what every scope user needs to know about transmission lines · eliminate cable reflections using...
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Teledyne LeCroy Signal Integrity Academy1
What Every Scope User Needs to
Know About Transmission Lines
Dr. Eric Bogatin, Signal Integrity EvangelistDean, Teledyne LeCroy Signal Integrity Academy www.beTheSignal.com
Director, Teledyne LeCroy Front Range Signal Integrity LabAdjunct Prof, Univ of Colorado, Boulder, ECEE
Editor, Signal Integrity Journal, www.SignalIntegrityJournal.com
Teledyne LeCroy Signal Integrity Academy2
A Confusing Aspect of Scope Measurements
When you measure the rise time of the cal signal (really the compensation reference signal) the rise time depends on the length of the cable.
How come? Are we seeing losses in the cable? Is it an RC charging effect?How do we interpret the source features from this sort of measurement?
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Looks like longer cables have longer rise times
3 ft cable6 ft cable
Cal out source is really RT = 5 nsec
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Outline
Five essential principles1. All interconnects are transmission lines2. Signals propagate3. Signals see an instantaneous impedance 4. Signals reflect when they encounter a change in the instantaneous
impedance5. ALL voltage sources have a Vth, Rth, 10-90 rise time
▪ Applied to interpreting scope measurements▪ Common artifacts▪ When to use 1 Meg input, when to use 50 Ohm input
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“A method is a trick that works more than once”- George Polya
Gary Larson:
We will encounter many tricks-some more valuable than others
Pay attention to the more valuable ones
“remember that spot”
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Rule #9: Never do a measurement or simulationwithout first anticipating what you expect to see.
If you are wrong, there is a reason- either the set up is wrong or your intuition is wrong. Either way, by exploring the difference, you will learn something
If you are right, you get a nice warm feeling that you understand what is going on.
Corollary to rule #9:There are so many ways of screwing up a measurement or
simulation, you can never do too many consistency checks
An important habit for ALL Engineers
• Get in the habit of:• Never passing an opportunity to apply rule #9. • Evaluating every measurement with rule #9• Practice thinking of new consistency tests you can perform, and doing them.• “Put in the numbers” at every opportunity
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Essential Principle: All interconnects are transmission lines
GROUND
Signal path
Return pathVVin
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Essential Principle: Signals are Dynamic
GROUND
Signal path
Return pathVVin
All interconnects are transmission lines
A signal as a voltage difference
Signals propagate
Dk
12v secn
inches
secn
inchessecninches
secninches
62
12
4
12In FR4 traces
inches inchesnsec nsec inches
nsec
12 128
1.52.2 In Coax cable
TD for 1 foot coax = 1.5 nsec, 3 feet in 4.5 nsec ~ 5 nsec
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Dynamic Simulation of Propagating and Reflected Signals
Download this free animation tool from
www.beTheSignal.com
VRPW-30-16: Yoshi’s Animations of
Reflections
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VVsignal
Essential Principle: Signals see an instantaneous impedance
GROUND
Signal path
Return pathVVin
ALL Signals ALWAYS propagate
The edge has a spatial extent, where the dV/dt, dI/dt is
The edge sees an instantaneous impedance
The dV/dt
The dI/dt
Frozen in time
I
V
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Really Simple View of the Impedance of a Transmission Line
throughCurrent
appliedVoltageZ
I
V
instantaneous impedance of the transmission line
x
C
C = CLx
Q = CV,
every t =xv
I = Qt
=vC
LxVx
= vCLV
I = Qt
LL vC
1
VvC
V
I
VZ
The characteristic impedance of a transmission line:
The one value of instantaneous impedance in a uniform transmission line
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Essential Principle: The Return Current is Just as Important
as the Signal Current
signal
The current loop has two directions associated with it:
1. A direction of propagation
2. A direction of circulation
They are independent!
+++
=
+++
- - - +++
I
displacement
current
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If the instantaneous impedance changes some of the signal reflects
Most important distinction: signals are dynamic! Don’t confuse the signal that propagates with the measured voltage at a node.
Signals reflect when the instantaneous impedance changes
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1. Keep track of all the reflections
2. Know your source impedance
3. Know the round trip time of the cable: round trip time for a reflection to come back to the scope is 2 x 1.5 nsec per foot of cable. • 2 foot cable, RT time ~ 6 nsec• 1 m cable, RT time ~ 10 nsec
Three Secrets to Understanding Scope Measurements
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Situational Awareness: ALWAYS be aware of your scope features
AND your DUT features
▪ Scope:▪ Sample rate▪ Time base▪ Number of samples in an acquisition▪ Vertical resolution▪ Analog bandwidth, instrument intrinsic rise time▪ Scope input impedance setting
▪ Cable (probe):▪ BW (from losses)▪ Z0, TD
▪ DUT (as a Thevenin Source)▪ Unloaded voltage▪ Source resistance▪ Rise time
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The Scope We Are Using Today
▪ Teledyne LeCroy WavePro HD
▪Main features▪ 4 channels
▪ 8 GHz analog BW
▪ 12 bit vertical resolution
▪ 20 Gsamples/sec (50 psec interval)
▪ 60 fsec rms sample clock jitter
▪ 5 G samples acquisition memory
▪ Longest acquisition time at max sample rate = 5 G samples / 20 Gsamples/sec = 0.25 sec @ 50 psec resolution!
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Six important cases: what we expect to see depends on
the source impedance!
Source Impedance Scope termination Expected behavior
50 Ohms 50 Ohms
50 Ohms 1 Meg
>> 50 Ohms 50 Ohms
>> 50 Ohms 1 Meg
<< 50 Ohms 50 Ohms
<< 50 Ohms 1 Meg
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How to Reverse Engineer the DUT Figures of Merit
scope
meas source
source scope
RV V
R R
source meas
source scope
meas
V VR R
V
Step 1: Measure the DUT output voltage with scope at 1 Meg input
Step 2: Measure the DUT voltage with scope at 50 Ohms(caution: make sure voltage is < 5 V rms!!)
Step 3: calculate the output source resistance
Measured voltage is the unloaded, open circuit, Thevenin voltage of source
Measured voltage is the voltage across a 50 Ohm load to the Thevenin circuit
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Example #1: LeCroy WaveStation 2052 50 MHz
Waveform Generator
▪On square wave output
▪What is ✓Vsource
✓Rsource
✓10-90 rise time
Set up for 1 V P-P output
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How to Understand the Voltages and Signals
When you set 1 V P-P, the Thevenin voltage is set to 2 V P-P
The source impedance is 50 Ohms. A 1 V P-P signal is launched into the 50 Ohm coax cable.
What you do with this 1 V P-P signal is up to you.
When scope is 50 Ohms, you measure the 1 V P-P signal
When scope is 1 Meg load, 1 V P-P enters 1 Meg resistor, 1 V P-P reflects. Scope measures sum of both waves
2 v P-P
50 Ohm source
7 nsec rise time
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The Cal Out Signal, 1 Meg Scope Input
▪ On square wave output
▪ What is ✓ Vsource
✓ Rsource
✓ 10-90 rise time
Scope input: 1 Meg OhmProbe: 1 m long RG58 cable with gripper tips
V_source = 1 VRT = 206 nsec
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Source impedance of Cal signal
V-measured, 1 Meg = 1 V
V-measured 50 Ohm load = 0.06 V
source meas
source scope
meas
V VR R
V
source
1 0.06R 50 780
0.06
780 Ohms >> 50 Ohms
Rule #9: What should we see?3 ft cable, TD = 4.5 nsec, round trip = 9 nsecWith 50 OhmsWith 1 Meg input?
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Looks like longer cables have longer rise times
3 ft cable6 ft cable
Cal out source is really RT = 5 nsec
▪ It is an artifact of the reflections in the cable▪ Eliminate the reflections (terminate at scope), you eliminate the artifacts
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Summary
▪ Always characterize your source: Know its Vth, Rth, RT▪ Eliminate cable reflections using 50 Ohm input to the scope▪ But this loads the source down▪ If you use 50 Ohm cable and 1 Meg input AND the source is
high output impedance▪ Be aware that you will have cable reflections▪ Rise time will look like it depends on the cable length▪ Rise time will look like an RC charging, but it is due to
reflections
▪ If you do not want to load your source with 50 Ohms▪ Use an active high bandwidth probe▪ Use a 10x high impedance probe, but be aware of its
artifacts!!
Teledyne LeCroy Signal Integrity Academy25
What Every Scope User Needs to
Know About Transmission Lines
Dr. Eric Bogatin, Signal Integrity EvangelistDean, Teledyne LeCroy Signal Integrity Academy www.beTheSignal.com
Director, Teledyne LeCroy Front Range Signal Integrity LabAdjunct Prof, Univ of Colorado, Boulder, ECEE
Editor, Signal Integrity Journal, www.SignalIntegrityJournal.com