Signal Integrity Seminar

7/31/2019 Signal Integrity Seminar

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Signal Integrity

Analysis

- MansiResearch


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Signal Integrity

What do you mean by Signal Integrity?

At what point does Signal Integritybecome a problem?


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SIGNAL INTEGRITYANALYSIS Signal Integrity means quality of signal

throughout the transmission line

When the rise time or fall time decreasesto the point where the parasiticinductances and capacitances on the

board begin to result in noise signal andtransients


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Parasitic element ofTransmission line

There are basically three parasiticelement of transmission line:

1. Resistor

2. Capacitor

3. Inductor


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Two more obvious and common ways asignal can lose integrity is

When it becomes distorted

When the signal to noise ratio begins todegrade quality of signal.


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In general, Signal integrity problems oncircuit boards fall into four areas, all ofwhich are related to rise time of signal.


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Transmission LineFundamentals

In general communication model there is (A) sender ofthat communication, (B) Message that is being sent, (C)The media over which the message is being sent, and (D)The receiver of the message.

Now on PCB, The sender is driver circuit, the message istypically the change in state from a high to low or from a

low to a high signal, the media is the PCB trace and thereceiver is the receiving circuit.


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Transmission Line Lumped Model

So, what make transmission line so special?

Consider a long, straight wire or trace with its return wireor trace nearby. The wire has some inductance along itslength and some capacitive coupling between the wire and

its return path. Fig shows what we call a lumped model ofthe wire pair.

F t


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Fun amenta s oTransmission line

F t


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Fun amenta s oTransmission line

As a PCB design engineer, keep in mind two things.

We need to make our traces look like transmission lines.

We need to terminate them in their characteristicimpedances, Zo.

There are certain types of transmission lines that arecommonly used around us every day like as.

The coaxial cable leading to our cable TV is a 75 ohmtransmission line.

10Base2 coaxial cable for networking, that is a 50 ohm

transmission line.


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Reflection Co efficient oftransmission line If we have use controlled impedance traces, at least two

things might still cause a reflection in the system;

An impedance discontinuity along the line, and

An improper termination.

Any change in geometry or in the dielectric coefficient,will cause a change in the characteristic impedance of theline at that point. That may cause a reflection at thatpoint.

The magnitude of the reflection is determined by a

measure we call the reflection coefficient, o. In the caseof a single, parallel terminating resistor at the far end ofthe trace, the voltage reflection coefficient is defined as

o = (RL-Zo)/(RL+Zo)

where RL is the terminating, or load, resistor


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Uniform Transmission Lines

When the cross section is the same down the length, as in a

coax cable, the transmission line is called uniform


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Uniform Transmission Lines

When the cross section is the same down the length, as in

a coax cable, the transmission line is called uniform uniform transmission lines are also called controlled

impedance lines

Reflections will be minimized and signal quality optimized,

if the transmission lines are uniform, or have controlledimpedance

All high-speed interconnects should be designed asuniform transmission lines.

Non-uniform transmission lines exist when some geometryor material property changes as we move down the lengthof the line

For example, if the spacing between two wires is notcontrolled but varies, this is a non-uniform line.

Whether the transmission line is uniform or nonuniform, ithas s o e ole o la : o a s a s al o o e e d


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Ideal Distortionless, LosslessTransmission Line

An ideal transmission line consists of two perfect

conductors and these conductors should have zeroresistance and uniform cross section and extend forever.

There are two types of transmission line (1) Balanced(Twisted Pair) and (2) Single-ended or Unbalanced (Coax,Microstrip and stripline).

In a balanced transmission line, signal current flows outalong one wire and back along the other.

In a single-ended transmission line, signal current flows insignal wire and back along a ground connection.

Ideal transmission line should have following threeproperty.

1. Infinite in extend it starts here and goes forever inone direction

2. Signals propagating on the line are not distorted ashe o ess.


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Propagation Delay OR Transmissionvelocity The amount of delay in signal propagation per unit length

along a transmission line is called propagation delay. Unit : Picoseconds/inch.

Propagation velocity and transmission velocity both areinverse of propagation delay.

There is three different method to measure propagationdelay along transmission line.

Some reference material rate transmission velocity inpercent, where 100% is the velocity of light in a vacuum.

The velocity of light in vaccum is 3 X 10E8 m/s or 0.0118inch/Ps or a delay of 84.7ps/inch.

For example, RG-58/U, a relative velocity of 66% wouldgive a longer delay per inch, equal to

Delay (ps/in) = 84.7 / 0.66 = 128ps/inch


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Propagation Delay OR Transmissionvelocity 2nd Method :

The propagation delay of any transmission line is relatedto its series inductance per unit length and its parallelcapacitance per unit length.

In transmission line, these factors are both proportional tolength, and their fine balance is responsible for thedistortion less propagation of signals.

Lets measure the capacitance and inductance of RG-58/Ucoaxial cable,


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Propagation Delay OR Transmissionvelocity According to Electromagnetic wave theory, The

propagation delay is equal to

The speed of a signal depends on the materials thatsurround the conductors and how quickly the changingelectric and magnetic fields associated with the signal canbuild up and propagate in the space around thetransmission line conductors.

The signal, after all, is a voltage difference between the

signal path and the return path. As the signal propagates, a voltage difference must be

created between the two conductors.

Accompanying the voltage difference is an electric fieldbetween the conductors and the current loop movingthrough the conductors will produce a magnetic field.


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Propagation Delay OR Transmissionvelocity How quickly the electric and magnetic fields can build up

is what really determines the speed of the signal. The

propagation and interaction of these fields is described byMaxwell's Equations

These say that if the electric and magnetic fields everchange, the kink they make will propagate outward at aspeed that depends on some constants and materialproperties

The speed of the change, or the kink, v, is given by:

where:

0 = permittivity of free space = 8.89 x 1012 F/m

r = relative dielectric constant of the material


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Propagation Delay OR Transmissionvelocity The propagation delay per inch for printed circuit board

traces depends on both the dielectric constant of the

printed circuit board material and the trace geometry.

Trace geometry determines whether the electric field stayin the board or goes into the air.

When the electric field stays in the board, the effective

dielectric constant is bigger and signals propagate moreslowly.

Propagation delay of electromagnetic fields in variousmedia.


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Setup and Hold time

The Concept of Setup and Hold Time can be bestunderstood with the picture shown below.


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Setup and Hold time

Set up time is the amount of time before the clock edgethat the input signal needs to be stable to guarantee it isaccepted properly on the clock edge

Hold time is the amount of time after the clock edge thatsame input signal has to be held before changing it tomake sure it is sensed properly at the clock edge.

Setup and Hold times are vigorously (strongly) simulatedat the Chip design level to ensure that they meet thespecification.

As PCB designers, we will be required to match the flighttime between a set of signals

Problem : IC A is the source of a Clock and a Data Signal.The IC B is the destination of the Clock and the datasignal. The Clock signal is routed all on outer signal andhas a length of 12 inches. The Data signal is all routed on

the inner signal. Find the length of the Data Signal tomatch the flight time of clock signal. Assume propagation


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Points to be remember:

Signal Integrity means quality of signal throughout thetransmission line

Signal Integrity becomes a problem, When the rise time orfall time decreases to the point where the parasiticinductances and capacitances on the board begin to resultin noise signal and transients

The wire has some inductance along its length and somecapacitive coupling between the wire and its return path

We need to make our traces look like transmission lines.

We need to terminate them in their characteristic

impedances, Zo.

cause a reflection in the system;

An impedance discontinuity along the line, and

An improper termination

The value of the reflection coefficient can ran e between


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Signal Integrity Seminar

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