Published by Eric Bogatin on 27 Jul 2009
Check out our next public classes: Essential Principles of Signal Integrity and Advanced Signal Integrity Design, Oct 11-14 in Hillsboro, OR.
Check out our next No Myths Allowed Webinar, “Stack-up Design for Differential Pairs”, presented free on Sept 16, 1 pm EDT.
The design process is a creative process and intuition is the most important skill you rely on first. Once you have a design, then you apply your analysis skills to evaluate the cost-performance tradeoffs.
Wikipedia defines intuition as the “ability to sense or know immediately without reasoning”. I like to think of intuition as what you take away after you have done the reasoning- what has become ingrained in your understanding and you trust to use every day. It becomes your “common sense” of the right way of doing things.
The better your intuition or common sense about signal integrity, the better your first pass design will be. This translates to a shorter and lower cost design process.
Because signal integrity is sometimes “anti-intuitive”, we sometimes have to “re-calibrate” our intuition for these analog electromagnetic effects important in interconnects. The Essential Principles class I teach, is really about building a strong foundation for your common sense about signal integrity. Here in an abbreviated list, are 13 of the most important principles to strengthen your common sense when dealing with signal integrity:
1. The fastest way of fixing a problem is by first identifying its root cause and then applying the Youngman Principle.
2. All interconnects are transmission lines, no matter how long or how short they are.
3. All signals are dynamic and constantly move along the transmission line at the speed of light in the dielectric, roughly 6 inches/nsec.
4. Signals sees an instantaneous impedance each step they takes along a transmission line.
5. The return current is exactly coincident with the signal current, flowing in the opposite direction, in the return conductor.
6. Reflections occur whenever the instantaneous impedance changes.
7. Don’t confuse the distributed cross talk between transmission lines which rarely extends beyond adjacent lines, with ground bounce cross talk which can extend to many adjacent transmission lines.
8. Ground bounce occurs due to the dI/dt of the return current passing through the total inductance of the return path.
9. The differential impedance of a differential pair can be just as well controlled for a tightly coupled as loosely coupled pair.
10. A real capacitor behaves like a series combination of ideal RLC elements even up to the GHz bandwidth.
11. Always try to place power and ground planes on adjacent layers with thin dielectric between them.
12. Use SPICE to simulate the parallel resonances when multiple capacitors and the power and ground planes are connected in parallel.
13. Assign return path layers and signal routing in the stack up based on the ability to provide a return via whenever a signal via changes layers.