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Product category: Design and Development Hardware
News Release from: Tektronix | Subject: iView
Edited by the Electronicstalk Editorial Team on 24 May 2001

Confronting signal integrity in digital
design

Signal integrity is a broad topic, one that impacts many electronic design disciplines, as Dave Ireland of Tektronix explains.

Signal integrity is a broad topic, one that impacts many electronic design disciplines But until a few years ago, it wasn't much of a problem for digital designers

They could rely on their logic designs to act like the Boolean circuits they were.

Noisy, indeterminate signals were something that occurred in high-speed designs - something for RF designers to worry about.

Digital systems switched slowly and signals stabilised predictably.

Market forces in the PC, networking, and telecommunications realms have changed everything, following a curve on which the pace of change itself is accelerating.

Processor clock rates have multiplied by orders of magnitude.

Computer applications such as 3D graphics, video, and server I/O demand vast bandwidth.

Much of today's telecommunications equipment is digitally based, and similarly requires massive bandwidth.

So too does digital high-definition TV.

At the functional circuit level, the term 'bandwidth' means speed - high data rates, narrow pulses, and fast rise/fall times.

Integrated circuit processes have evolved, generation by generation, to meet the market's speed demands.

The current crop of microprocessor devices handles data at rates up to 2, 3 and even 5Gbyte/s, while some memory devices use 400MHz clocks as well as data signals with 200ps rise times.

Importantly, speed increases have trickled down to the common IC devices used in automobiles, VCRs, and machine controllers, to name just a few applications.

A processor running at a 20MHz clock rate may well have signals with rise times similar to those of an 800MHz processor.

We have crossed a performance threshold that means, in effect, almost every design is a high-speed design.

Without some precautionary measures, high-speed problems can creep into otherwise conventional digital designs.

If a circuit is experiencing intermittent failures, or if it encounters errors at voltage and temperature extremes, the chances are there are some hidden signal integrity problems.

These can affect time to market, product reliability, EMI compliance, and more.

It's time to face signal integrity problems head on.

Let's look at some of the specific causes of signal degradation in today's digital designs.

Why are these problems so much more prevalent today than in years past? The answer is, again, speed.

In the 'slow old days', maintaining acceptable digital signal quality meant paying attention to details like clock distribution, signal path design, noise margins, loading effects, transmission line effects, bus termination, decoupling, and power distribution.

All of these rules still apply, but - Bus cycle times are up to a thousand times faster than they were 20 years ago! Transactions that once took microseconds are now measured in nanoseconds.

To achieve this improvement, edge speeds too have accelerated: they are up to 100 times faster than those of two decades ago.

This is all well and good; however, certain physical realities have kept circuit board technology from keeping up the pace.

The propagation time of inter-chip buses has remained almost unchanged over the decades.

Geometries have shrunk, certainly, but there is still a need to provide circuit board real e