate very high amplitude, high-frequency shock. Special facilities are required for this testing.

Shaker-generated shock: Electrodynamic shakers can generate shock pulses by appropriate drive signals. This approach allows shock testing in an integrated vibration-EMC facility but is limited in peak acceleration and high-frequency content.

EMC Monitoring During Shock

Capturing EMC performance during shock events requires high-speed instrumentation:

High-speed sampling: Shock durations are typically milliseconds, and EMC effects may be even shorter. Sampling rates must be sufficient to capture transient events with adequate resolution.

Trigger synchronization: EMC measurements must be synchronized with the shock event. Pre-trigger capture ensures that the complete event, including the onset, is recorded.

Post-shock measurement: Some shock-induced changes (such as connector degradation or component damage) persist after the shock. Comparing pre-shock and post-shock EMC performance reveals permanent changes.

Multiple shock accumulation: Repeated shocks can cause progressive degradation that may not be apparent from a single shock event. Testing protocols often include repeated shock application with periodic EMC assessment.

Shock-Induced EMC Failures

Common EMC-related failure modes revealed by shock testing include:

Connector intermittents: Shock can cause momentary contact separation in connectors, generating transient interference.

Component damage: Ceramic components can crack, solder joints can fracture, and other mechanical damage can occur that affects EMC performance.

Displacement effects: Large displacements during shock can cause shorting to enclosure walls or contact between adjacent components.

Relay chatter: Electromechanical relays may chatter during shock, causing control signal glitches and conducted interference.

Random Vibration Fundamentals

Understanding random vibration concepts is essential for combined testing:

Power spectral density: Random vibration is characterized by the PSD, which specifies the power (acceleration squared) per unit frequency. The area under the PSD curve equals the mean square acceleration.

RMS and peak levels: The RMS (root mean square) acceleration is the square root of the area under the PSD. Peak levels in random vibration follow a statistical distribution, with peaks occasionally reaching 3-4 times the RMS level.

Frequency content: The PSD defines which frequencies are present and their relative energy. Specifications typically include break frequencies where the PSD slope changes and defined levels in different frequency bands.

Duration effects: Random vibration test duration affects the statistical confidence that all frequency components have been adequately tested. Longer durations are required to characterize low-probability events.

EMC Implications of Random Vibration

Random vibration affects EMC testing in several ways:

Continuous excitation: All mechanical resonances within the PSD bandwidth are excited simultaneously. This is more representative of real environments than sinusoidal testing at discrete frequencies.

Statistical nature: EMC effects during random vibration are statistical. Emissions may vary randomly, and susceptibility thresholds may be occasionally exceeded during peaks.

Correlation challenges: Relating a specific EMC event to a specific mechanical condition is difficult in random vibration because the instantaneous state is continuously changing.

Test coverage: Random vibration efficiently covers a wide frequency range but may not dwell at specific resonances long enough to reveal all failure modes. Some specifications combine random and sine-on-random vibration.

Random Vibration EMC Test Procedures

Effective random vibration EMC testing requires appropriate procedures:

Pre-test characterization: Perform EMC baseline measurements before vibration to establish reference levels.

Continuous monitoring: Monitor EMC parameters continuously during random vibration. Time-stamped data allows correlation with vibration parameters.

Statistical analysis: Analyze EMC data statistically to characterize the distribution of emissions or susceptibility during vibration.

Post-test comparison: Repeat baseline EMC measurements after vibration to identify permanent changes.

Axis sequence: Perform vibration in each of three orthogonal axes. Different failure modes may appear in different axes depending on component orientation and resonance modes.

Sine Sweep Parameters

Key parameters define a sine sweep test:

Frequency range: The sweep covers from the minimum to maximum frequency of interest, typically from a few Hz to several hundred or thousand Hz.

Sweep rate: The rate of frequency change, often expressed in octaves per minute. Slower sweep rates allow more time at each frequency but increase test duration.

Amplitude: The vibration amplitude may be specified as constant acceleration, constant velocity, or constant displacement across different frequency ranges.

Sweep direction: Sweeps may be upward (low to high frequency), downward, or both. Some resonance behaviors differ depending on sweep direction.

Dwell requirements: Some specifications require dwelling at specific frequencies (such as resonant frequencies or operating frequencies) for extended duration.

EMC Correlation in Sine Sweep

Sine sweep testing offers advantages for EMC correlation:

Frequency identification: EMC events occurring during sine sweep can be directly correlated to the vibration frequency at that instant. This identifies which resonances are EMC-significant.

Threshold determination: By varying amplitude at specific frequencies, the vibration level that causes EMC effects can be determined.

Mode characterization: The phase relationship between mechanical motion and electrical effects can be measured at different frequencies, helping to understand the coupling mechanism.

Resonance mapping: Recording transmissibility (response/input) at the EUT and EMC response simultaneously creates a map of mechanical-electrical coupling across frequency.

Resonance Dwell Testing

When sine sweep identifies critical frequencies, resonance dwell testing provides more detailed characterization:

Extended exposure: Dwelling at a resonance for extended time allows fatigue effects to develop that may not appear during a brief sweep through the resonance.

Amplitude variation: Varying the excitation amplitude while dwelling at a resonance establishes the relationship between vibration level and EMC effect.

Thermal effects: Extended dwell allows thermal equilibrium to be reached, revealing temperature-dependent effects that may combine with vibration effects.

Statistical characterization: For intermittent effects, extended dwell provides enough events for statistical characterization of failure rate versus vibration level.

SRS Fundamentals

Understanding the SRS concept is essential for shock specification and testing:

SDOF response: Each point on the SRS represents the maximum absolute acceleration experienced by a single-degree-of-freedom oscillator (mass on spring) when subjected to the shock. The oscillator natural frequency determines the frequency axis location; the peak response is the ordinate.

Damping: The SRS depends on the damping ratio assumed for the oscillators. Common specifications use Q = 10 (5% critical damping), which is representative of many electronic structures.

Maximax versus primary: The maximax SRS uses the absolute maximum response regardless of when it occurs. The primary SRS uses only the maximum during the shock; residual oscillations are not counted.

Interpretation: High SRS values at a given frequency indicate that components with resonances near that frequency will experience high stress. The SRS helps identify the frequencies where damage is most likely.

SRS for EMC Analysis

The SRS concept applies to understanding vibration-induced EMC effects:

Component sensitivity: Just as the SRS predicts structural damage, it can predict EMC problems if component EMC sensitivity is related to mechanical response.

Comparing environments: The SRS allows comparison of different shock environments (pyrotechnic, drop, transportation) to identify which is most severe for a specific product.

Test specification: SRS-based specifications ensure that test shocks are at least as severe as expected field environments at all relevant frequencies.

EMC-SRS correlation: By measuring EMC response to shocks with different SRS characteristics, the frequency-dependent relationship between mechanical input and EMC output can be established.

SRS Synthesis and Testing

Creating test shocks with specified SRS characteristics:

Classical pulses: Standard shock waveforms (half-sine, terminal sawtooth, etc.) have predictable SRS characteristics. The pulse amplitude and duration determine the SRS shape.

Complex waveforms: Real-world shocks often have complex SRS characteristics that cannot be matched by classical pulses. Synthesized waveforms can be designed to match specified SRS.

Multiple pulses: Some SRS characteristics require multiple overlapping pulses to achieve the required frequency content.

Shaker versus mechanical: Electrodynamic shakers can generate low-frequency SRS content but are limited in high-frequency capability. Mechanical shock machines or pyrotechnic devices are needed for high-frequency, high-amplitude SRS.

Reverberant Field Characteristics

Reverberation chambers produce distinctive acoustic conditions:

Diffuse field: Multiple reflections from hard walls create a sound field where energy arrives from all directions with equal probability. This provides uniform exposure regardless of equipment orientation.

High levels: The reverberant buildup of sound energy allows very high sound pressure levels (140 dB and above) to be achieved with reasonable acoustic source power.

Frequency limitations: Below the Schroeder frequency (determined by room volume and reverberation time), the chamber supports discrete room modes rather than a diffuse field. Testing below this frequency is less uniform.

Spatial uniformity: In the diffuse field region, sound pressure level is nearly constant throughout the chamber, simplifying measurement and ensuring uniform EUT exposure.

Acoustic-Vibration Coupling

Intense acoustic fields cause structural vibration in the EUT:

Panel modes: Flat panels (enclosure walls, PCBs) are efficiently excited by acoustic pressure fluctuations, vibrating in their natural modes.

Broadband excitation: The broadband acoustic spectrum excites all structural modes within the acoustic bandwidth simultaneously.

Response prediction: Finite element analysis can predict structural response to acoustic excitation, enabling design optimization before testing.

Measurement challenges: Attaching accelerometers to measure structural response can affect local response. Non-contact methods (laser vibrometry) avoid this mass-loading issue.

EMC Effects of Acoustic Exposure

High-level acoustic environments can affect EMC performance:

Microphonic noise: The structural vibration induced by acoustic pressure creates microphonic noise in sensitive circuits.

Component stress: Intense vibration can damage components or degrade connections, causing permanent EMC changes.

Resonance excitation: Acoustic excitation of component or PCB resonances may reveal EMC sensitivity at specific frequencies.

Combined with other environments: Launch and flight environments include acoustic exposure combined with vibration, thermal extremes, and other stresses. Representative testing should address these combinations.

Mechanical-Electrical Correlation

Relating mechanical events to electrical responses:

Time domain correlation: Time-synchronizing mechanical and electrical measurements allows direct correlation. When an EMC event occurs, the simultaneous mechanical state is known.

Frequency domain correlation: Spectral analysis of both mechanical and electrical signals reveals frequency relationships. EMC effects at harmonics or subharmonics of mechanical frequencies indicate specific coupling mechanisms.

Transfer function: For linear systems, the transfer function relating electrical output to mechanical input can be measured. This enables prediction of EMC behavior for different mechanical inputs.

Statistical correlation: For random processes, statistical techniques (cross-correlation, coherence) quantify the relationship between mechanical and electrical signals.

Multi-Point Correlation

Understanding how vibration propagates through a system requires multiple measurement points:

Input-response correlation: Comparing vibration at the input (shaker attachment) with response at component locations reveals amplification and attenuation throughout the structure.

Mode shape measurement: Measuring response at multiple points allows determination of structural mode shapes. Knowing which modes are excited when EMC problems occur helps identify sensitive locations.

Path analysis: Vibration may reach a sensitive component through multiple paths. Multi-point measurement enables path analysis to identify the dominant paths for targeting mitigation efforts.

Environmental Correlation

Relating test conditions to real-world environments:

Field data collection: Measuring actual environmental conditions in the intended application provides the basis for realistic test specifications.

Environment synthesis: Creating test inputs that match field-measured environments, accounting for the differences between laboratory test setup and actual installation.

Damage equivalence: Establishing the relationship between test duration/severity and field life allows accelerated testing while maintaining relevance.

Margin verification: Testing beyond expected environmental levels verifies design margin and establishes failure thresholds.

Military and Aerospace Standards

Military and aerospace applications have the most developed combined testing requirements:

MIL-STD-810: This comprehensive environmental testing standard includes methods for vibration, shock, acoustic, and other environmental stresses. While focused on environmental testing rather than EMC, it provides the mechanical test framework often used in combined testing.

MIL-STD-461: The military EMC requirements standard includes provisions for operation during vibration for certain equipment classes. The combination of MIL-STD-461 EMC requirements with MIL-STD-810 environmental requirements addresses multi-physics testing.

RTCA DO-160: This standard for airborne equipment includes both environmental and EMC sections. Some sections explicitly require operation during vibration or temperature extremes.

NASA standards: Space flight hardware is subjected to extensive combined testing, including acoustic-vibration-thermal-vacuum combinations. NASA technical standards and program-specific requirements define these tests.

Automotive Standards

Automotive electronics face demanding combined environments:

ISO 16750: This series of standards addresses environmental conditions for electrical and electronic equipment in vehicles, including vibration and mechanical shock.

CISPR 25 and ISO 11452: These EMC standards for vehicles and components may be applied in combination with environmental testing per manufacturer requirements.

OEM specifications: Vehicle manufacturers often have proprietary requirements for combined testing that go beyond generic standards.

Tailoring and Justification

Effective test programs tailor standard requirements to specific applications:

Environment analysis: Understanding the actual operational environment enables selection of appropriate test levels and combinations.

Failure mode identification: Prior knowledge of likely failure modes helps focus testing on relevant combinations rather than testing all possible combinations.

Risk assessment: The consequences of failure influence the extent of testing. Safety-critical systems require more comprehensive combined testing than non-critical systems.

Documentation: Tailoring decisions and their justification should be documented in the test plan to support review and approval.

Shaker Capabilities

Electrodynamic shaker requirements for combined testing:

Force capacity: The shaker must provide sufficient force to achieve required vibration levels with the combined mass of fixture and EUT.

Frequency range: Low-frequency capability (below 20 Hz) and high-frequency capability (above 2000 Hz) extend the useful test range but increase shaker complexity and cost.

Displacement: Low-frequency vibration requires large displacements. Shaker displacement limits may constrain low-frequency testing.

Control system: Sophisticated control systems are needed for random vibration and complex swept-sine profiles. Multi-axis and multi-shaker control enables more realistic excitation.

Electromagnetic compatibility: The shaker system itself should not generate excessive electromagnetic interference that corrupts EMC measurements.

Shielded Enclosure Integration

Combining shielded room capabilities with vibration facilities:

Shielding effectiveness: The shielded room must maintain adequate shielding across the frequency range of interest while accommodating vibration equipment and motion.

Anechoic or semi-anechoic: For radiated testing, RF absorbing materials create anechoic or semi-anechoic conditions. These materials must be secured against vibration-induced motion.

Measurement equipment location: EMC receivers, amplifiers, and other measurement equipment should be located outside the vibration field to prevent vibration effects on measurements.

Power and signal feedthroughs: Filtered feedthroughs for power and signals must accommodate the cabling requirements of both vibration and EMC test systems.

Measurement Systems

Instrumentation for combined testing:

Data acquisition: High-speed, multi-channel data acquisition captures both mechanical and electrical parameters simultaneously. Adequate sampling rates and resolution are essential.

Synchronization: All measurement channels must be synchronized to enable accurate correlation. A common time base ensures that events on different channels can be related.

Environmental sensors: Temperature, humidity, and other environmental parameters should be monitored throughout testing.

Analysis software: Software for processing vibration data (PSD calculation, SRS, transmissibility) and EMC data (spectrum analysis, limit comparison) should be integrated for efficient analysis.

Test Sequence Considerations

The order of testing can affect results and efficiency:

Baseline first: Establish baseline EMC performance before any environmental exposure to provide a reference for detecting changes.

Non-destructive before destructive: Perform less stressful tests before more severe tests to avoid damaging the EUT before all data is collected.

Cumulative damage: Consider that earlier tests may affect results of later tests. Damage accumulation may require multiple test articles or careful test sequencing.

Combined versus sequential: True combined testing (simultaneous stresses) is more representative but more complex. Sequential testing (one stress at a time) is simpler but may miss interaction effects.

Resource Planning

Combined testing requires significant resources:

Facility access: Specialized combined test facilities are less common than single-discipline facilities. Early scheduling is essential.

Personnel: Combined testing requires expertise in multiple disciplines (vibration, EMC, environmental). Coordinating multi-disciplinary teams requires planning.

Test articles: Multiple test articles may be needed if testing is destructive or if tests must be conducted in parallel to meet schedule.

Fixturing: Fixtures that provide both mechanical support and appropriate RF conditions add complexity and cost.

Risk Management

Combined testing carries risks that should be managed:

Test article damage: Combined tests can damage or destroy test articles. Having backup articles or the ability to repair and retest mitigates this risk.

Facility problems: Complex combined test setups have more potential failure points. Contingency plans for facility issues should be in place.

Schedule impact: Failures discovered in combined testing may require design changes and retesting, affecting project schedule.

Interpretation challenges: Complex combined test data can be difficult to interpret. Adequate analysis time should be planned.