156376194 Crosstalk OnPCB

Patrick CarrierProduct Marketing ManagerPCB Analysis ToolsMentor Graphics

Controlling Crosstalk in PCB Designs

2© 2010 Mentor Graphics Corp. Company Confidentialwww.mentor.com

Signal Integrity Concerns

Signal Quality— Noise margin

– Relative to Vih/Vil– Voltage swing at receiver– Ringback/non-monotonicities

— OvershootTiming— Flight times, Setup/Hold Times

Crosstalk— Noise induced by aggressor signal to victim

Electromagnetic Compatibility (EMC)— Radiated Emissions, Signal strength

at various frequencies

http://www.mentor.com/products/pcb-system-design/series/si-basics-seriesand ../si-basics-pcb-series


ClockAClockB

Net Topologies

CoupledRegion

Signal Integrity and CrosstalkCrosstalk

Crosstalk occurs when 2 or more neighboring traces couple together

Net ClockA inducing crosstalk on ClockB

ClockA(Aggressor)

ClockB(Victim)

Sending a signal down one tracecauses a signal to appear on the 2nd trace


The “aggressor” signal or trace — Switches and causes crosstalk

The “victim” signal or trace — Responds by developing

an unintended signal

The effect is 3-dimensional— Victims can be adjacent,

above or below the Aggressor

Background


E-fields (electric)— Capacitive coupling between trace and plane, between

trace and other traces— Voltage injected onto victim

B-fields (magnetic)— Inductive coupling between traces— Current injected onto victim

E-Fields and B-Fields

electric field lines (blue)

magnetic field lines (red)


Crosstalk Overview

Two types of coupling— Mutual Inductance— Mutual Capacitance

Two types of crosstalk— Far-end crosstalk

– A.k.a. Forward crosstalk– A.k.a. FEXT

— Near-end crosstalk– A.k.a. Reverse crosstalk– A.k.a. NEXT


FEXT

Propagates with aggressor signal edgeHas same width as aggressor signal edgeAmplitude determined by coupling — Grows continuously— Negative coupling caused by mutual inductance— Positive coupling caused by mutual capacitance


FEXT

Crosstalk pulses “stack” to form a larger pulse


NEXT

Propagates in the reverse direction of aggressor signal edgeHas width equal to twice the signal propagation timeAmplitude determined by coupling— Saturates when

parallelism length = aggressor edge length— Positive coupling caused by mutual inductance— Positive coupling caused by mutual capacitance


NEXT

Crosstalk pulses “line up” to form a longer pulse


Crosstalk Examples

NEXT and FEXT from real simulationNEXT has width equal to twice the line length (5”or 768ps)FEXT has same width as aggressor signal edge (200ps)


Crosstalk Examples

Highlight areas of layout with high crosstalk


Differential crosstalk

Equal and opposite pulses of crosstalk can be induced on either side of the differential pairAlso need to be concerned about higher-voltage aggressor signals – leave extra spacing


Differential crosstalk

Differential signal = DIFFERENCE of single-ended signalsAdiff = Aplus – Aminus = 2*Asingle

— Both crosstalk and signal amplitude are twice their single-ended counterparts

— Differential crosstalk just like single-ended


Crosstalk in PCI Express

Major design concern— Same as in PCI and PCI-X

– Large number of signals– Signals need to go to

the same placeLarge amount of parallelism

Edge rates ~ 50ps— About 1/3 of an inch— More crosstalk for given parallelism

Main method of control = increased spacing



PCI Express consists of unidirectional differential pairs— TX— RX

Main concern is crosstalk at receiverCrosstalk can ALSO be controlled by altering aggressor directionality



On Microstrip Routing— Interleave TX and RX

differential pairs

On Stripline Routing— Interleave RX and TX pairs for long routes— Do not interleave RX and TX pairs for short routes— Use simulation to determine NEXT/FEXT crossover point

– Can vary based on length, spacing, stackup– Model different dielectric layers with appropriate dielectric

constants– FEXT is not zero


Controlling Crosstalk

Space signals further apart— Weaker field interaction

Minimize parallelism— Allows less time for coupled

energy to build up— Shorter lengths— Spread out when able


Controlling Crosstalk

Minimized through trace spacing— Smaller dielectric heights = less spacing required

Typically good to have at least 3 times the dielectric height for spacingShould do analysis on signals using a simulator like HyperLynx


Sweeping Trace Spacing


Sweeping Coupling Length


Other methods of noise coupling

Simultaneous switching noise (SSN)— Looks just like crosstalk in the lab

– Edge-aligned phenomena

— I/Os switching all at once create excessive power draw which can show up on other signals

– Caused by high power distribution network impedance (PDN)– Can be prevented with proper decoupling analysis

Via noise coupling— Vias can have mutual inductance and capacitance— Vias radiate noise into plane pairs, which can couple onto other

vias and pins

Check out our webinar later this month:http://www.mentor.com/products/pcb-system-design/events/analyze-stop-coupling-noise-webinar

23© 2010 Mentor Graphics Corp. Company Confidentialwww.mentor.comYour Initials, Presentation Title, Month Year

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156376194 Crosstalk OnPCB