Transcript TDR Techniques

Impedance Measurements
on a PCB
PCB Transmission Lines Impedance
and Interconnects Measurements
Using TDR-Techniques
Tuomo Heikkilä
Systems Applications Engineer
Network Solutions
Applications Project Center
p
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 1
Tektronix Oy
Piispantilankuja 2A
02240 ESPOO
puh:
GSM:
email:
09-4783 400
040-506 4401
[email protected]
Impedance Measurements on PCB
–
–
–
–
–
–
–
–
–
Speed of Signal
Transmission Line on a PCB
TDR Concept
Microstrip Line Impedance Measurement
Differential Line on PCB
The Impedance?
Differential TDR
CrossTalk on PCB
Tektronix CSA8000 – TDR Sampling Oscilloscope
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 2
Speed of the Signal
– Edge Rise Time
0.5
fknee =
tr
tr
– Circuit design must be extended up to the Knee
Frequency!
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 3
sin(x)/x ; tr = 0
sin(x)/x [tr = 0]
*
sin(x)/x [tr = 1.3ns]
fknee = 0.5/tr
(385MHz)
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 4
Signal Speed vs Physical dimension
– Edge Length over Substrate
D
Q
C
– If length of line longer than
1/6 of rise time, line behaves
as a Transmission Line
– Propagation Delay Time
– Change of Impedance causes
a Reflection
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 5
tr
Two models for transmission lines
– Lumped model: single component value for the line
–short line, long rise time
–no indication of location
– Distributed model: each section of line separated and
valued for the transmission line
–long line, short rise time
–individual values at each
location
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 6
PCB Microstrip – Transmission Line
Strip Width
PCB Thickness
e
Impedance level of
the microstrip line
is a function of:
• Dielectricity (e) of the substrate
• Board Thickness
• Strip Width.
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 7
Ground Plane
Coupling in Microstrip line
Microstrip Line
Any time when there is Current flow in two
Conductor system inside the Magnetic
field, there is an Inductance involved
Ground Plane
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 8
Any time when there is Voltage
between Planes, there is a
Capacitance involved
Microstrip – Line Discontinuities
Z
u
Connector
Capacitive
Discontinuity
Inductive
discontinuity
Open
circuit
(no
termination)
u/2
t
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 9
Time Domain Reflectometry – TDR
u
Z
u
Connector
Capacitive
Discontinuity
Inductive
discontinuity
Open
circuit
(no
termination)
u
u/2
2*t
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 10
TDR Overview - Reflection Coefficient
and Impedance
Rho () =
Reflected Amplitude
Incident Amplitude
=
Z T - Z0
Z T + Z0
Tek CSA800 calculates -profile into display waveform:
ZT= Z0
1 +
1 -
Where ZT represents the trace impedance
Z0 is a known impedance
(the characteristic impedance of the TDR system)
 is measured by the oscilloscope
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 11
TDR Overview - Typical System
BW and Rise Time of system must support the resolution : 20GHz = 17.5ps
Sampler
Incident Step
Reflections
50 
Step Generator
Rise Time = 17.5 ps
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 12
Tektronix CSA8000 – with TDR Sampler
50GHz mainframe
80E04:
– 20GHz BW
– 17ps tr TDR-step
generator
– Dual-TDR with
deskew adjustment
– 35ps SystemRiseTime
– ~ 3mm resolution
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 13
TDR – How to set it up?
–
–
–
–
–
–
–
Open Setup Dialog (Windows-like)
Click the TDR Tab
TDR modules only active
Switch On TDR Step Generator
Separately activate acquisition
Select vertical scale Volt,  , 
Click Cx (C3) Preset for TDR Autoset
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 14
Transmission Line
Open end of
Transmission Line
PCB Edge connector
TDR-Step
50  Coax Cable to PCB
Front Panel Connector
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 15
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 16
Differential Line on PCB
Two Microstrip Lines close
enough to have both
inductive and capacitive
coupling;
They also have
coupling to the
ground plane.
Ground Plane
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 17
Differential Line with Coupling to Ground
– Modes of Signal
Differential Mode
(Odd Mode)
+ drive
- drive
– Modal Propagation
(dispersion, wave
shape distortion)
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 18
Common Mode
(Even Mode)
Ground Plane
Impedance?
Definition of Impedance may vary within various
applications > check the application.
For TDR-measurements on HW-design, following is used:
• Zo; Characteristic Impedance
• Differential Impedance
• Commom Mode Impedance
• Odd Mode Impedance
• Even Mode Impedance
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 19
Characteristic Impedance
Zo is the Impedance
between the conductors
when there is no Coupling
to Ground
+ drive
- drive
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 20
This is the same as an
ordinary Transmission Line
Differential Impedance and
applies to Unshielded
Twisted Pair
Differential Mode Impedance
Differential Impedance is the impedance between the
conductors when there is a coupling to Ground.
+ drive
- drive
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 21
Common Mode Impedance
Common Mode Impedance is the impedance between the
short connected conductors and the Ground (when there is a
coupling to Ground)
Short connection
+ drive
+ drive
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 22
Even Mode Impedance
Even Mode Impedance is the impedance between one
conductor and the ground plane when both conductors are
driven with same polarity signal against the ground.
Virtual Short due to equal
voltages on both conductors
+ drive
+ drive
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 23
Odd Mode Impedance
Odd Mode Impedance is the impedance between one
conductor and the ground plane when the coductors are
driven with opposite polarity signal against the ground.
Diff Imped divided into two
Virtual Ground due to opposite
voltages on the conductors
+ drive
- drive
Ground Plane
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 24
CSA8000 TDR – Which does it measure?
To CSA
waveform
Sampler
CSA8000 TDR measures:
• TDR-Profile
• Only against Ground
50 Ohm
Terminator
Transmission Line under test
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 25
CSA8000 TDR – Differential TDR?
CSA Differential TDR measures:
• ODD Mode TDR-Profiles: When conductors are driven
with opposite polarity steps that are simultaneous in time
• EVEN Mode TDR-Profiles: When conductors are driven
with same polarity steps that are simultaneous in time
TDR CH2
TDR CH1
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 26
CSA8000 – Averaged Differential
Impedance
From the ”T” and ”p”-models
following can be calculated:
Averaged Differential Impedance
= TDR odd1 + TDR odd2
Averaged Common Mode Impedance
= TDR even1 // TDR even2
These can be directly acchieved by
the CSA8000 by:
• setting the TDR Polarity
• adjusting the TDR Skew
• Waveform Math
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 27
”T”
p
CSA8000 Waveform Math Dialog
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 28
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 29
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 30
Crosstalk – Why to consider?
While speed in Digital HW increases, one of the
consequencies is, and will be more and more in the
future, that Crosstalk becomes (one of) the major new
bottleneck in succesfull product launches.
HW designers everywhere meet, or will soon meet a
need to measure Crosstalk in PCB’s, in Buses, and
Cable Sets.
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 31
Modeling CrossTalk in Microstrip Bus
• Mutual Inductance and Mutual Capacitance are causes
• Data Edge travelling in the Transmission Line generates:
• Capacitively coupled CrossTalk
• Inductively, ie Transformer coupled CrossTalk
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 32
Capacitively Coupled CrossTalk
– Data Edge arrives an
empty capacitor at the
Aggressor Line
– Edge couples via C
onto the Victim Line
u
Aggressor Line
– Victim Line has a Spike
of Rise Time length
– Polarity is the same
– Spike divides to Two:
one travels to Reverse,
other to Forward
direction
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 33
Victim Line
u
Inductively (Transformer) Coupled CrossTalk
– Data Edge arrives an empty
location
– Current spike fills the location
– Magnetic Field Spike is
generated
– Transformer couples the
voltages on to Victim line
– Positive polarity travels to
Reverse direction
– Negative polarity travels to
Forward, along with the
aggressor spike
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 34
u
i
i
u
u
Two Types of Crosstalk
– Forward CrossTalk, to the destination direction
– Reverse CrossTalk, to the source direction
Aggressor Line
Victim Line
Reverse CrossTalk:
Amplitude is low and
only based on mutual
impedance value.
Capacitive and Inductive
sum up.
Source receives a pulse
that equals length of line.
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 35
Forward CrossTalk:
Faster Edge -> Higher Amplitude.
Amplitude grows up while
travelling the line.
Capacitive and Inductive cancel
out if L and C in balance.
Destination receives a spike.
Effect of CrossTalk
– Depends on:
– is it Forward or Reverse
– drive source impedance
– destination termination impedance
– Consequencies are:
–
–
–
–
Unwanted spikes are generated
Edges suffer shape distortion and jitter
Noise level increases
Reflections are generated if improper terminations
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 36
Measuring CrossTalk
with the CSA8000
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 37
Example 1: Forward with all Terminated
Aggressor Line
Termination 50 
Victim Line
Termination 50
Termination 50 
CSA8000
Filter – function in
CSA8000 will display the
TDR step Gen
aggregated CrossTalk at
the amplitude of
”Filtered Output
Equivalent Aggregated”
XrossTalk
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 38
Example 2: Reverse with all Terminated
Aggressor Line
Termination 50 
Victim Line
Termination 50 
CSA8000
Filter – function in
CSA8000 will display the
TDR step Gen
aggregated CrossTalk at
the amplitude of
”Filtered Output
Equivalent Aggregated”
XrossTalk
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 39
Termination 50 
Example 3: Forward with no Termination
at the end of Aggressor Line
Aggressor Line
Termination 50
Victim Line
Termination 50 
CSA8000
Ordinary forward CrossTalk
Secondary Reverse CrossTalk
from backreflection on Aggressor line
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 40
Line amplitue
reaches full value if
aggressor is not
terminated.
Similar Edge starts
propagation to the
source.
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 41
Example 4: Reverse at the end of Victim
with Low- Drive at Source
Aggressor Line
Termination 50Ohm
Victim Line
ECL source Z = 15..25 Ohms
Line Z = 50 Ohms.
Reverse pulse will reflect
with more than half amplitude
and opposite polarity.
Noise Level Increases.
If destination is not properly
terminated, portion of reflected
reverse pulse will reflect third
time.
Noise Level increases more
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 42
Termination 50 
CSA8000
Example 5: Reverse CrossTalk with
Unterminated Lines
Aggressor Line
Open end reflects original pulse back.
Victim Line
Open end reflects alll pulses back.
Termination 50 
CSA8000
Original Reverse CrossTalk
Forward from Reflected Pulse
added with Reflection of
Original Forward CrossTalk
Reflected Reverse
CrossTalk from Reflected
Original Pulse
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 43
Example 6: Unterminated Lines
Open source reflects
all CrossTalk pulses
back.
Aggressor Line
Victim Line
Open end reflects
original pulse back.
Termination 50 
CSA8000
Several reflections
will occur due to
CrossTalk between
lines from earlier
CrossTalk signals
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 44
Tektronix Solution: The CSA8000 TDR
–
–
–
–
CSA8000 provides with 80E04 dual TDR capability
Can resolve Differential Line TDR Profile
Measures Odd and Even Mode TDR profiles
Common Mode and Differential Impedance Profiles
can be calculated by Waveform Maths
– High Accuracy by 17ps Rise and Skew Adjustment
– CrossTalk Measurements are easy
– Filter transforms from TDR BW down to actual BW
with no errors in LTI-circuits (LTI=Linear Time Invariant)
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 45
Tektronix CSA8000
TDR & CrossTalk Measurements – April 2001, Tuomo Heikkilä
Slide 46