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EMC coupling from Australian uni

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EM Coupling Phenomena General Remarks • One of the most fundamental issues in EMC is coupling. We can distinguish four coupling mechanisms: – Galvanic or conductive coupling. – Capacitive near field coupling. – Inductive near field coupling. – Electromagnetic far field coupling. • A way the coupling can be quantified is by using the concept of transfer impedance ZT. ZT = Vsink I source c) WATRI – 2007 Part II - EM Coupling Phenomena 1 http://www.watri.org.au Dr Franz Schlagenhaufer Electromagnetic Basics (1) James Clerk Maxwell (1831 – 1879), Coulomb (1736-1806), Ampere (17751836), Faraday (1791-1867), Oersted (1777-1851) Volta (1745-1827), Gauss (1777-1855), Hertz (1857-1894), Marconi (1874-1937). ∇ × H = J + ∂D ∂t ∇ × E = − ∂B ∂t J = σE ∇⋅B = 0 ∇2Es −γ 2Es = 0 ∇⋅D = ρv c) WATRI – 2007 Part II - EM Coupling Phenomena 2 http://www.watri.org.au Dr Franz Schlagenhaufer Electromagnetic Basics (2) • Electric charges are associated with electric fields. • Moving electric charges (currents) are associated with magnetic fields. • Time varying electric fields are associated with changing magnetic fields. • Time varying magnetic fields are associated with changing electric fields. • This inter-dependence between varying electric and magnetic fields yields to wave propagation. c) WATRI – 2007 Part II - EM Coupling Phenomena 3 http://www.watri.org.au Dr Franz Schlagenhaufer Electromagnetic Theory – Motivation • Emissions, in the radiated or conducted form, are usually an unwanted side effect caused by physical and electrical characteristics of circuits. • These characteristics are often related to parasitic parameters and the non-ideal behavior of components. • These parameters are not explicitly shown in schematics and layout diagrams. • In the case of immunity problems, the source of the disturbance and the coupling path are not always obvious. • To avoid EMC problems, a few fundamental electromagnetic principles must be followed. • Knowledge of these principles will give an understanding, why certain guidelines and rules should be applied. • It will also allow to chose the best solution for each particular case, and to develop tailored solutions for special problems. c) WATRI – 2007 Part II - EM Coupling Phenomena 4 http://www.watri.org.au Dr Franz Schlagenhaufer Principle of Cancellation (1) z z I H Pr Ampere: ∫ Hr ⋅ dlr = Ienclosed 2h loop H Pr One line current: Hr = I 2π r Two parallel line currents Hr = 2π ⋅ I (r − h) − 2π ⋅ I (r + h) = I 2π (r + h) r2 − − (r h2 − h) ≈ I 2π 2h r2 h (for : r >> h) c) WATRI – 2007 Part II - EM Coupling Phenomena 5 http://www.watri.org.au Dr Franz Schlagenhaufer Principle of Cancellation (2) Dependency on distance: single and dual conductors, influence of separation c) WATRI – 2007 Part II - EM Coupling Phenomena 6 http://www.watri.org.au Dr Franz Schlagenhaufer Basic Radiators - Hertzian dipole +Q  q(t)  = I0 ω sin ωt   l I0 (i(t) = I0 cosωt) -Q  q(t)  = − I0 ω sin ωt   Er = Z0 pe cosθ ⋅ − e j r r0 2π r02  (r 1 / r0 )2 − (r / j r0 )3  Eθ = Z0 pe sin θ ⋅ − e j r r0 4π r02  (r j / r0 ) + 1 (r / r0 )2 j − (r / r0 )3  Hφ = pe sin θ ⋅ − e j r r0 4π r02  (r j / r0 ) + 1 (r / r0 )2  z θ r Dipole moment: pe = I0 ⋅ l (l: dipole length) Impedance of free space: Z0 = 120π Ω ≈ 377 Ω Transition point: near - far field: r0 = λ 2π c) WATRI – 2007 Part II - EM Coupling Phenomena 7 http://www.watri.org.au Dr Franz Schlagenhaufer Basic Radiators – Small current loop i(t) = I0 cosωt A Hr = pm cosθ ⋅ − e j r r0 2π r03  (r 1 / r0 )2 − (r / j r0 )3  Hθ = pm sin θ ⋅ − e j r r0 4π r03  (r j / r0 ) + (r 1 / r0 )2 − (r / j r0 )3  Eφ = Z0 pm sin θ ⋅ − e j r r0 4π r03  (r j / r0 ) + 1 (r / r0 )2  z θ r Dipole moment: pm = I0 ⋅ A (A: loop area) Impedance of free space: Z0 = 120π Ω ≈ 377 Ω Transition point: near - far field: r0 = λ 2π c) WATRI – 2007 Part II - EM Coupling Phenomena 8 http://www.watri.org.au Dr Franz Schlagenhaufer Near/Far field behaviour ~1/r3 ~1/r2 r = r0 E H = r0 r Z0 ~1/r E H = Z0 ~1/r3 ~1/r2 r = r0 E H = r r0 Z0 ~1/r E H = Z0 c) WATRI – 2007 Part II - EM Coupling Phenomena 9 http://www.watri.org.au Dr Franz Schlagenhaufer Current Return Path (1) The return current will usually take the path of least impedance! At low frequencies, this path will depend on the resistance characteristic of the structure. At high frequencies the impedance characteristics are dominated by inductances. (At very high frequencies, also stray capacitances may play a role.) Mother Nature sets up the return path in order to obtain the lowest possible impedance for the pair. c) WATRI – 2007 Part II - EM Coupling Phenomena 10 http://www.watri.org.au Dr Franz Schlagenhaufer Current Return Path (2) c) WATRI – 2007 Part II - EM Coupling Phenomena 11 http://www.watri.org.au Dr Franz Schlagenhaufer Reduction due to Ground Plane (1) Current loop (0.2m X 0.2m) with uniform current of 1mA. Maximum electric field in 10m distance for: • Loop in free space • Loop 0.1m above perfect, infinite ground plane • Loop 0.1m above finite ground plane (1m X 1m) • Slot in ground finite ground plane. c) WATRI – 2007 Part II - EM Coupling Phenomena 12 http://www.watri.org.au Dr Franz Schlagenhaufer Reduction due to Ground Plane (2) Current in ground plane (200 MHz) Current in ground plane with slot (200 MHz) Note the high current density along the edges of the slot. c) WATRI – 2007 Part II - EM Coupling Phenomena 13 http://www.watri.org.au Dr Franz Schlagenhaufer Reduction due to Ground Plane (3) Free space Infinite ground plane Slotted ground plane Finite ground plane c) WATRI – 2007 Part II - EM Coupling Phenomena 14 http://www.watri.org.au Dr Franz Schlagenhaufer Reduction due to Ground Plane (4) Excitation in each case: Current source: 20 mA The difference between large and small loop should always be a factor of 10. However, the near field observation point is very close to the large loop (> 1 ωC2  ⇒ Z2 ≈ 1 jωC2 ⇒ V2 VSource ≈ C12 C2 + C12 To reduce capacitive coupling: - decrease C12 (increase distance, decrease cross section) - increase C2 (decrease distance to ground, increase cross section) c) WATRI – 2007 Part II - EM Coupling Phenomena 27 http://www.watri.org.au Dr Franz Schlagenhaufer Capacitive coupling – Asymptotic behaviour (2) V2 VSource ≈ C12 C2 + C12 V2 VSource ≈ jωC12 R2 c) WATRI – 2007 Part II - EM Coupling Phenomena 28 http://www.watri.org.au Dr Franz Schlagenhaufer Capacitive coupling - Influence of termination Vsource = 1 V l = 0.5 m 1 MΩ RNear = RFar = 1 MΩ RNear RFar RNear = RFar = 10 kΩ RNear = RFar = 100 Ω c) WATRI – 2007 Part II - EM Coupling Phenomena 29 http://www.watri.org.au Dr Franz Schlagenhaufer Inductive Coupling General remarks • Coupling due to currents (magnetic fields); • Assumption: electrically short conductor (much shorter than a wavelength); • Uniform current along the conductors; • Critical parameter: inductance; • Most effective for low impedance circuits (source and sink). c) WATRI – 2007 Part II - EM Coupling Phenomena 30 http://www.watri.org.au Dr Franz Schlagenhaufer Inductive coupling – Model (1) L1 M L2 RFar ( ) RNear a = h1 − h2 2 + d 2 a' = (h1 + h2 )2 + d 2 Per-unit-length inductances for two cylindrical conductors above a perfect ground plane L1' = µ 2π ln 2h1 r1 ; L'2 = µ 2π ln 2h2 r2 ; M' = µ 2π ⋅ ln a' a 1 a12 2 a12’ M'= µ 2π ⋅ ln a12' ⋅ a1'2 a12 ⋅ a1'2' a1’2 2’ 1’ a1’2’ 1 2 1’ 2’ Strongest coupling 1 2 2’ 1’ Weakest coupling c) WATRI – 2007 Part II - EM Coupling Phenomena 31 http://www.watri.org.au Dr Franz Schlagenhaufer Inductive coupling – Model (1) Coupling model: simple – ideal current source driving circuit 1 V=jωMI1 L2 Znear+Zfar Low Frequency: I2 High Frequency: (R2 >> ωL2 ) ⇒ (R2 << ωL2 ) ⇒ I2 I1 ≈ jωM R2 I2 I1 ≈ M L2 Coupling model: considering real current/voltage source driving circuit 1 Z s1 ~ L I 1 1 Z L1 M V s L 2 Z s2 V i Z L2 c) WATRI – 2007 Part II - EM Coupling Phenomena 32 http://www.watri.org.au Dr Franz Schlagenhaufer Inductive coupling - Influence of termination Isource = 20 mA l = 0.5 m 1 mΩ RNear RFar RNear = RFar = 1 mΩ RNear = RFar = 10 Ω RNear = RFar = 100 Ω c) WATRI – 2007 Part II - EM Coupling Phenomena 33 http://www.watri.org.au Dr Franz Schlagenhaufer Capacitive and inductive coupling Isource = 20 mA l = 0.5 m 1 kΩ 100 Ω 100 Ω c) WATRI – 2007 Part II - EM Coupling Phenomena 34 http://www.watri.org.au Dr Franz Schlagenhaufer Example: Capacitive Coupling C 12 ZL1 ZL2 C12 Vs C1 C2 Vi Vi = Vs ⋅ C C12 2 + C12 C1 Vi Equivalent electric circuit Zs1 Vs C2 ~ Zs2 74 HC/HCT family or equivalent: ZS1 = 150 Ω   C1 = C2 ≅ l[m] ⋅    9wε h r  ⋅ ln 56(ε r 2h t + − 1) 4h 2 t2   − 1    pF track width w = 0.5 mm, distance between the tracks d = 0.5 mm, track thickness t = 35 µm, track height above reference plane h = 1.8 mm track length l = 100 mm and l << λmin, (smallest wavelength considered) C12 ≅ l[m]⋅ 6.41 +  εa + εr 2  ⋅  w d pF C1 = C2 = 5.1 pF C12 = 2.5 pF Vs C1 C12 C2 Vi Ceff = C12 + C1C2 C1 +C2 Note: The use of not grounded land can increase the coupling between tracks! c) WATRI – 2007 Part II - EM Coupling Phenomena 35 http://www.watri.org.au Dr Franz Schlagenhaufer Example: Capacitive Coupling Zs1 Vs ~ C12 C1 Zs2 ZL1 ZL2 Vi C2 5.0V 2.5V 0V 0s Uout 5.0V 2.0ns Uin 4.0ns (6.0ns,0.20V) 6.0ns 8.0ns Time 2.5V c) WATRI – 2007 http://www.watri.org.au Dr Franz Schlagenhaufer (2.4ns,0.35V) 0V 0s Uout 2.0ns Uin Part II - EM Coupling Phenomena 4.0ns 6.0ns 8.0ns Time 36 Example: Inductive Coupling Zs1 Vs ~ Zs2 ZL1 ZL2 Vi Zs1 ~ I1 L1 ZL1 M Vs L2 Zs2 ZL2 Equivalent electric circuit k= Vi M L1L 2 k = 0.78 74 HC/HCT family or equivalent: ZS1 = 150 Ω track width w = 0.5 mm, distance between the tracks d = 0.5 mm, track thickness t = 35 µm, track height above reference plane h = 1.8 mm track length l = 100 mm and l << λmin, (smallest wavelength to be considered) Self inductance: (t << w) L1 = L2 ≅ 0.2 ⋅ l[m]⋅   ln 8⋅l w − 1  µH Mutual inductance M12 ≅ 0.2 ⋅ l[m]⋅ ln  2⋅l d + d l − 1  µH L1 = L2 = 0.13 µH M12 = 0.10 µH c) WATRI – 2007 Part II - EM Coupling Phenomena 37 http://www.watri.org.au Dr Franz Schlagenhaufer Example: Inductive Coupling ZL1 ZL2 Vi Zs1 Vs ~ Zs2 5.0V 2.5V 0V 0s Uout 2.0ns Uin 5.0V 4.0ns (6.0ns,0.14V) 6.0ns 8.0ns Time c) WATRI – 2007 http://www.watri.org.au Dr Franz Schlagenhaufer 2.5V (2.0ns,0.39V) 0V 0s Uout 2.0ns Uin Part II - EM Coupling Phenomena 4.0ns 6.0ns 8.0ns Time 38 Example – Coupling (1) Source (digital signal) Termination Radio Antenna receiver Termination Wire attached to passive trace Variants:Distance between traces Ground plane: no / yes Shielding trace: no / yes (not grounded) / yes (grounded) c) WATRI – 2007 Part II - EM Coupling Phenomena 39 http://www.watri.org.au Dr Franz Schlagenhaufer Function generator RG = 50 Ω f = 18.9 MHz EM Coupling (Demo) Measurement setup Source trace Oscilloscope RI = 50 Ω 50 Ω Termination Sink trace Oscilloscope RI = 50 Ω c) WATRI – 2007 Part II - EM Coupling Phenomena 40 http://www.watri.org.au Dr Franz Schlagenhaufer Example – Coupling (3) PCB Rise time for No. source trace Ns 1 6.7 2 6.6 5 6.5 6 5.5 11 5.5 12 5.5 15 5.5 16 5.4 PCB No. Trace width mm 1 0.25 2 0.25 5 0.25 6 0.25 11 0.25 12 0.25 15 0.25 16 0.25 Fall time for source trace Ns 7.2 7.0 6.9 5.6 5.8 5.6 5.7 5.4 Distance between traces Land fill between traces, width Reference plane mm mm 0.25 Nothing No 2.5 Nothing No 2.5 Floating, 2.0 No 2.5 Grounded at both ends, 2.0 No 0.25 Nothing Yes 2.5 Nothing Yes 2.5 Floating, 2.0 Yes 2.5 Grounded at both ends, 2.0 Yes Maximum voltage on sink trace mV 340 260 275 16 120 35 48 4.5 Interference to 94.5FM Radio with the shown setup Yes Yes Yes No Yes No No No c) WATRI – 2007 Part II - EM Coupling Phenomena 41 http://www.watri.org.au Dr Franz Schlagenhaufer Cable Transfer Impedance ~ Ioutside Vinside Z 'T = dVinside dz 1 I outside Table: Typical values for cable transfer impedance Z’T in mΩ/m Cable type 10 kHz 100 kHz 1 MHz 10 MHz 100 MHz Wire with metallic film 100 200 500 5,000 30,000 Single shield (poor quality) Single shield (good quality) Double shield 10 15 8 30 500 9 10 7 2 20 3 2 0.3 0.3 3 Double shield with magnetic film 0.6 0.06 0.002 0.01 0.1 c) WATRI – 2007 Part II - EM Coupling Phenomena 42 http://www.watri.org.au Dr Franz Schlagenhaufer Cable Transfer Impedance c) WATRI – 2007 http://www.watri.org.au Dr Franz Schlagenhaufer Phase [°] |ZT`| [Ohm/m] 10 1 0.1 RG 178 RG 180 RG 316 RG 174 RG 62 RG 179 0.01 RG 58 RG 11 0.001 1e+4 15 0 -15 -30 -45 -60 -75 -90 -105 -120 -135 1e+4 RG 59 RG 63 RG 213 1e+5 1e+6 1e+7 Frequenz [Hz] 1e+8 RG 174 RG 180 RG 11 RG 213 RG 59 RG 62 RG 63 RG 179 RG 58 RG 178 RG 316 1e+5 1e+6 1e+7 Frequenz [Hz] 1e+8 Part II - EM Coupling Phenomena 43 Cable Transfer Impedance Z 'T = dVinside dz 1 I outside Short cable: Vinside = Z 'T I outside l Example: Z’T = 500 mΩ/m; Ioutside = 10 mA; l = 10 m ⇒ Vinside = 50 mV With connector: Vinside = (Z 'T l + Z connector )I outside c) WATRI – 2007 Part II - EM Coupling Phenomena 44 http://www.watri.org.au Dr Franz Schlagenhaufer

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