Signal Integrity

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.
Finger the culprit: Debugging rare modes of failure
By Howard Johnson, PhD, 6/21/2007
When debugging a rare mode of failure, never attempt a direct fix. The test cycles associated with each attempted improvement will kill your development schedule. Your first order of business is to make the problem worse.
In-between spaces: Understanding inductance in high-speed digital circuits
By Howard Johnson, PhD, 5/24/2007
In high-speed electronics, you must supplement Kirchhoff’s laws with parasitic capacitance, due to electric fields, and parasitic inductance, due to magnetic fields.
Diagnostic testing (and tasting?)
By Howard Johnson, PhD, 4/26/2007
Human senses play a smaller role than they once did in diagnosing digital systems, but the philosophy remains the same: Use every tool at your disposal.
Using pulse-width compression as an indicator of transmission-line performance
By Howard Johnson, PhD, 3/29/2007
Pulse-width compression works well as a measure of system bandwidth because it overcomes the limitations of probe placement and loading.
OFC madness: Facts, not fantasy, regarding power cables for high-end audio equipment
By Howard Johnson, PhD, 3/1/2007
Several suppliers would have you believe your audio amplifier's cable should be an OFC (oxygen free copper) cryogenic model with shielding, silver plating, and EMI noise rejection.
Why teach science?
By Howard Johnson, PhD, 2/1/2007
The primary purpose of science education is to seek out and find those few curious, self-motivated people who want to know how everything works and encourage them to save the rest of humanity.
Two-way street: Transmission lines support traffic in two directions
By Howard Johnson, PhD, 1/4/2007
A voltage probe connected to a transmission line shows an aggregate-voltage waveform, but doesn't say which way the waveform is moving.
Eye of the probe
By Howard Johnson, PhD, 12/1/2006
My differential probe touches two surface-level signal traces, directly adjacent to the input balls of a 2.5-Gbps digital deserializer in a large BGA package. The signal arrives from an optical-to-electrical converter some 6 in. away.
Eye don't like it
By Howard Johnson, PhD, 11/9/2006
My heart sank when I first saw the hideous jitter and loss of noise margin evident in the eye diagram of Figure 1. The figure shows the received eye diagram for a simple serial connection operating at 2.5 Gbps. This differential link connects a fiber-optic receiver module (O/E) to a large protocol processing chip (PHY).
Frequent obsession
By Howard Johnson, PhD, 10/12/2006
I recently encountered yet another pc-board-trace specification written in the frequency domain. Why do people do that?
Voltage-regulator droop
By Howard Johnson, PhD, 9/14/2006
The circuit model in Figure 1 captures the important low-frequency behavior of most voltage regulators (Reference 1). Parameters C2, R2, and L2 represent a typical bulk decoupling-capacitor array. To model the response below 100 kHz, you can set L2 to 0. Parameter R1 models the regulation resistance, or stiffness, of a small switching-regulator module.
Voltage-regulator model
By Howard Johnson, PhD, 8/17/2006
I love switching-regulator modules. They are efficient, you can configure them for many uses, and you can easily model them. Figure 1 shows a typical characterization test for a regulator module—a Texas Instruments PTH08T220W switching-regulator module. The module is subject to an 8A step load, with a maximum dI/dt of 2.

Next >>

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.

Signal Integrity

Please visit these other Reed Business sites