Date post: | 22-Oct-2015 |
Category: |
Documents |
Upload: | jagadees21 |
View: | 51 times |
Download: | 0 times |
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
3© 2010 Mentor Graphics Corp. Company Confidentialwww.mentor.com
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
4© 2010 Mentor Graphics Corp. Company Confidentialwww.mentor.com
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
5© 2010 Mentor Graphics Corp. Company Confidentialwww.mentor.com
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)
6© 2010 Mentor Graphics Corp. Company Confidentialwww.mentor.com
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
7© 2010 Mentor Graphics Corp. Company Confidentialwww.mentor.com
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
8© 2010 Mentor Graphics Corp. Company Confidentialwww.mentor.com
FEXT
Crosstalk pulses “stack” to form a larger pulse
9© 2010 Mentor Graphics Corp. Company Confidentialwww.mentor.com
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
10© 2010 Mentor Graphics Corp. Company Confidentialwww.mentor.com
NEXT
Crosstalk pulses “line up” to form a longer pulse
11© 2010 Mentor Graphics Corp. Company Confidentialwww.mentor.com
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)
12© 2010 Mentor Graphics Corp. Company Confidentialwww.mentor.com
Crosstalk Examples
Highlight areas of layout with high crosstalk
13© 2010 Mentor Graphics Corp. Company Confidentialwww.mentor.com
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
14© 2010 Mentor Graphics Corp. Company Confidentialwww.mentor.com
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
15© 2010 Mentor Graphics Corp. Company Confidentialwww.mentor.com
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
16© 2010 Mentor Graphics Corp. Company Confidentialwww.mentor.com
Crosstalk in PCI Express
PCI Express consists of unidirectional differential pairs— TX— RX
Main concern is crosstalk at receiverCrosstalk can ALSO be controlled by altering aggressor directionality
17© 2010 Mentor Graphics Corp. Company Confidentialwww.mentor.com
Crosstalk in PCI Express
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
18© 2010 Mentor Graphics Corp. Company Confidentialwww.mentor.com
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
19© 2010 Mentor Graphics Corp. Company Confidentialwww.mentor.com
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
20© 2010 Mentor Graphics Corp. Company Confidentialwww.mentor.com
Sweeping Trace Spacing
21© 2010 Mentor Graphics Corp. Company Confidentialwww.mentor.com
Sweeping Coupling Length
22© 2010 Mentor Graphics Corp. Company Confidentialwww.mentor.com
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
w w w . m e n t o r . c o m
Thank you