Signal Integrity

Power-aware Signal Integrity and EMI/EMC On High-speed Digital Chip-to-Chip Links

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The Microwave Crisis

Posted July 9th, 2008 · 2 Comments · Calculator

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No, it doesn’t mean a hotdog has exploded in your microwave oven 🙂 . In chip-to-chip links, you hit the “microwave crisis” when your bit rate increases to the point that the bit period becomes equal to or less than the flight time of the interconnect, or, equivalently, when the EM waves in the dielectric are equal to or less than the physical size of the interconnect. Here is a simple calculator that gives a order-of-magnitude illustratation of this rule of thumb.

Relative dielectric constant:
Interconnect length (cm):
Interconnect length (inches):
Flight time (ps) = bit period at critical bit rate:
Critical bit rate (gigabit/s):

If you have bit rate faster than the critical bit rate, reflections must be minimized by careful impedance matching of source, load, trace segments, vias, and connectors using EDA tools like Agilent ADS.

For a more accurate estimate, you have to consider the frequency spectrum of a pseudo-random binary signal with finite rise time.


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