Uncertainty evaluations in EMC measurements
Transcript of Uncertainty evaluations in EMC measurements
Politecnico di Milano - 20 Feb. 2009 1
Uncertainty evaluations in EMC measurements
Dipartimento di Elettronica e Telecomunicazioni Università degli Studi di Firenze
Carlo Carobbi
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Non - reproducibility of radiated emission tests
Presentation mainly focused on this topicThe contribution due to test site imperfections will be analyzedImportant subject per seAll the typical ingredients of EMC measurement uncertainty evaluation are involved
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Sources of non – reproducibility of radiated emission tests
Imperfections of:Test sitesAntennasReceiversConnectionsSet-up (geometry)
EUT (intrinsic instability, layout of cables and auxiliary equipment)
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Isolating the contribution due to test site imperfections
Collaborative exercise performed: radiated emission measurement repeated in several different test sitesSame instrumentation involved in each site (field source, receiving antenna, spectrum analyzer, cables)Same geometry, same measurement procedure (pre-defined measurement protocol), same personnel
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Test sites involved
Compact size fully anechoic rooms14 nominally equivalent sites investigated
30 – 300 MHz frequency rangeVertical polarizationReceived power (dBm) is the measured quantity
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Resolving power of the method
Ability to discriminate a site from anotherLimited by measurement non repeatability
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Achieving repeatability
Stable field source, battery operatedCare of positioning (distance and mutual alignment)Well balanced receiving antennaWeak coupling with and reflections from the length of cable inside the roomHigh signal to noise ratio and numerical averagingSpectrum analyzer warm-up and self calibrationAutomatic measurement
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Repeatability quantifiedSpectrum Analyzer noise ± 0.2 dB (± 0.02 dB)
Spectrum Analyzer amplitude resolution and repeatability ± 0.1 dB
Generator instability(intrinsic + thermal fluctuations) ± 0.05 dB
Positioning uncertainty ± 0.02 dB
Inversion test ± 0.14 dB
Total ± 0.27 dB (± 0.18 dB)
Repeatability better than 0.3 dB or 0.2 dB (1 std. dev.), depending on the signal to noise ratio
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Results: dispersion among sites
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30 60 90 120
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f (MHz)
dB
Standard deviation 14 sites
Repeatability
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Results: mean received power vs. prediction in ideal empty space
-120
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-90
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-60
30 60 90 120
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f (MHz)
pow
er (d
Bm
)
Average noise level
Ideal
Mean
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Results: source + spectrum analyzer observed instability
-0.5-0.4-0.3-0.2-0.1
00.10.20.30.40.5
30 60 90 120
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f (MHz)
dB
Observed0.16 dB std. dev.Predicted0.11 dB std. dev.
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Questions
Sites investigated “equivalent”?Deviation dominated by a minority of bad performing sites?Correlation with sites’ physical structure possible?Site correction factor?
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Deviations from the mean
-15
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-5
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30 60 90 120
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f (MHz)
dB
# 4# 5
# 10 # 1
# 6
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Site # Volume (m^3) Absorbing lining1 63 Ferrite
2 93 Ferrite + Pyramid 45/60 cm
3 101 Ferrite + Pyramid 50 cm
4 105 Pyramid 60 cm
5 112 Ferrite + Pyramid 32/50 cm
6 134 Pyramid 30/55/65 cm
7 150 Ferrite + Pyramid
8 216 Ferrite + Pyramid 10/30/50 cm
9 321 Ferrite + Pyramid 45 cm
10 323 Ferrite
11 324 Ferrite + Pyramid 50 cm
12 371 Ferrite + Pyramid 50/200 cm
13 743 Pyramid 60 cm
14 1152 Pyramid 60 cm
Sites’ structural characteristics
small
medium
large
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Deviations from the mean(removed small sites without ferrite)
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30 60 90 120 150 180 210 240 270 300f (MHz)
dB
# 5
# 10
# 14
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Rejecting outliers
# outliers site #6 54 143 102 132 12 71 81 21 121 11
Chauvenet’s rejection criterion applied at each frequency23 outliers distributed over 10 sitesSite #5 worst performing: 6 outliersDecision to reject 3 measured values: 2 (site 5) + 1 (site 10)We are not rejecting blunders (risk missing information)
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Dispersion:2 sites removed and 3 outliers rejected
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30 60 90 120 150 180 210 240 270 300
f (MHz)
dB
Standard deviation 12 sites
Repeatability
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Mean:2 sites removed and 3 outliers rejected
-120
-110
-100
-90
-80
-70
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30 60 90 120 150 180 210 240 270 300
f (MHz)
pow
er (d
Bm
)
Average noise level
Ideal
Mean
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What we conclude
Need of inter-laboratory comparisons (both collaborative exercises and proficiency tests)
Reproducibility quantifiedGet physical insightUncertainty evaluation appliedLab personnel involved in non-standard experimentsNot expensive practiceIf well designed can cover any type of EMC test (RE, RS, CE, CS)
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A note on uncertainty calculations in EMCQuite large deviationsExtensive use of dB units
A problem when mixing natural and logarithmic quantities
Specific asymmetric probability density functions involved
Log-normalRice (weak signal plus receiver noise, strong multipath interference)
All these analytical aspects dealt with in GUM supplement 1
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FM
Thank you for your kind attention