Bogatin: 3 Confusing Topics Slide -1web.mst.edu/~jfan/slides/Bogatin1.pdf · Bogatin: 3 Confusing...
Transcript of Bogatin: 3 Confusing Topics Slide -1web.mst.edu/~jfan/slides/Bogatin1.pdf · Bogatin: 3 Confusing...
Slide -1Bogatin: 3 Confusing Topics
IEEE EMC Distinguished Lecturer Series
The Three Most Confusing Topics in Signal Integrity
and how not to be confused
with
Dr. Eric Bogatin, Signal Integrity Evangelist,
Bogatin Enterprises, www.beTheSignal.com
Slide -2Bogatin: 3 Confusing Topics
IEEE EMC Distinguished Lecturer Series
Overview
• Why do we care?
• Specific examples:
Characteristic Impedance
Differential Impedance
Inductance
• Got your own? Send me a note
Slide -3Bogatin: 3 Confusing Topics
IEEE EMC Distinguished Lecturer Series
What are the Designer’s Most Important Tools?
From Here To Here Or Here
?
Creativity and intuition are the key ingredients to the design process
Slide -4Bogatin: 3 Confusing Topics
IEEE EMC Distinguished Lecturer Series
What Does it Mean to Refer to a Cable as a “50 Ohm Cable”?
Ω
3 foot long 50 Ohm coax
Slide -5Bogatin: 3 Confusing Topics
IEEE EMC Distinguished Lecturer Series
What is the Most Important Electrical Quality the Signal Cares About?
GROUND
VVsignal
Signal path
Return pathVVin
Ans: the instantaneous impedance
Slide -6Bogatin: 3 Confusing Topics
IEEE EMC Distinguished Lecturer Series
Electrical Model of a Lossless Transmission Line
( ) ( )t,xIt
Lt,xVx ∂
∂−=
∂
∂ ( ) ( )t,xVt
Ct,xIx ∂
∂−=
∂
∂
( ) ( )t,xVxLC
1t,xV
t 2
2
2
2
∂
∂=
∂
∂ ( ) ( )t,xIxLC
1t,xI
t 2
2
2
2
∂
∂=
∂
∂
Telegraphers’ equation
Wave equation
C
LZ0 = LCTD =derive
Slide -7Bogatin: 3 Confusing Topics
IEEE EMC Distinguished Lecturer Series
“…be the signal”
+ + + + +
- - - - -
Charging up a transmission line
Instantaneous Impedance, I
VZ =
Slide -8Bogatin: 3 Confusing Topics
IEEE EMC Distinguished Lecturer Series
Geometry, Current and Impedance
Line width decreases, capacitance ______________, impedance ______________
Dielectric thickness increases, capacitance ___________, impedance, ___________
Line width increases, capacitance ______________, impedance______________decreases
increases
increases
decreases
decreases
increases
Slide -9Bogatin: 3 Confusing Topics
IEEE EMC Distinguished Lecturer Series
What Does it Mean to Have a 50 Ohm Line?
Ω
Verrrry longgggg 50 Ohm coax
Slide -10Bogatin: 3 Confusing Topics
IEEE EMC Distinguished Lecturer Series
The Input Impedance of a Transmission Line is Time Dependent
Imp
ed
an
ce (
Oh
ms)
TimeMany round trip time of flights
open
Round trip time of flight
Characteristic impedance
Slide -11Bogatin: 3 Confusing Topics
IEEE EMC Distinguished Lecturer Series
“…the impedance” of a Transmission Line is Ambiguous
• The input impedance of the transmission line - may be time dependent
• The instantaneous impedance of the transmission line
• The Characteristic impedance of the transmission line
Slide -12Bogatin: 3 Confusing Topics
IEEE EMC Distinguished Lecturer Series
2nd topic: Differential Impedance
• What is differential impedance and how does coupling affect it?
Difference signal
Slide -13Bogatin: 3 Confusing Topics
IEEE EMC Distinguished Lecturer Series
Differential mode
Common mode
A Secret to Minimize Confusion About Differential Impedance
ThinkThink::
Differential signalsDifferential signals
Common signalsCommon signals
Odd modeOdd mode
Even modeEven mode
Slide -14Bogatin: 3 Confusing Topics
IEEE EMC Distinguished Lecturer Series
Essential Principle
Differential impedance is the instantaneous impedance the
differential signal sees
Slide -15Bogatin: 3 Confusing Topics
IEEE EMC Distinguished Lecturer Series
The Differential Signal
What is the impedance the difference signal sees?What is the impedance the difference signal sees?
V = 0v1vV = 1v0v
Differential signal
1
2
Difference voltage = 2v : -1v +1v
-1
+1
Slide -16Bogatin: 3 Confusing Topics
IEEE EMC Distinguished Lecturer Series
Zdiff
Differential Impedance and Series Impedances
What is the equivalent impedance between the two signal lines?
Zdiff = Z0 + Z0
Zdiff = 2 x Z0
with no coupling:with no coupling:
Z0 Z0
What happens to the impedance of one line when we turn on coupling?
Slide -17Bogatin: 3 Confusing Topics
IEEE EMC Distinguished Lecturer Series
Other Line Is Tied Low
Z0, Second Trace Pegged Low
40
42
44
46
48
50
52
54
56
58
0 5 10 15 20 25 30 35 40 45 50
Edge to Edge Spacing Between the Traces (mils)
Sin
gle
-En
ded
Im
pe
dan
ce (O
hm
s)
Z0, Second Trace Pegged Low
Slide -18Bogatin: 3 Confusing Topics
IEEE EMC Distinguished Lecturer Series
Other Line Driven Opposite
40
42
44
46
48
50
52
54
56
58
0 5 10 15 20 25 30 35 40 45 50
Edge to Edge Spacing Between the Traces (mils)
Sin
gle
-Ended Im
pedance (O
hm
s)
Z0, Second Trace Pegged Low
Z0, Both Traces Driven Opposite
return
+1 v -1 v
Vdiff = V1 – V2
ODD MODE
Slide -19Bogatin: 3 Confusing Topics
IEEE EMC Distinguished Lecturer Series
Relating the Modes’ Impedance to the Impedance the Signals See
ZoddZodd
Zdifferential
oddaldifferenti Zx2Z =
What is the equivalent impedance between the two signal lines?
with coupling:with coupling:
As coupling increases, differential impedance will: _______________decreasedecrease
Slide -20Bogatin: 3 Confusing Topics
IEEE EMC Distinguished Lecturer Series
What’s Inductance?
Slide -21Bogatin: 3 Confusing Topics
IEEE EMC Distinguished Lecturer Series
Deficiencies with Text Book Definitions
• Too mathematical to provide insight
• Deals with coils, not traces on a board
Slide -22Bogatin: 3 Confusing Topics
IEEE EMC Distinguished Lecturer Series
Inductance Principles -1
photo source: Halliday and Resnick, Physics, 1962
wire carrying a current
Rings of magnetic field lines are Rings of magnetic field lines are around all current carrying around all current carrying
conductorsconductors
What influences the total number of rings of field lines?
Right hand rule
Slide -23Bogatin: 3 Confusing Topics
IEEE EMC Distinguished Lecturer Series
Inductance Principles -2
2. Inductance is the number of rings of magnetic field 2. Inductance is the number of rings of magnetic field lines around a conductor, per amp of current lines around a conductor, per amp of current through itthrough it
Units: Webers/amp = Henry
nH more common
Many flavors of inductance:
self self mutualmutual
loop loop partialpartial
total, net or effectivetotal, net or effective
Inductance is a measure of the efficiency of a conductor to create
rings of magnetic field lines at the cost of current
- high inductance, lots of field lines
Slide -24Bogatin: 3 Confusing Topics
IEEE EMC Distinguished Lecturer Series
Inductance Plays a Pivotal Role in Signal Integrity
• Signal propagation:
loop self inductance
• Discontinuities:
loop self inductance
• Cross talk:
loop mutual inductance
• PDN and rail collapse:
loop self inductance
• Ground bounce:
total inductance of the return path
• Hacking interconnects: performance physical design
Partial self and partial mutual inductance
Slide -25Bogatin: 3 Confusing Topics
IEEE EMC Distinguished Lecturer Series
Most Important Principle (#3) of Inductance
1831 Christmas lecture to the Royal Society:
Faraday demonstrates “switching noise”
Michael Faraday's discovery:
Induction: dB/dt V I
ChangingChanging magnetic field
lines around a conductor
induce a voltage, which
drives a current.
V I B ?B V
Slide -26Bogatin: 3 Confusing Topics
IEEE EMC Distinguished Lecturer Series
Summary
• Why do we care?
• Specific examples:
Characteristic Impedance
Differential Impedance
Inductance
• Got your own? Send me a note
Slide -27Bogatin: 3 Confusing Topics
IEEE EMC Distinguished Lecturer Series
Thanks for listening!