Verifying Simulation Results with Measurements. Scott Piper General Motors

Transcription

1 Verifying Simulation Results with Measurements Scott Piper General Motors

2 EM Simulation Software Can be easy to justify the purchase of software packages even costing tens of thousands of dollars Upper management tends to be more forward thinking The potential benefit of these tools are clear Difficult to implement in a company There are doubts Many will ask for real world data to validate simulation results 2

3 The Real World 3

4 The EMC Real World 4

5 EMC Component Level Measurements A form of modeling and simulation The component s actual environment is replaced by a test chamber Efforts have been made to remove the influence from other devices Cable harness lengths may be altered The priority of compliance testing is repeatability over accuracy 5

6 Measurements vs. Simulation Measurement Techniques Used and relied upon for over 50 years Numerical Modeling Rarely used in industry until 7-8 years ago Measurement devices are physical objects Measurement devices can be infinitely small Ideal conditions are difficult to produce Ideal conditions are easy to produce (non-ideal is difficult) Information about the product is not necessary Only as accurate as the information provided 6

7 Can EM simulation provide useful information on EMC performance of complex electronic products? You Need to convince Your Company? & Yourself 7

8 Design of Experiment An experiment (or more likely a series of experiments) needs to be designed to show that the simulation tool correctly handles the EM principles for the intended application The experiment must be: Simple (shows only one principle at a time) Relevant to your application Verifiable (a correct answer is known) 8

9 Example: Crosstalk on a Printed Circuit Board A signal travelling on a conductor has the ability to transfer energy to an adjacent conductor This phenomenon is due to electromagnetic fields radiated by the signal Cross coupling Aggressor Signal Victim Line 9

10 Electromagnetic Cross Coupling A simple circuit board was designed to show the principle 10

11 PCB Model 11

12 Test Setup 12

13 Initial Results Measured Simulated 13

14 Which is Right? Inductive and capacitive coupling contributions are direct functions of the excitation frequency. Therefore they increase linearly with an increase in frequency Clayton R Paul (Introduction to Electromagnetic Compatibility) Simulation data doesn t follow this it must be incorrect 14

15 PCB Model 15

16 Starting Discretization of Volume 16

17 Finer Discretization of Volume 17

18 Final Results Measured Simulated Finer Cells Simulated Larger Cells 18

19 Crosstalk PCB The dominant feature in this investigation is cross coupling This example would work well for issues dealing with printed circuit boards The results of this example can be checked with a network analyzer and textbook equations 19

20 Test Setup simulation 20

21 Test Setup Simulation EMC tests try to mimic ideal conditions as much as possible but the ideal is unattainable When attempting to simulate a test setup, certain non-ideal elements will need to be remembered and included Some aspects of the test setup will make a large difference in the results Other aspects of the test setup will only add complexity to the model and consume resources 21

22 Wire above a Metal Plane 22

23 Measurement Setup 23

24 Experiment Check A known signal was exciting a known geometry An aerospace standard test setup was used The goal of this simulation is to reproduce the measurement 24

25 Model from 1-30 MHz 25

26 Unterminated Wire Measured Simulated 26

27 Comparison Results Frequency Measurement Simulation Difference 150 khz 68 dbµv/m 64 dbµv/m -4 db 1 MHz 52 dbµv/m 47 dbµv/m -5 db 10 MHz 40 dbµv/m 37 dbµv/m -3 db 15 MHz 21 dbµv/m 22 dbµv/m 1 db Minimum (19.7 MHz measured vs 19 MHz simulated) Maximum (30 MHz) 11 dbµv/m 9 dbµv/m 2 db 40 dbµv/m 37 dbµv/m 3 db 27

28 Wire Terminated into 100 Measured Simulated 11 db difference NOT GOOD 28

29 Test Setup Simulation When verifying the accuracy of a simulation tool, one must consider the setup specifics When these specifics change, re-verify! If the goal is to support test data, certain aspects of the test setup (wires, support equipment) should be considered even though it goes against the intent of the test Insight can be gained into test setups by attempting to simulate their results 29

30 Extra Coaxial Cables Added Coax shield for antenna connection Without Cable 22 MHz With Cable 30

31 Comparison with Coax Cables 31

32 Test Setup Simulation If the goal is to support test data, certain aspects of the test setup (wires, support equipment) should be considered even though it goes against the intent of the test Insight can be gained into test setups by attempting to simulate their results 32

33 Limitations of Measurements Exploring 33

34 PCB Shield 34

35 Field Probe Setup - Simulation 35

36 Plane Wave Excitation 36

37 Electric Field Plot 37

38 Shielding Effectiveness vs. Frequency At Different Locations 38

39 Real World Data? 39

40 Real World Problems Point electric field sensors are typically too large to fit underneath the shield A noise generating circuit attached to a small antenna could be placed inside the shield The circuit would be battery powered Emissions from the circuit could then be measured 40

41 PCB Shield 41

42 Noise Generating Circuit 42

43 Noise Generating Circuit Model 43

44 Shielding Effectiveness vs. Frequency 44

45 Measured Results Shielding Effectiveness = Unshielded-Shielded Unshielded Shielded 45

46 Simulated Results Shielding Effectiveness 46

47 9V Battery Added to Model 47

48 New Results 48

49 Measured Results 49

50 Electric Field Plot at 480 MHz Shield 50

51 Battery Shielded 51

52 Measured Results with Shielded Battery Measured Emissions (dbm) No Shield Full Shield Ambient Frequency (MHz) 52

53 Shielding Effectiveness of PCB Shields There are some investigations that are difficult to use measurement equipment While measurements can be a good way to verify the results of a simulation, simulations can also be used to verify the results of measurements! 53

54 Clues from Measurements Simplifying Simulations: 54

55 Lumped Elements A capacitor is typically represented as a series LRC circuit Parasitic R and L is typically provided on a vendor s datasheet A circuit simulator would require all three of these values in order to produce a correct answer When performing EMC simulations using a full wave solving method, is it important to include these values? 55

56 Experiment Design - ESL A test PCB will be designed such that capacitor impedance can be measured easily by a network analyzer The design will then be simulated with and without capacitor ESL and compared with the measured result 56

57 Simulation Setup FIT Solver SMA Connector 4 Layer PCB 10 nf 0402 capacitors 57

58 Measurement Setup 58

59 Results Measured ESL Dominates Impedance Simulated With Lumped ESL Simulated Without Lumped ESL 59

60 Plates with Ceramic Material in-between Capacitor ESL Leads 60

61 Simulated Capacitor Lumped Element 61

62 Different Simulation Tool 3D finiteelement method solver intended for PCB problems 62

63 Results Simulated With Lumped ESL Measured Simulated Without Lumped ESL What is right for one simulation tool is not always right for another! 63

64 Conclusion - ESL A setup was used that isolated one variable A multi-layer PCB with ceramic SMD capacitors was used for this investigation The results were in harmony with VNA measurements and existing knowledge 64

65 Product Immunity Investigations 65

66 Measurements are the Standard If you want to know if a product will operate normally in a field of 100 V/m at 400 MHz what do you do?. Build the product, then expose it to 100 V/m at 400 MHz and see if it works of course! Can we replace this with a simulation? 66

67 Example Pair of wires Transmitter Receiver What is the optimal configuration for the wire pair in order to carry the signal from the transmitter to the receiver? 67

68 To Twist or Not to Twist? Twisted Separated by 2cm 68

69 To Twist or Not to Twist? S21 Comparision for Wiring Configurations S21 (db) Frequency (MHz) Side by Side Twisted Separated by 2cm 69

70 To Twist or Not to Twist? Measurement/Simulation Comparison Twisted Wires Untwisted Wires Separated by 2cm Measurement Untwisted Measurement Twisted Measurement Separated 70

71 To Twist or Not to Twist? 0 S21 Comparision for Wiring Configurations S21 (db) Frequency (MHz) Side by Side Separated Twisted Does this prove that if wires are separated by 2 cm it won t work? What if you tried this wiring configuration and the communication failed? 71

72 Immunity Simulation Can Still be Valuable Measurements typically provide YES or NO answers Simulations fill in the gaps when conducting troubleshooting investigations Root cause is the circuit weak at this frequency? Or is there a large amount of RF coupling into the circuit caused by a resonance? Is this relating to the test setup or product geometry? Often one can find a fix to the problem while the product is still in the lab. This usually requires cuts and jumpers as a lab fix. If a problem can be reproduced in simulation, then the lab fix can then be simulated along with a production fix. Therefore we have a better idea that the production fix works just as well or better than the lab fix. 72

73 Measurements & Simulations 73

74 Fishing Someone may throw a product layout over the wall and this person would like some simulation done of their product Simulation tools are not very effective at discovering problems More useful information can be obtained by keeping a narrow scope for the analysis Measurements can help bound the investigation 74

75 Actual Product 2 Layer PCB Had sensor malfunctions during immunity testing from MHz Near-field probe investigation has determined that the area near the sensor s data trace is susceptible to EMI 75

76 Near Field Probe Position Initial Position Moved 5cm 76

77 Initial Position Antenna Hidden Moved 5cm 77

78 Magnitude Comparison Initial Position Shifted 5cm 78

79 Antenna 90 with Trace 79

80 Comparison Initial Position Shifted 5cm 80

81 Simulation Setup Replicating a near-field probe investigation would take much time to perform using a simulation tool A diagnostic method that is convenient for laboratory measurements is not always best with simulation A plane wave source will first be used in the model to see if the problem can be duplicated without constructing the near-field antenna 81

82 Simulation Result of Baseline PCB 379 MHz (3pF load assumed at trace ends) Sensor Processor The product had sensor malfunctions during immunity testing from MHz 900 MHz 82

83 Product Product PCB vs. Ideal PCB Ideal 83

84 Comparison Product PCB Ideal PCB 84

85 Make Ideal PCB Product Size Product PCB Product Size Infinite PCB 85

86 Comparison Finite Size Trace near PCB center Finite Size Trace near PCB Edge 86

87 Comparison Results Product PCB Close to Edge Far From Edge 87

88 Conclusion The immunity problem appears to be caused by EM coupling from the edge of the PCB near 900 MHz The Immunity problem would be reduced if the trace is moved away from the PCB edge The problem at 380 MHz was able to be reproduced in the full PCB simulation but the cause has not been found 88

89 Schematic Sensor Processor Resistors will make trace lossy 89

90 Resistors Product Sensor Trace on Ideal PCB 90

91 With and Without Resistors R= 0 R= 47 Resistors appear to do a great job with dampening resonance why is there a problem with this trace? 91

92 Electric Field at 379 MHz on Product PCB Sensor Trace It appears at 379 MHz the power trace is resonant and runs parallel to the sensor trace 379 MHz Power Trace 92

93 Comparison with Power Trace Removed With Power Trace W/O Power Trace 93

94 380 MHz Immunity Problem Upon further investigation it is shown that the immunity problem at 380 MHz is caused by cross coupling from a nearby resonant power trace. Either the resonance can be dampened or the trace can be moved further away from the sensor trace to solve this problem 94

95 Summary Simulation can be used to find the root causes of problems that may exist in products in the testing phase of development by breaking down the problem into a simpler situation These simulations are more effective if guided by measurement data 95

96 EMC Simulation and Measurements EMC simulation and measurement methods both have their strengths and weaknesses but they do not have many weaknesses in common Using EMC simulation and measurement together can provide useful data more than what one or the other can provide by itself. 96

97 References / Acknowledgements Piper, S. and Teune, J., "Verifying Electromagnetic Simulation Results with Measurements," SAE Int. J. Passeng. Cars - Electron. Electr. Syst. 5(1):2012, doi: / Paul, Clayton. Introduction to Electromagnetic Compatibility. Wiley, 2006 Test PCBs were provided by Gentex Corporation Test facilities were provided by Gentex Corporation and Woodward MPC, Inc. 97