High-Speed Noise and Grounding

Howard Johnson's new advanced course for Digital and Analogue Designers

1-2 July 2010

Please send me an email about future High-Speed Noise and Grounding courses as I cannot make this date.

This course is part of Oxford University's High-Speed Digital Engineering Week

About the course

This is an advanced-level course for experienced digital designers who want to press their designs to the upper limits of speed and distance. It is an advanced sequel to the 2 courses High-Speed Digital Design and Advanced High-Speed Signal Propagation by Dr Howard Johnson.

Focusing on interactions between your system and the natural world around it, including:

Radiation
Susceptibility
Electrostatic discharge

Who is it for?

This is an advanced course for Digital logic engineers, chip designers, system architects, EMC specialists, and applications engineers; anyone working with digital logic at speeds in excess of 1 GHz.

Benefits

This is a practical course, filled with practical examples and explanations.
Delegates without the benefit of formal training in analog circuit theory can use and apply the formulas and examples from this course to determine which of their circuits will encounter difficulties and how to fix them.
Delegates who have completed (at least) a first-year university level class in introductory linear circuit theory will comprehend the material at a deeper level.

Questions & Comments: all students who attend our High-Speed Noise and Grounding course have the opportunity to talk directly with Dr Johnson

All students who attend our High-Speed Noise and Grounding course will receive a free copy of Dr Howard Johnson's High-Speed Noise and Grounding DVD.
(click on the image below for more details)

Course presenter

Dr Howard Johnson, Signal Consulting Inc.

Dr Johnson has taught thousands of students at companies all over the world, including: 3Com Corp.; IBM; Allied-Signal; Intel; Bay Networks; Mentor Graphics; Cisco Systems; Motorola; Compaq Computer; NASA; Network Equipment Technologies; Level One Communications; Siemens; DEC; Northern Telecom; Dell Computer; Sandia Nat'l Labs; Ericsson Telecom; Sun Microsystems; GEC Marconi; Texas Instruments; Hewlett-Packard; VLSI Technology.

Course content

Principles of Mixed Signal Isolation

Forms of Coupling

Common Impedances
Parasitic Mutual Capacitance
Parasitic Mutual Inductance
E&M Field Radiation

Points to Remember

If my current path overlaps yours, we crosstalk.
Some systems are orders of magnitude more sensitive to noise than others.
The limits of acceptable crosstalk depend on your signal level and its required noise-free dynamic range.

Facts Necessary for Understanding High-Speed Noise and Grounding

Current flows in loops.
Loops may be completed by parasitic capacitance.
All loops have inductance.

Mutual Inductance Matters

Among high-speed, low impedance digital circuits, mutual inductance is often a worse problem than mutual capacitance.

Points to Remember

All circuits suffer from parasitic shunt capacitance and parasitic series inductance.
Mutual capacitance between devices can cause crosstalk.
Mutual inductance can cause crosstalk as well.
Among high-speed, low impedance digital circuits, mutual inductance is often a worse problem than mutual capacitance.

Frequencies That Matter for Digital Signals

The power in a digital signal concentrates in the band from DC up to the Knee Frequency.
Knee frequency is a function of rise/fall time, not repetition rate.

Five Ways to Reduce Crosstalk

Shrink the Aggressor
Reduce the Coupling
Change the Timing
Improve Receiver Margins
Reduce the Number of Aggressors

Points to Remember

A solid plane makes a very good defense against crosstalk.
Synchronous signals suffer crosstalk only when sampling.
Asynchronous signals remain sensitive to crosstalk at all times.

Ground Bounce (SSO)

In the presence of fast-changing magnetic fields,

You can only measure voltages between nearby points.
Use the shortest ground attachment possible on your probe.

Distributed nature of inductance

The energy stored in an inductor is nothing more, and nothing less, than the energy stored in the magnetic field surrounding that inductor.
The magnetic field exists in the spaces between conductors.
Inductance is not a property of the conductors.
Inductance is a property of the spaces between conductors.

Points to Remember

Displacement current (due to parasitic capacitance) flows between otherwise unconnected regions.
Magnetic induction from fast-changing magnetic fields induces voltages in your circuit.
Never measure voltages between widely separated points.

BGA package crosstalk (SSO coupling) varies with:

Separation between signal pathways
Height of via + ball + package thickness
Rise or fall time of the aggressive signal (specifically, the di/dt)

Points to Remember

SSO is one of many forms of inductive crosstalk.
SSO is caused by outgoing signal currents, and also by returning signal currents.
SSO varies pin by pin.
It increases as you approach aggressive signals.
It decreases as you approach ground balls (ergo, use a lot of ground balls).

BGA Ground Ball Placement

This 40 min. film presents the theory of SSO crosstalk in BGA packages with examples, giant scale models, and measured lab results.

Outline of Film

What causes crosstalk in BGA packages?
What can be done to improve it?
Demonstrate crosstalk in a scale-model package.
Compare alternative ground-ball layouts.

PCB Layer Stack

The Path of Return Signal Current
High-speed current follows the path of least inductance

Measuring Tiny Amounts of Crosstalk

Counteracting parasitic effects of your probes

Differential Probing

The differential probe does not require a ground wire, but it works better if you provide one directly from the scope to the device under test.

Points to Remember

Do not trust "ground" symbols on your schematic.
Draw out the whole circuit, including your scope.

Stacking Connectors

Ground-transfer impedance

Points to Remember

Object radiate in proportion to their height above the nearest solid plane.
Stacking connectors exhibit a very significant ground transfer impedance.
Returning signal current, multiplied times the ground transfer impedance, creates a measurable voltage on the piggyback card reference plane.

When to Segment the VCC Plane

IC's with different power voltages
PLL's or clock generators with special sensitivity to power supply noise
I/O chips that must not have noise on their power terminals

Moat and Drawbridge Construction

Can help reduce stray currents, but creates slots in your reference system

Points to Remember

A slot in your reference plane creates unwanted inductance.
Slot inductance slows down rising edges.
Slot inductance creates mutual inductive crosstalk.
If you use your power plane as a signal reference, the boundary from one power region to the next works like a slot or gap—routing across the boundary can dramatically increase crosstalk (see High-Speed Digital Design class notes, chapter 5).

Good High-Speed Practices for Big, Fat Boards

Use ground planes to isolate pairs of routing layers.
Lay power layers adjacent to grounds.
Connect ground layers with numerous vias.

System-Level Grounding

Picture Frame Analysis

A simplified method for understanding how common-mode noise moves in a complex system.

Three Big Ideas For Isolation (i.e., Reducing the Impact of Stray Current)

High impedance blocks current
Low impedance shunts current
Change in topology

Points to Remember

If you can't explain the operation of your isolation technique using a simple picture-frame model, you probably don't understand how it works.

Capacitance Between Boards

Measurement technique and examples

Points to Remember

Capacitance between boards conveys current from one board to another.
Shunt that current to the reference plane using broad, flat straps or short, thick standoffs.

Interconnections Between Boxes

Coaxial
Twisted-pair
Fiber-optic

Points to Remember

The interconnection topology determines the magnitude of currents flowing on a coaxial shield
The shield transfer impedance determines the noise pickup internal to a coaxial cable
A twinax cable further attenuates the received noise
Always test for crosstalk!

Crosshatched Ground: To what degree does patterning affect…

Trace impedance
Crosstalk

Clock-Related Noise Issues

Metastable MTBF: Points to Remember

Slow down the sampling clock
Get a better flip-flop
Use multiple stages

Unexpected Synchronization Difficulties: Points to Remember

Test your system with links unplugged
Test your system with killer packets
Test your system with input clocks adjusted to slightly different frequencies

Do Terminations Reduce Emissions: Points to Remember

An long unterminated transmission line harbors resonances that exacerbate radiation at certain frequencies.
Terminations can help.

In a differential link: Points to Remember

Skew creates a common-mode signal
DCD (individually on each output) creates a common-mode signal
Common-mode signals radiate far more than differential signals

Jitter Effects in Modern System Design II

Outline of Presentation

Tracking down jitter, like medicine, is a diagnostic science.
You must make independent tests to see what actually causes your jitter.
Planning for these tests is the most important thing you can do to address jitter in your high-speed system architecture.

Electromechanical Issues

Board-to-board Connector Electrical Noise Performance: Points to Remember

The measure of connector noise-generating capability in a mixed-signal system is ground-transfer impedance.
The ground-transfer impedance is a function of connector design and board layout.

Shielded Cabling Connectors: Points to Remember

The ground-transfer impedance of a shielded connector determines the amount of stray current emanating from that location.
Conversely, the ground-transfer impedance also determines the susceptibility of the interface.

Mechanical Issues: Key environmental parameters

Shock and vibration
Temperature
Humidity
Sand and dust
Durability
Electrostatic discharge (ESD)

Points to Remember

Everything shakes
Everything corrodes
Every good ground contact eventually works free
Everything intentionally not grounded eventually becomes so

Trends in Interconnect Design

Smaller (better for RF); show examples
Multiple contacts; show examples of advanced contact design

Standards-Based Design

Why compete? Pick one of the standards and use most (if not all) of its standard parts.

Example of standards-based system: Firewire (IEEE 1394)

Points to Remember

Easy: Figure out how to pass your data through a standard LAN architecture
Hard: Design a new LAN

Tin Whisker Alert

What is a tin whisker, and why does it matter to my design?

Points to Remember

Keep abreast of the fast-changing "tin whisker" subject.
In situations that require long-term reliability, do not use pure tin.

System Test

Eye Don't Like It

An eye diagram makes a terrible diagnostic tool.

Compliance Testing:

Testing a data link in isolation is never sufficient; links must be tested in combination with other noise sources.
Isolate Effects to pinpoint sources of error
Quantify Your Results with stress testing

Debugging Hardware

Chop the System into small pieces for initial tests
Remain Ever Vigilant for even the smallest clue about system behavior.
Keep Meticulous Records
Use your simulator to maximum effect

Measurement Problems

Measuring tiny voltages
ESD Induces Large Voltages
ESD Induces Large Currents

Personnel issues

Working with Consultants
Assigning your best engineers to critical design tasks

Practical System-Design Advice (Wrap-up)

With the development of any new product, limit your use of new and untried technical innovations to five.
Never fully utilize any ASIC or custom IC. As you approach 100% utilization, the difficulty of routing and placement approaches infinity.
Ignore the maximum toggle frequency for a flip-flop.
Once you obtain a decent probe, immediately pin a big REWORK tag on it. The tag may discourage others from taking it.
Stress Test Everything
Build plenty of test outputs into every ASIC.
With passive components, never specify the most extreme value listed in a catalog.
Always include a few spare traces on a backplane. You never know what additional features you may need later.
Your first version of a product need not adhere to the final form factor. If it does, you have wasted a lot of valuable engineering time by mechanically squeezing everything into the final packaging.
Hire Only First-Rate People

Venue

Department for Continuing Education, Rewley House, Oxford

First day registration from 8.30am when course materials will be distributed.

Refreshments from 8.30am on the first day plus two 30 minute breaks during the day and a one-hour lunch break.

The course will begin at 9.00am and end at approximately 5.00pm on each day.

Certification

Delegates will receive a University of Oxford Certificate of Attendance.

Fee options

£1045 - standard course fee.

Fees include course materials, tuition, refreshments and lunches. The price does not include accommodation.

All delegates will receive a free copy of Howard Johnson's High-Speed Digital Design DVD.

All courses are VAT exempt.

How to apply

If you would like to discuss your application or any part of the application process before applying, please contact:
Emma Haslam (Course Administrator) - Tel: +44 (0)1865 286958 - Email: electronics@conted.ox.ac.uk

If you would like to pay using a company purchase order / bank transfer, please see the guidance notes below.