Download Test Methodologies for Today`s Fastest Digital Data Interfaces

Time-domain Reflectometry
(TDR) Measurements
Test Methodologies for Today’s
Fastest Digital Data Interfaces
Intel Developer’s Forum
Asia Pacific
Spring 2000
Z1
Z2
TDR Test Methodologies for
Today's Fastest Digital Data Interfaces
Time-Domain Reflectometry (TDR) test methods are
used to measure the signal transmission path
characteristics in high-speed digital systems.
This lab will review the basic concepts of TDR, then
present some fundamental TDR measurement methods.
These methods will next be applied to some of the
fastest digital data interfaces used in today’s computer
systems
Typical TDR Applications
TDR Measurements Are Used to Characterize the
Signal Transmission Properties Of:
–
–
–
–
Printed Circuit Boards
Connectors
IC Packages
Cables and Interconnects
Typical TDR Measurements
Types of TDR Measurements used to characterize
signal transmission properties include:
–
–
–
–
–
Signal characteristic impedance
Differential signal characteristic impedance
Signal-signal Crosstalk
Signal propagation delay
Inductance and capacitance characterization
When Are TDR Measurements Needed?
Typical applications where TDR Measurements are
needed include:
– To characterize electrical transmission properties in
high bandwidth and high data-rate applications.
– To guarantee the transmission properties meet the
system performance requirements.
– To verify manufacturing processes of PC boards, IC
packages and connectors.
TDR Fundamentals - Applied Step Signal
– Starting with a transmission line with a characteristic
impedance Z equal to Z0
– A fast rise-time step signal is applied at the transmission
line input point.
– The step signal will propagate down the line.
Propagating
Step
Step
Source
Transmission
Line
Characteristic
Impedance Z = Z0
TDR Fundamentals - Impedance Change
– An impedance change in the transmission line will cause
a change in the amplitude of the propagating step.
Impedance
Change Point
Step
Source
Characteristic
Impedance Z1 = Z0
Characteristic
Impedance Z2 > Z0
TDR Fundamentals – Transmitted and
Reflected Signals
– The change in impedance causes some of the power to
be reflected back to the source.
– The remainder of the power will be transmitted.
Transmitted
Step
Incident
Step
Step
Source
Reflected
Step
Z1 = Z0
Z2 > Z0
TDR Fundamentals – Oscilloscope
Monitoring
– Use an oscilloscope to monitor the transmission line
signal at the step source input point.
– The oscilloscope waveform will show the combined sum
of the incident and reflected propagating signals.
Oscilloscope
Measurement
Point
Step
Source
Z1 = Z0
Z2 > Z0
TDR Fundamentals – Oscilloscope
Waveform
– The oscilloscope waveform will show the impedance
change as a change in signal amplitude occurring in
twice the propagation time, Tpd, from the incident step.
Propagation time = 2Tpd
Time ( Sec/Div )
Propagation time = Tpd
Z1 = Z0
Z2 > Z0
TDR Fundamentals - Typical TDR System
– A Typical TDR system consists of TDR sampling head that
generates fast step signals, samples the incident and
reflected signals that are digitized by the oscilloscope.
Transmission Line
50 W
Step
Source
SMA
Connector
To Oscilloscope
Mainframe
Sample-Hold
Gate
TDR Sampling Head
ZLoad
Z0
TDR Measurements – Typical System
– TDR measurements provide a means to get quantitative
characterization data of the transmission system.
– The measurement comparison of the incident and
reflected signals provide the data for analysis.
TDR
Sampling
Head
Load
Z0
ZL
TDR Measurements - Reflection
Coefficient – r (rho)
r is the ratio of the reflected pulse amplitude to the
incident pulse amplitude.
Vreflected
=
r =
Vincident
r can be expressed in terms of the transmission line
characteristic impedance, Z0 , and the load impedance, ZL.
r
=
Vreflected
Vincident
=
( ZL – Z0 )
( ZL + Z0 )
TDR Measurements - Reflection
Coefficient for Matched Load
There are some interesting boundary conditions for the
value of the reflection coefficient, r.
Load
Z0
ZL
– When ZL is equal to Z0 – Matched Load
VReflected = 0 and r = 0
r
=
Vreflected
Vincident
=
0
V
=
0
Reflected Wave is
equal to zero.
No Reflections.
TDR Measurements - Reflection
Coefficient Boundary Values
– When ZL is equal Zero – Shorted Load
VReflected = -VIncident and r = -1
r
=
Vreflected
Vincident
=
-V
=
-1
V
– When ZL is Infinite – Open Load
VReflected = VIncident and r = +1
r
=
Vreflected
Vincident
Reflected Wave is
equal but negative
of incident wave.
=
V
V
=
1
Reflected Wave is
equal to the
incident wave.
TDR Measurements – Open and Shorted
Load TDS 8000 Display
TDR Measurements - Reflection Coefficient
and Impedance
The characteristic impedance, Z0 , or the load
impedance, ZL, can be calculated with the value of r.
ZL = Z0 * ( 1 + r )
Z0 = ZL * ( 1 - r )
(1–r )
(1+r )
Z0
ZL
TDR Measurements – Oscilloscope
Waveform Measurement Units
– Oscilloscope TDR Measurements can use units of Volts,
Ohms or r (Rho) for the vertical magnitude scale.
– The horizontal axis represents unit of time.
Magnitude Units
Volts/Div
-orRho/Div
-orOhms/Div
Time Units - Sec/Div
TDR Measurements – Impedance
Measurements With Cursors
– Using a properly calibrated TDR Oscilloscope the
horizontal waveform cursors can be used to make
impedance measurements.
Measurement
Readouts
Ohms/Div
Cursor 1 = 50.0 W
Cursor 2 = 95.3 W
Cursor 2
Delta 2-1 = 4.7 W
Cursor 1
Sec/Div
TDR Measurements – Propagation Delay
Measurements With Cursors
– The horizontal waveform cursors can be used to make
time and propagation delay measurements.
– The one-way propagation delay is half the time
measured between the incident and reflected waves.
Measurement
Readouts
Ohms/Div
Cursor 1 = 50.0 pS
Cursor 2 = 201.5 pS
Delta 2-1 = 151.5 pS
Prop Delay = 75.75 pS
Cursor 1
Cursor 2
Sec/Div
TDR Measurements – Propagation Delay
and Dielectric Constant
– The relationship between propagation delay,
– Using the medium dielectric constant and the fact the
measured time is twice the propagation time of the
signal.
Propagation Delay = TPD = L * ( eEFF )1/2
VC
Where:
e
• EFF is the effective dielectric constant which is a function
of materials and type of transmission line, strip-line, microstrip, etc.
• L is the length of the trace
• VC is the velocity of light
TDR Measurements – Location
Calculation with Propagation Delay
– The location of a transmission line impedance change can
be calculated.
– Using the medium dielectric constant and the fact the
measured time is twice the propagation time of the signal.
Location of
Impedance
Changed
= D = TMeas * VC
2*(
e
EFF
)1/2
Where:
• D is the distance to the point of
the impedance change that
caused the reflection.
Cursor 1
TMeas
Cursor 2
TDR Measurments – Reflection Diagrams
– Reflection diagrams are useful for calculating and
understanding the propagating wave reflections.
Z2
Z1
Transmitted
Wave
Incident
Wave
Time
Reflected
Wave
Z1
Z2
TDR Measurement – Reflection Diagram
Calculations
– Reflection diagram calculations determine the magnitude
of each reflection using the values of the reflection
coefficient, r and the incident voltage.
Time
Z1
Vreflected =
Z2
Vincident *
r
TDR Measurements – Diagrams for
Multiple Reflections
– Reflection diagrams are useful for calculating and
understanding transmission line with multiple discontinuities.
Z1
Z2
Z4
Z3
Time
Z1
Z2
Z3
Z4
TDR Measurements – Waveform for
Multiple Reflections
– The TDR waveform for multiple reflections shows the
results of the combined reflections from all the impedance
discontinuities.
Z1
Ohms
Time
Z2
Z4
TDR Waveforms - Shorted and Open
Terminations
– Short Circuit Termination
TP
2TP
V
Z
ZL = 0
0
0
– Open Circuit Termination
TP
2V
2TP
V
Z
0
0
ZL = Open
TDR Waveforms - Matched and
Mismatched Load Terminations
– Matched Load Termination
TP
2TP
V
Z
ZL = Z0
0
0
– Mismatched Load Termination
TP
V + VR
ZL > Z0
2TP
V
Z
0
0
ZL < Z0
ZL <> Z0
TDR Waveforms - Capacitor and Inductor
Terminations
– Capacitor Load Termination
TP
2V
2TP
V
Z
ZL = C
0
0
– Inductor Load Termination
TP
2TP
V
Z
0
0
ZL = L
TDR Waveforms - Shunt Capacitance and
Series Inductance Discontinuities
– Shunt Capacitance Discontinuity
TP
2TP
V
Z
0
C
0
Z
0
– Shunt Inductance Discontinuity
TP
2TP
L
V
0
Z
Z
0
0
TDR Waveforms - Inductance and
Capacitance Discontinuities
– Series Inductance – Shunt Capacitance
TP
2TP
L
V
Z
0
C
0
Z
0
– Shunt Capacitance – Series Inductance
TP
2TP
L
V
0
Z
0
C
Z
0
TDR Waveforms - Multiple Inductance
and Capacitance Discontinuities
– Capacitance – Inductance - Capacitance
TP
2TP
L
V
Z
0
C
Z
C
0
0
– Inductance – Capacitance - Inductance
TP
2TP
V
0
L
L
Z
0
C
Z
0
TDR Waveforms - PC Board
Transmission line
– A typical PC board will have impedance controlled PCB
micro-strip and strip-line transmission lines.
– The transmission lines will have components, vias,
connectors, etc., that will create impedance
discontinuities.
Input
TDR Waveforms - PC Board Impedance
Model
– These impedance discontinuities can be modeled as
inductors, capacitors and resistors.
Input
TDR Waveforms - PC Board Impedance
Measurement
– The TDR Oscilloscope waveform will display the
reflections created by the impedance discontinuities.
– Measurements can be made to determine how much the
impedance deviates from the nominal value.
Rambus TDR Measurements – Channel
Impedance Requirements
– The Rambus channel has an impedance specification
of 28 Ohms with a 10% tolerance, +/- 2.8 Ohms.
– Meeting this requirement means careful design and
testing of the PCB transmission lines.
28 W within
10% over
entire system
Rambus TDR Measurements – Multiple
Simultaneous Data Transfers
– Multiple bits can be simultaneously propagating on a single
Rambus channel data line – multiple clock domains.
– Unwanted signal reflections from one data transfer would
effect multiple data transfers on the channel date line.
– Assuring PCB impedances meet specifications is a
requirement for successful Rambus design.
Rambus
Controller
RDRAM
Rambus TDR Measurements – Channel
Timing and Voltage Requirements
– The Rambus 800Mbps data transfer rate requires timing
margins to be maintained with high degree of accuracy.
– Rambus signals use low voltage swings of 0.8V.
– Signal reflections caused by impedance discontinuities
reduce timing and voltage margins.
– All the Rambus channel signals and clock lines must
also be accurately matched to maintain timing margins.
Data
Clock
0.8 V
1.25 nS
Rambus TDR Measurements – PCB
Transmission Line Design
– Determine Microstrip and Stripline PCB designs with
consideration of the PCB fabrication materials and
constraints.
– Use impedance equations, simulation tools and
reference designs for best results – 28 Ohms is low.
– Include a test coupon for testing impedance and
monitoring PCB production lots.
Microstrip
Stripline
Rambus TDR Measurements – TDR Test
Coupon
– A TDR test coupon can be designed on to PCB for testing
impedance and monitoring PCB production lots.
– The coupon should use the same routing guidelines used
for the Rambus channel to represent process.
– The probe land pattern used with the coupon needs to
match the type and method of TDR probing used.
– The far end of the coupon should be open – no pad or via.
Probe
pads
Open
end
Ground
Rambus TDR Measurements –
Instrumentation Setup
– The basic TDR instrumentation for Rambus TDR consist
of a sampling oscilloscope with a TDR sampling head,
with TDR probe and cable.
– Other methods of connecting to the circuit board can be
used including SMA connectors, specialized RIMM
modules, probe fixtures, etc.
Rambus TDR Measurements –
TDR 28 Ohm Calibration
– The TDR measurement system should be calibrated with
a 28 Ohm standard.
– This could be a certified test coupon, specialized
calibration air line, etc.
– TDR instrumentation are 50 Ohm systems. So a 28 Ohm
impedance will be displayed as a downward step.
28 W
Standard
50 W TDR
Sampling Head
50 W
28 W
Rambus TDR Measurements – 28 Ohm
Calibration TDS8000 Display
Rambus TDR Measurements – TDS8000
Display of Rambus Impedance Range
Rambus TDR Measurements – TDR
Probing
– One method to measure the circuit board trace
impedance is to probe the trace with a TDR probe.
– The probe is used to apply the TDR step and measure
the reflections.
– The probe ground lead needs to be as short as possible
and connected to trace ground plane.
TDR probe with
short ground lead
probing trace
Rambus TDR Measurements –
Minimizing Probing Aberrations
– When making 28 Ohm measurements aberrations from
the probe launch can be minimized with careful probing
techniques.
Rambus TDR Measurements – Probing
28 Ohm Trace TDS8000 Display
I used my
finger to lower
bandwidth and
ringing
Rambus TDR Measurements – Rambus
Differential Clock
– There are two pairs of 400MHz differential clock lines,
ClockToMaster and ClockFromMaster.
– Clock and data travel in parallel to minimize timing skew.
– An RDRAM sends data to the memory controller synchronously
with ClockToMaster.
– The controller sends data to the RDRAMs synchronously with
ClockFromMaster.
– Data and Clock transmission lines must be matched in
impedance and length to maintain timing margins.
28 W within
10% over
entire system
Rambus TDR Measurements –
Differential Clock TDR Measurement
– A differential TDR measurement is performed much like
a single-ended TDR measurement.
– Use two TDR sampling head channels with the step
generators set to opposite polarities.
Rambus TDR Measurements –
Differential Clock Coupling
– Attempting to measure the two halves of the differential
pair separately can produce misleading results.
– Two traces in close proximity tend to read a lower
impedance than there characteristic impedance as a pair.
– Proper characterization of the differential impedance of the
Rambus clock traces to maintain voltage and timing
margins.
Rambus TDR Measurements –
Differential TDR Step Timing Skew
– Another important consideration when making differential
TDR measurement is the alignment of the TDR step
pulses.
– The positive and negative going TDR steps must be
adjusted so there is not any time skew between them at
the transmission line launch point.
Rambus TDR Measurements – TDS8000
Differential TDR Display
Rambus TDR Measurements Propagation Velocity Measurements
– TDR is used to make propagation velocity
measurements to compare electrical signal length
of the Rambus data and clock lines.
– Matching the electrical signal length – propagation
time is important to maintain voltage and timing
margins.
Measurement
Readouts
Dielectric K = 4.50
Delta 2-1 = 822.6 pS
Cursor 1
Cursor 2
Distance = 58.17 mm
Sec/Div
More TDR Measurements – Rambus
Coupling and Crosstalk
– Mutual coupling and crosstalk between signal lines
can be characterized with TDR measurements.
– Apply the TDR step on one signal line and measure
the signal strength on the other.
Sampling Oscilloscopes - Low Voltage
Differential Signaling (LVDS)
– Low voltage differential signaling (LVDS) is being used in
many high-speed interface applications.
– LVDS using low voltage swing, high-speed, differential
signaling – similar to Rambus.
– High speed buses are formed by using multiple sets of
LVDS data lines along with a differential clock.
– Transmit and Receive pairs for the bi-directional bus.
TX
RX
RX
TX
Sampling Oscilloscopes - Low Voltage
Differential Signaling (LVDS)
– Sampling oscilloscopes have the features required to
characterize LVDS interfaces.
– LVDS requires differential transmission line
characterization - impedance, coupling, etc.
– LVDS also requires timing and jitter characterization
measurements – eye patterns, noise, etc.
– Sampling oscilloscopes have very high timing accuracy
and low system jitter.
Conclusion – Other TDR Measurement
Considerations
– TDR improved accuracy with baseline level and incident
Amplitude correction.
– Fast TDR step rise-time allows for high resolution
measurements to characterize short transmission line
segments, connectors, etc.
– Parametric modeling with third-party software available
to develop SPICE model, etc.
– Easy to determine the transmission line characteristics
and Instrumentation for measurements can be applied to
manufacturing environments.