Uncertainty evaluations in EMC measurements Carlo Carobbi Dipartimento di Elettronica e Telecomunicazioni Università degli Studi di Firenze Politecnico di Milano - 20 Feb. 2009 1
Non - reproducibility of radiated emission tests Presentation mainly focused on this topic The contribution due to test site imperfections will be analyzed Important subject per se All the typical ingredients of EMC measurement uncertainty evaluation are involved Politecnico di Milano - 20 Feb. 2009 2
Sources of non reproducibility of radiated emission tests Imperfections of: Test sites Antennas Receivers Connections Set-up (geometry) EUT (intrinsic instability, layout of cables and auxiliary equipment) Politecnico di Milano - 20 Feb. 2009 3
Isolating the contribution due to test site imperfections Collaborative exercise performed: radiated emission measurement repeated in several different test sites Same instrumentation involved in each site (field source, receiving antenna, spectrum analyzer, cables) Same geometry, same measurement procedure (pre-defined measurement protocol), same personnel Politecnico di Milano - 20 Feb. 2009 4
Test sites involved Compact size fully anechoic rooms 14 nominally equivalent sites investigated 30 300 MHz frequency range Vertical polarization Received power (dbm) is the measured quantity Politecnico di Milano - 20 Feb. 2009 5
Resolving power of the method Ability to discriminate a site from another Limited by measurement non repeatability Politecnico di Milano - 20 Feb. 2009 6
Achieving repeatability Stable field source, battery operated Care of positioning (distance and mutual alignment) Well balanced receiving antenna Weak coupling with and reflections from the length of cable inside the room High signal to noise ratio and numerical averaging Spectrum analyzer warm-up and self calibration Automatic measurement Politecnico di Milano - 20 Feb. 2009 7
Repeatability quantified Spectrum Analyzer noise Spectrum Analyzer amplitude resolution and repeatability Generator instability (intrinsic + thermal fluctuations) Positioning uncertainty Inversion test Total ± 0.2 db (± 0.02 db) ± 0.1 db ± 0.05 db ± 0.02 db ± 0.14 db ± 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 Politecnico di Milano - 20 Feb. 2009 8
Results: dispersion among sites 7 6 db 5 4 3 Standard deviation 14 sites 2 1 0 Repeatability 30 60 90 120 150 180 210 240 270 300 f (MHz) Politecnico di Milano - 20 Feb. 2009 9
Results: mean received power vs. prediction in ideal empty space -60-70 Ideal power (dbm ) -80-90 -100-110 Mean Average noise level -120 30 60 90 120 150 180 210 240 270 300 f (MHz) Politecnico di Milano - 20 Feb. 2009 10
Results: source + spectrum analyzer observed instability db 0.5 0.4 0.3 0.2 0.1 0-0.1-0.2-0.3-0.4-0.5 Observed 0.16 db std. dev. Predicted 0.11 db std. dev. 30 60 90 120 150 180 210 240 270 300 f (MHz) Politecnico di Milano - 20 Feb. 2009 11
Questions Sites investigated equivalent? Deviation dominated by a minority of bad performing sites? Correlation with sites physical structure possible? Site correction factor? Politecnico di Milano - 20 Feb. 2009 12
Deviations from the mean 15 10 # 4 # 5 # 6 5 db 0-5 -10-15 30 60 90 120 150 180 210 240 270 300 # 10 # 1 f (MHz) Politecnico di Milano - 20 Feb. 2009 13
Sites structural characteristics l a r g e Site # Volume (m^3) Absorbing lining 1 63 Ferrite s m a l l 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 m e d i u m 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 Politecnico di Milano - 20 Feb. 2009 14
Deviations from the mean (removed small sites without ferrite) 15 10 # 5 # 14 5 db 0-5 -10 # 10-15 30 60 90 120 150 180 210 240 270 300 f (MHz) Politecnico di Milano - 20 Feb. 2009 15
Rejecting outliers # outliers site # 6 5 4 14 3 10 2 13 2 1 2 7 1 8 1 2 1 12 1 11 Chauvenet s rejection criterion applied at each frequency 23 outliers distributed over 10 sites Site #5 worst performing: 6 outliers Decision to reject 3 measured values: 2 (site 5) + 1 (site 10) We are not rejecting blunders (risk missing information) Politecnico di Milano - 20 Feb. 2009 16
Dispersion: 2 sites removed and 3 outliers rejected 7 db 6 5 4 3 Standard deviation 12 sites 2 1 0 Repeatability 30 60 90 120 150 180 210 240 270 300 f (MHz) Politecnico di Milano - 20 Feb. 2009 17
Mean: 2 sites removed and 3 outliers rejected -60-70 Ideal power (dbm) -80-90 -100-110 Mean Average noise level -120 30 60 90 120 150 180 210 240 270 300 f (MHz) Politecnico di Milano - 20 Feb. 2009 18
What we conclude Need of inter-laboratory comparisons (both collaborative exercises and proficiency tests) Reproducibility quantified Get physical insight Uncertainty evaluation applied Lab personnel involved in non-standard experiments Not expensive practice If well designed can cover any type of EMC test (RE, RS, CE, CS) Politecnico di Milano - 20 Feb. 2009 19
A note on uncertainty calculations in EMC Quite large deviations Extensive use of db units A problem when mixing natural and logarithmic quantities Specific asymmetric probability density functions involved Log-normal Rice (weak signal plus receiver noise, strong multipath interference) All these analytical aspects dealt with in GUM supplement 1 Politecnico di Milano - 20 Feb. 2009 20
Thank you for your kind attention FM Politecnico di Milano - 20 Feb. 2009 21