what every scope user needs to know about transmission lines

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What Every Scope User Needs to Know About Transmission Lines

Dr. Eric Bogatin, Signal Integrity EvangelistDean, Teledyne LeCroy Signal Integrity Academy

www.beTheSignal.comDirector, Teledyne LeCroy Front Range Signal Integrity Lab

Adjunct Prof, Univ of Colorado, Boulder, ECEEEditor, Signal Integrity Journal, www.SignalIntegrityJournal.com

http://www.bethesignal.com/

http://www.signalintegrityjournal.com/

Teledyne LeCroy Signal Integrity Academy

A Confusing Aspect of Scope Measurements

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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?


Looks like longer cables have longer rise times

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3 ft cable 6 ft cable

Cal out source is really RT = 5 nsec


Outline Two engineering principles to keep in mind Five essential principles

1. 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”


Rule #9: Never do a measurement or simulation without 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


Essential Principle: All interconnects are transmission lines

GROUND

Signal path

Return pathVVin


Essential Principle: Signals are Dynamic

GROUND

Signal path

Return pathVVin

All interconnects are transmission linesA signal as a voltage differenceSignals propagate

Dk12v secn

inches

= secninchessecn

inchessecn

inches

62

124

12===In FR4 traces

inches inchesnsec nsec inches

nsec12 12 8

1.52.2= = =In Coax cable

TD for 1 foot coax = 1.5 nsec, 3 feet in 4.5 nsec ~ 5 nsec


Dynamic Simulation of Propagating and Reflected Signals

Download this free animation tool fromwww.beTheSignal.com

VRPW-30-16: Yoshi’s Animations of Reflections


VVsignal

Essential Principle: Signals see an instantaneous impedance

GROUND

Signal path

Return pathVVin

ALL Signals ALWAYS propagateThe edge has a spatial extent, where the dV/dt, dI/dt isThe edge sees an instantaneous impedance

The dV/dtThe dI/dt

Frozen in time

=ZIV


Really Simple View of the Impedance of a Transmission Line

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throughCurrentappliedVoltageZ =

I

V

instantaneous impedance of the transmission line

∆x

C

C = CL ∆x

∆Q = CV,

every ∆t = ∆xv

I = ∆Q∆t =

vCL∆xV∆x = vCLV

I = ∆Q∆t

LL vC1

VvCV

IVZ ===

The characteristic impedance of a transmission line:The one value of instantaneous impedance in a uniform transmission line


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 propagation2. A direction of circulation

They are independent!

+++

=+++

- - - +++

Idisplacement

current


Signals reflect when the instantaneous impedance changes

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


Three Secrets to Understanding Scope Measurements

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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


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!

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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


How to Reverse Engineer the DUT Figures of Merit

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scopemeas source

source scope

RV V

R R=

+

( )source meassource 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


Example #1: LeCroy WaveStation 2052 50 MHz Waveform Generator

On square wave output What is

Vsource Rsource 10-90 rise time

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Set up for 1 V P-P output


How to Understand the Voltages and Signals

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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


The Cal Out Signal, 1 Meg Scope Input

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Scope input: 1 Meg OhmProbe: 1 m long RG58 cable with gripper tips

V_source = 1 VRT = 206 nsec

On square wave output What is

Vsource Rsource 10-90 rise time


Source impedance of Cal signal

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V-measured, 1 Meg = 1 V

V-measured 50 Ohm load = 0.06 V

( )source meassource scope

meas

V VR R

V−

=

( )source

1 0.06R 50 780

0.06−

= Ω = Ω

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?


Looks like longer cables have longer rise times It is an artifact of the reflections in the cable Eliminate the reflections (terminate at scope), you eliminate the artifacts

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3 ft cable 6 ft cable

Cal out source is really RT = 5 nsec


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!!

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What Every Scope User Needs to Know About Transmission Lines

Dr. Eric Bogatin, Signal Integrity EvangelistDean, Teledyne LeCroy Signal Integrity Academy

www.beTheSignal.comDirector, Teledyne LeCroy Front Range Signal Integrity Lab

Adjunct Prof, Univ of Colorado, Boulder, ECEEEditor, Signal Integrity Journal, www.SignalIntegrityJournal.com

http://www.bethesignal.com/

http://www.signalintegrityjournal.com/

what every scope user needs to know about transmission lines

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