Electronics Guide

Mode-Tuned Operation

In mode-tuned operation, the stirrer is moved to discrete positions and held stationary while measurements are made:

1. Position stirrer at starting angle
2. Allow field to stabilize (several chamber time constants)
3. Perform measurement (power, field strength, or device response)
4. Record data and stirrer position
5. Advance stirrer to next position
6. Repeat until full rotation completed

Mode-tuned operation provides the highest measurement accuracy because each sample is taken at steady-state conditions. It enables correlation between stirrer position and measured values, allowing identification of worst-case positions for equipment under test.

Typical mode-tuned measurements use 100-400 stirrer positions per rotation, with angular increments of 0.9 to 3.6 degrees. The number of positions should exceed the number of independent samples to avoid aliasing effects.

Mode-Stirred Operation

In mode-stirred operation, the stirrer rotates continuously while measurements are made:

1. Set stirrer to continuous rotation at specified speed
2. Apply excitation signal
3. Continuously sample measurement data
4. Process samples to extract statistics (mean, maximum, standard deviation)
5. Complete full rotation(s) before stopping

Mode-stirred operation is faster but requires careful attention to measurement bandwidth and sampling rate. The detector bandwidth must be wide enough to follow field variations as the stirrer rotates, and the sampling rate must capture the relevant statistics.

Typical stirrer speeds range from 0.5 to 10 RPM. Faster speeds reduce test time but may cause field variations faster than detector response. Slower speeds provide more accurate results but increase test duration.

Sample Size Requirements

The number of independent samples determines measurement uncertainty:

• Minimum: 12 independent samples (IEC 61000-4-21 requirement)
• Typical: 50-100 independent samples for 1-2 dB uncertainty
• High accuracy: 200+ samples for less than 1 dB uncertainty

The number of independent samples is not simply the number of stirrer positions; it depends on the correlation between adjacent positions. A 360-position measurement may yield only 30-80 independent samples depending on stirrer effectiveness and frequency.

Combining multiple stirring methods (mechanical, frequency, and source position) can increase the effective number of independent samples without proportionally increasing test time.

Frequency Stepping

When testing across a frequency range, the frequency step size affects both accuracy and efficiency:

• Step size should be small compared to chamber Q bandwidth for accurate resonance characterization
• At frequencies well above LUF, larger steps are acceptable for general characterization
• Logarithmic frequency stepping (constant percentage bandwidth) is common
• Critical frequencies (near device resonances or limit frequencies) may require finer steps

Typical frequency steps range from 0.5% to 2% of center frequency. Standards often specify minimum steps (such as 1% or 50 frequencies per decade).

Measurement Configuration

IEC 61000-4-21 specifies a standard uniformity measurement procedure:

1. Define working volume where equipment under test will be placed
2. Position reference antenna at each of 8 locations (working volume corners)
3. Orient antenna in three orthogonal directions at each location (24 total measurements)
4. At each configuration, record maximum received power over full stirrer rotation
5. Calculate standard deviation of the 24 maximum values

The measurement locations form a rectangular volume with antenna positions at each corner. The working volume should be the maximum region where equipment will be tested.

Uniformity Criteria

Standard uniformity requirements specify maximum standard deviation of the 24 maximum measurements:

• Above 400 MHz: Standard deviation not exceeding 3 dB
• 100-400 MHz: Standard deviation up to 4 dB may be acceptable
• Near LUF: Higher values may be observed; interpret results accordingly

If uniformity does not meet requirements, potential causes include:

• Insufficient mode density (frequency too close to LUF)
• Inadequate stirrer effectiveness
• Direct coupling between transmit and receive antennas
• Loading effects from objects in working volume
• Standing wave patterns from symmetric chamber dimensions

Calibration Frequency

Uniformity validation should be performed:

• Initially after chamber construction or major modification
• Annually as part of routine calibration
• After changes to stirrer, antenna, or chamber configuration
• Whenever measurement validity is questioned

Documentation of uniformity results provides traceability and supports measurement uncertainty analysis.

Chamber Loading Effects on Uniformity

Large equipment under test affects chamber uniformity:

• Increased absorption reduces Q and changes field distribution
• Scattering from equipment may improve or degrade uniformity
• Shadow regions behind large absorbing objects may have reduced fields

When testing large equipment, chamber loss measurements with the equipment in place help characterize these effects. Standards typically limit acceptable loading to preserve uniformity (often 3 dB maximum change in chamber loss).

Test Setup

Equipment under test (EUT) setup requires:

1. Position EUT in working volume, away from walls and stirrer
2. Connect EUT cables as in normal operation, routed to penetration panels
3. Establish EUT normal operating condition with monitoring
4. Document EUT configuration including software version and operating mode

EUT orientation is typically not critical due to statistical isotropy, but specific orientations may be required by product standards or customer requirements. Support structures should be non-metallic to minimize field perturbation.

Field Level Establishment

The required field level must be established considering chamber calibration:

E-average = sqrt(CCF * P-input)

Where CCF is the chamber calibration factor (V^2/m^2/W) and P-input is the input power. The chamber calibration factor is determined during calibration by measuring received power with a reference antenna of known efficiency.

Because the field is statistical, the average field level differs from the peak field level. For immunity testing, the required level is typically specified as the average field, with understanding that peak excursions above this level will occur.

Test Execution

Immunity test procedure:

1. Set frequency to first test point
2. Apply power to achieve required average field level
3. Execute stirring sequence (mode-tuned or mode-stirred)
4. Monitor EUT for performance degradation or malfunction
5. Record any deviations from normal operation
6. Step to next frequency and repeat

Dwell time at each frequency must be sufficient for the EUT to respond to the field and for adequate stirrer sampling. Typical dwell times range from 1 second per stirrer position (mode-tuned) to several full rotations (mode-stirred).

Performance Monitoring

EUT performance monitoring during testing requires:

• Functional parameters relevant to normal operation
• Monitoring circuits isolated from chamber fields (optical links preferred)
• Automatic logging of monitored parameters
• Defined pass/fail criteria before testing begins

Performance criteria typically reference product standards (IEC 61000-4-3 for general immunity, automotive standards, military standards, etc.) which define acceptable degradation levels during and after exposure.

Results Interpretation

Interpreting immunity test results requires understanding the statistical nature of the exposure:

• EUT experienced fields from all directions and polarizations
• Worst-case exposure occurred at some stirrer position(s)
• Average exposure was at the specified level
• Peak exposure exceeded average by statistical factor (typically 6-10 dB)

Failure at a specific stirrer position may indicate a directional or polarization sensitivity that could be further investigated with anechoic chamber testing if needed.

Total Radiated Power Measurement

The total radiated power (TRP) is measured using the chamber as an integrating device:

1. Position EUT in working volume with normal operating configuration
2. Operate EUT in mode that produces maximum emissions
3. Measure average received power at reference antenna over stirrer rotation
4. Calculate TRP using chamber calibration factor

The relationship between received power and TRP is:

TRP = P-received * (lambda^2 / (8*pi)) * (1/eta-antenna) * (1/CCF)

Where eta-antenna is the reference antenna efficiency and CCF is the chamber calibration factor.

Comparison with Limit Specifications

Most EMC emissions limits are specified as field strength at a distance (typically 3 or 10 meters) assuming a reference antenna gain. Converting TRP to equivalent field strength:

E = sqrt(30 * TRP * G-antenna) / d

Where G-antenna is the equivalent antenna gain (typically 1-2 for dipole/monopole approximation) and d is the measurement distance. This conversion assumes specific EUT radiation characteristics that may not match actual equipment.

Some standards now include reverberation chamber methods with limits expressed as TRP, avoiding the conversion uncertainty.

Frequency Scanning

Emissions measurements typically scan a frequency range to identify emission peaks:

1. Set measurement receiver to starting frequency
2. Record maximum received power over stirrer rotation
3. Step to next frequency
4. Repeat across entire frequency range
5. Identify frequencies with highest emissions
6. Perform detailed measurements at peak frequencies

Initial scans may use wider frequency steps for efficiency, with finer steps near peaks for accurate level determination.

Advantages and Considerations

Reverberation chamber emissions testing offers:

• Speed: No need to rotate EUT or scan antenna position
• Total power: Captures all radiated energy regardless of direction
• Repeatability: Less sensitive to EUT positioning

However, limitations include:

• No directional information about emissions
• Results not directly comparable to anechoic measurements
• May not be accepted for all compliance purposes
• LUF limits low-frequency coverage

Enclosure Shielding Measurement

For enclosure shielding effectiveness (SE):

1. Place receive antenna inside enclosure under test
2. Position enclosure in chamber working volume
3. Measure received power with enclosure (P-shielded)
4. Remove enclosure, measure received power at same location (P-unshielded)
5. Calculate SE = 10*log(P-unshielded / P-shielded)

Multiple stirrer rotations and frequency points characterize SE across the test range. Results represent the average SE over all angles of incidence and polarizations.

Nested Reverberation Chamber Method

For larger enclosures that operate as reverberation chambers themselves, the nested chamber method applies:

1. Place enclosure (inner chamber) inside main reverberation chamber (outer chamber)
2. Excite outer chamber with transmit antenna
3. Measure field inside inner chamber with receive antenna
4. Stir both outer and inner chambers (if inner has stirrer)
5. Compare to reference measurement without inner enclosure

This method accounts for the mode structure of the inner enclosure and provides results applicable to real-world shielded room performance.

Cable and Connector Shielding

Shielded cable effectiveness is characterized by transfer impedance, measurable in a reverberation chamber:

1. Install cable sample through chamber wall with proper shield terminations
2. Illuminate cable shield with chamber field
3. Measure voltage coupled to inner conductor
4. Calculate transfer impedance from field strength and coupled voltage

This method tests the cable under realistic multi-path illumination conditions that may reveal weaknesses not apparent in traditional single-direction testing.

Material Shielding Effectiveness

Shielding effectiveness of materials can be measured using aperture methods:

1. Create a reference aperture in a shielded enclosure
2. Measure power transmitted through open aperture (P-open)
3. Cover aperture with material under test
4. Measure power transmitted through covered aperture (P-material)
5. Calculate SE = 10*log(P-open / P-material)

Results include contributions from both material properties and aperture diffraction effects. The method is most appropriate for materials used to cover openings in shielded enclosures.

Data Recording

Measurements should record:

• Received power (or field strength) at each stirrer position
• Stirrer position angle or index
• Frequency for each measurement set
• Forward and reflected power at transmit antenna
• Environmental conditions (temperature, humidity)
• EUT status and monitored parameters (for immunity)

Raw data should be preserved for subsequent analysis, even when only summary statistics are needed for reports.

Statistical Quantities

Key statistical parameters extracted from reverberation chamber data:

Mean (average) power:

P-mean = (1/N) * sum(P-i)

Maximum power:

P-max = max(P-i)

Standard deviation:

sigma = sqrt((1/(N-1)) * sum((P-i - P-mean)^2))

Coefficient of variation:

CV = sigma / P-mean

For an ideal chamber with exponentially distributed power samples, CV approaches 1. Significantly different values indicate non-ideal behavior.

Distribution Verification

Verifying that measured data follows expected distributions validates chamber operation:

Chi-squared test: Compare measured distribution to theoretical exponential distribution

Kolmogorov-Smirnov test: Compare cumulative distribution functions

Probability plots: Graphical comparison of data to theoretical distribution

Departures from expected distributions may indicate insufficient mode density, inadequate stirring, direct coupling, or excessive loading.

Outlier Detection

Statistical outliers can affect results and should be investigated:

• Values more than 3 standard deviations from mean warrant examination
• Consistent outliers at specific stirrer positions suggest direct coupling
• Outliers at specific frequencies may indicate chamber resonances

Outliers should not be automatically removed; they may represent valid worst-case conditions relevant to the test objective.

Uncertainty Sources

Major uncertainty contributors include:

• Statistical sampling: Finite number of independent samples creates uncertainty in estimated mean and maximum values
• Field uniformity: Spatial variation within working volume
• Antenna calibration: Uncertainty in reference antenna factors
• Instrumentation: Power meter, network analyzer, and receiver uncertainties
• Mismatch: Reflections between components
• Loading effects: EUT loading changes from calibration conditions
• Chamber calibration: Uncertainty in CCF determination

Statistical Uncertainty

The statistical uncertainty in mean power estimate depends on sample size:

u(P-mean) / P-mean = 1 / sqrt(N-independent)

For 100 independent samples, the relative standard uncertainty is 10% (0.4 dB). For 400 samples, it decreases to 5% (0.2 dB).

Maximum value estimates have higher uncertainty:

u(P-max) / P-max is approximately 1 / sqrt(N) for large N

The actual distribution of maximum values follows an extreme value distribution, and confidence intervals can be calculated accordingly.

Combined Uncertainty

Individual uncertainty components are combined using root-sum-of-squares:

u-combined = sqrt(u1^2 + u2^2 + u3^2 + ...)

Expanded uncertainty for a 95% confidence level uses coverage factor k=2:

U = 2 * u-combined

Typical expanded uncertainties for reverberation chamber measurements:

• Immunity testing: 3-4 dB
• Emissions testing: 4-6 dB
• Shielding effectiveness: 2-4 dB

Uncertainty Budget Documentation

A complete uncertainty budget should document:

• All identified uncertainty sources
• Estimated magnitude of each source
• Type (A or B) classification
• Probability distribution assumed
• Sensitivity coefficients if applicable
• Combined and expanded uncertainty

Uncertainty analysis per GUM (Guide to Expression of Uncertainty in Measurement) principles provides traceable, defensible uncertainty estimates.

Measurement Procedure

1. Measure average received power with empty chamber (P-empty)
2. Place object under test in working volume
3. Measure average received power with object present (P-loaded)
4. Calculate absorption cross-section from power ratio and chamber parameters

The absorption cross-section is given by:

sigma-a = (lambda * V) / (2*pi*c*tau) * (1/P-loaded - 1/P-empty) / (1/P-empty)

Where V is chamber volume and tau is the chamber time constant.

Applications

Absorption cross-section measurements are used for:

• Characterizing loading for chamber calibration correction
• Dosimetry for biological exposure studies
• Material absorber characterization
• Antenna efficiency measurement

The technique provides absorption data averaged over all angles and polarizations, complementing single-angle measurements in anechoic chambers.