Signal Integrity

Real signals
By Howard Johnson, PhD, 10/8/2009
The piecewise-linear step starts with a jerk, mindlessly follows a perfectly uniform ramp, and smacks hard into its upper limit. Real signals don't do that.

Sliding edge
By Howard Johnson, PhD, 9/3/2009
Boards with different transmission-line geometries require special techniques. Here's how to cope when high-speed signals reflect due to the sudden change in board properties as they move across the connection.

The nature of ESD
By Howard Johnson, PhD, 8/6/2009
To control ESD (electrostatic-discharge) transient currents, you must design the system so that ESD events have only one clear path to ground at every single point.

Holding on
By Howard Johnson, PhD, 7/9/2009
The tristate feature can help you extend the hold time of your driver.

Endpoint distortion
By Howard Johnson, PhD, 6/11/2009
The end termination improves short-term response, but round-trip reflections may interfere with subsequent bits.

Driving-point impedance
By Howard Johnson, PhD, 5/14/2009
The endpoint effect delays the received signal.

Dangerous games
By Howard Johnson, PhD, 4/9/2009
Budding engineers benefit from physically challenging games.

Not your fault
By Howard Johnson, PhD, 3/5/2009
Green safety wires do not form a reliable single-point-ground reference system.

Unified electrodynamic force
By Howard Johnson, PhD, 2/5/2009
The magnetic force is a relativistic effect.

Differential transitions
By Howard Johnson, PhD, 1/8/2009
Signal transitions occur wherever your signal passes through a package body, a connector, or a pair of vias.

Visualizing differential crosstalk
By Howard Johnson, PhD, 12/5/2008
To reduce intertrace crosstalk, you must enforce spacing rules between the aggressor and the victim, much as you would with single-ended signals.

Differential coupling
By Howard Johnson, PhD, 11/13/2008
Differential links do not need tight coupling to work effectively.

Reverberant signals in digital designs: The shot heard ’round the world
By Howard Johnson, PhD, 10/16/2008
Wallace Clement Sabine's 19th-Century theory of acoustics, which he vividly demonstrated by firing a pistol in a cathedral, can help engineers who need to dampen reverberating signals in digital systems today.

Twisted impedance: Wire proximity impacts line-impedance-approximation formula
By Howard Johnson, PhD, 9/18/2008
In a situation with closely spaced wires, the “proximity effect” generates a nonuniform distribution of current, with the greatest preponderance of current occurring on the inside-facing surfaces of the two conductors.

All about surface-mount ferrites
By Lee Hill, 8/21/2008
You may be surprised to learn that the performance of a ferrite bead in any power-filtering application can vary by more than a factor of 10, depending on the magnitude of dc current passing through the part during actual operation.

Crossing the river: The hazards of crossing a split-plane gap with a high-speed signal
By Doug Smith, 7/24/2008
Next time you analyze a PCB layout, keep in mind that crossing splits in the planes can cause problems with radiated emissions, immunity to external signals, crosstalk, jitter, and degraded rise and fall times.

EM-simulation software
By Howard Johnson, PhD, 6/26/2008
Bruce Archambeault, PhD, distinguished engineer at IBM, IEEE fellow, and the author of the EMI/EMC Computational Modeling Handbook, responds to questions about EM (electromagnetic)-simulation software.

Pointy tips: How to straighten bent oscilloscope probe tips
By Howard Johnson, PhD, 5/29/2008
My high-speed-oscilloscope probes were bent so badly that they looked like elf shoes. Here's how I fixed them.

Scrape it: How to probe a microstrip trace with no accessible test points or vias
By Howard Johnson, PhD, 4/28/2008
The lowly scraper is the best tool for the job. Given the right curvature, you can scrape a path just wide enough to reveal a trace under test without exposing other nearby features.

Designing a split termination
By Howard Johnson, PhD, 4/2/2008
If no suitable voltage source exists for a relatively simple end-terminating structure, then you have no choice: You must synthesize the Thevenin equivalent.

Z_MIN, a very special value
By Howard Johnson, PhD, 2/27/2008
A special value that I call 'minimum impedance' is the secret to successful end-termination design.

Yao! What a handshake!
By Howard Johnson, PhD, 2/7/2008
Imagine a handshake between Yao Ming of the Houston Rockets and diminutive actor Danny DeVito. When a logic driver meets its load, it behaves in a similar way.

Initial condition: The advantage of symmetric end termination in transmission lines
By Howard Johnson, PhD, 1/10/2008
A circuit with a symmetric end termination enjoys the benefit of sinking 20 mA the entire time it holds low.

Lossless propagation
By Howard Johnson, PhD, 12/3/2007
The properties of lossless propagation and resistive input impedance are inextricably linked.

Aunt Judy: Beware relatives' repairs
By Howard Johnson, PhD, 11/8/2007
Engineers tend to get a lot of requests to fix electronic devices for friends and relatives. These tips will help you keep your sanity without letting down Aunt Judy.

Setting up your oscilloscope to measure jitter
By Gary Giust, PhD, 10/3/2007
Because real-time oscilloscopes are workhorses in any laboratory, it’s important to know how to get the most out of them. Jitter measurements are particularly sensitive to their environment.

Jitter peaking and PLLs
By Gary Giust, PhD, 9/13/2007
Jitter peaking with each PLL degrades the timing of the input signal. As this signal passes through subsequent PLLs, jitter peaking can accumulate to cause instability or timing failures.

Roll back the lead-free initiative: 12 ROHS myths
By Howard Johnson, PhD, 9/13/2007
I'm all for environmental legislation when it actually helps. When it doesn't help, and it harms something else, I question the result.

Uncertainty principle: Time and frequency in high-speed digital design
By Howard Johnson, PhD, 7/19/2007
The proof of the uncertainty principle rests on properties of the Fourier transform that apply just as well to electrical engineers as to physicists.

Howard Johnson, PhD, of Signal Consulting, guides readers through the peaks and valleys of design for signal integrity at the board level.

Johnson frequently conducts technical workshops for digital engineers at Oxford University and other sites worldwide. In addition to this popular regular column, Signal Integrity, Johnson also has a video channel on EDN's Tech Clips page. You can email Johnson at howie03@sigcon.com.