ECE414/514 Electronics Packaging Spring 2012 Lecture 5 Electrical C: Transmission lines (Transmission line reflections) Lecture topics

Transcription

1 ECE414/514 Electronics Packaging Spring 2012 Lecture 5 Electrical C: Transmission lines (Transmission line reflections) James E. Morris Dept of Electrical & Computer Engineering Portland State University 1 Lecture topics Reflection concepts Reflection coefficients Basic cases: Matched and open circuit Generalized mismatches Source and load Bounce chart/lattice diagram Reflections from discontinuities EMI and EMC 2 1

2 Objectives Understand origins of reflections Determine reflection waveforms by bounce chart Calculate effects of discontinuities Calculate EM radiation and susceptibility in basic case 3 1. Reflection Coefficient 4 2

3 5 2. Basic examples 3 significant cases: VL = (R L -Z 0 )/ (R L +Z 0 ) Matched load R L = Z 0 VL = 0 Open circuit load R L = VL = +1 Short circuit load R L = 0 VL = -1 Similarly at the source end: VS = (R S -Z 0 )/ (R S +Z 0 ) R S = Z 0 VS = 0 R S = VS = +1 R S = 0 VS =

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6 3. Mismatched load &/or source 1 volt pulse, Z 0 = 78 Consider R L = 78 cases first, then vary R L (a) R S =Z 0 (b) R S << Z 0 (c) R S >> Z

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8 4. Bounce chart/lattice diagram Calculate reflection coefficients Distance / time diagram Transfer data to waveform plots Examples: (1) Open circuit line (2) Two segment line 15 (1) Γ VS =(80-50)/(80+50) = Γ VL =

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11 21 5. Discontinuities Capacitive loads (MOSFETs) Stubs Multiple loads Line geometry changes bends vias Non-linear loads 22 11

12 Initially R L =0 Γ L =

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18 Summary 35 EMI/EMC Models: Loop/Dipole Antennas 36 18

19 L I 1 d I 2 L R s d H n E t R L Electromagnetic compatibility/interference models EMI/EMC Models: Emissions At far field distance r ( =c/f) from the line of length L and area A = L.d, the radiated electric field strength is E = E D + E CM where E D due to the differential current I D is E D = x f 2 A I D /r V/m and E CM due to the common mode current I CM is (for L ) E CM = 4 x 10-7 f 2 L I CM /r V/m where I D = (I 1 + I 2 )/2 and I CM = (I 1 -I 2 )/2. Common mode currents are ideally zero, but small values can lead to CM dominance over differential. For the differential current, the maximum value can be taken to be the supply current, but the user must specify a non-ideal common mode estimate. For digital systems, use f = 2 /t r. For other geometries, other expressions for A are valid. There are many different standards for EM radiation limits, but for guidance the EU limit is E 100 V/m at r = 10m (class A) or 3m (class B)

20 EMI/EMC Models: Susceptibility jw o L d H n + V s - R s -jwcl d E t R L V L Susceptibility For the line shown, with capacitance C per unit length, e.g. C = r 0 /ln(d/r w ) for parallel wires, the induced voltages are V S = -j [R L R S / (R L + R S )] [Ld] [C - ( 0 / 0 )/R L ] V L = -j [R L R S / (R L + R S )] [Ld] [C + ( 0 / 0 )/R S ] where E = E t = 0 H n, and 0 = 120 = 377. As an example of an EU standard, the device must function in a field of E = 3V/m