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Bill Schweber writes about analogue design trends and their impact on the industry, and gives us his expert insight on the role that analogue plays in our largely digital world.
Bill is the editor-in-chief of Planet Analog and a registered professional engineer.
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Posted: 11:52:20 AM, 21/06/2011

Minimizing EMI with spread-spectrum clocking: clever or cheat?

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Most designers know the technique of using a spread-spectrum clock to minimize apparent EMI/RFI emissions. By deliberately dithering the system clock, the radiated energy is spread across the spectrum and thus its peaks are diminished, which allows the product to meet regulatory or industry specifications. The technique is now well-established, and vendors offer clock ICs with adjustable spread widths and rates, as well as advanced pseudorandom spread algorithms.

But when discussing the use of spread -spectrum clocking to meet EMI/RFI requirements, I find that engineers are divided on the approach. Some feel it is a legitimate, very-low-cost tool in the designer's kit that helps the design meets market requirements. Others feel it is short-cut cheat, too often used instead of proper EMI design methods. Still others feel it should be used only after all other conventional steps have been taken, such as shielding, grounding, layout changes, ferrite beads, to cite a few.

One argument against using spread-spectrum is that you are very likely actually making your design "problem" into someone else's. As you spread the energy, yes, you may meet a specification, but you also introduce the likelihood of unexpected problems when your spread energy mixes with as-yet unknown or undefined energy in other nearby or connected systems, each with their own frequencies and amplitudes. In short: "hey, I met the specafter that, it's your problem, not mine!"

As with most engineering designs, there is no right or best answer. What makes sense depends for your project's priorities, budget, constraints, market forces, and the balance among all the tradeoffs which every design encompasses. Perhaps in the ideal world, the design would first be made as EMI-robust as possible, and then spread spectrum would be added for a little extra insurance, but only if needed.

[Ironically, using spread spectrum is absolutely contrary to another engineering imperative, central to test equipment and many data links: to have a clock which is as perfect and jitter-free as possible. That's one of the many contradictions and sources of discomfort engineers see with it.]

Have you ever used spread spectrum to reduce EMI and meet a spec? What's your view on spread spectrum as an EMI reduction technique?

- It's a great idea, go ahead and use it right away;
- Use it only after everything else that should be done has been done, and you are still stuck;
- Use it only after you have already met the spec, just to buy a little extra margin;
- Or don't use it at all, since the unforeseen consequences to the overall system and broader application are too risky?

What do you think?


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