PCB design begins off being a comparatively simple affair — you create an oblong define, assign some element footprints, run some traces, and dump out some Gerber recordsdata to ship to the fab. Then as you get extra skilled and start attempting more durable circuits, dipping into switching energy provides, excessive velocity digital and low noise analog, issues get progressively harder; and we haven’t even talked about RF or microwave design but, the place issues can get simply plain bizarre from the uninitiated viewpoint. [Robert Feranec] is not any stranger to such issues, and he’s teamed up with one among main consultants (and one among this scribe’s private electronics heroes) in sign integrity issues, [Prof. Eric Bogatin] for a deep dive into the how and why of controlled impedance design.
One fascinating a part of the dialogue is why is 50 Ω so prevalent? The reply is firstly historic. Back within the Thirties, coaxial cables wanted for radio purposes, had been designed to reduce transmission loss, utilizing cheap dimensions and polyethylene insulation, the impedance got here out at 50 Ω. Secondarily, when designing PCB traces for an affordable value fab, there’s a trade-off between energy consumption and noise immunity.
As a rule of thumb, reducing the impedance will increase noise immunity at the price of extra energy consumption, and better impedance goes the opposite manner. You have to steadiness this with the ensuing hint widths, separation and general routing density you’ll be able to tolerate.
Another enjoyable story was when Intel had been designing a excessive velocity bus for graphical interfaces, and created a simulation of a typical bus construction and parameterized the bodily constants, such because the hint line widths, dielectric thickness, by way of sizes and so forth, that had been viable with low-cost PCB fab homes. Then, utilizing a Monte Carlo simulation to run 400,000 simulations, they positioned the candy spot. Since the by way of design appropriate with a budget fab design guidelines resulted usually in a by way of attribute impedance that got here out fairly low, it was really useful to scale back the hint impedance from 100 Ω to 85 Ω differential, relatively than strive tweak the by way of geometry to deliver it as much as match the hint. Fun stuff!
We admit, the video is from the beginning of the 12 months and very lengthy, however for such vital primary ideas in excessive velocity digital design, we expect it’s nicely price your time. We actually picked up a few helpful titbits!
Now we’ve obtained the PCB building nailed, why circle again and go check those cables?