Impedance profiles, and a bug in Altium's calculator
Set up the impedance profiles for the new board today, hit a bug in Altium's impedance calculator, found a way around it, and filed a report.
Quick one today.
Most of the day went into the impedance profiles for the new multi-channel board. This is the step where you tell the layout tool what the stackup looks like, the copper weights, the dielectric thicknesses, which layers are the reference planes, and it works out how wide a trace needs to be and how far apart a differential pair has to sit to land on the target impedance. Get it right and your numbers and the fab’s numbers agree. Get it wrong and you find out after the boards come back, which is the expensive way to find out.
Which is where Altium got annoying. Partway through, the impedance calculator started giving me results that were clearly wrong for one of the profiles. I spent longer than I’d like to admit assuming the mistake was mine. It wasn’t. Once I was sure it was the tool, I found a way to set the profile up that sidesteps the problem, so the board isn’t blocked on it.
I also filed a bug report with Altium. It might go nowhere, but if it saves one other person an afternoon of second-guessing their own stackup, it was worth the ten minutes.
Net result: the profiles are done, the layout rules can now be driven off them, and I have a new entry on my list of things to double-check by hand instead of trusting the button.
If you’ve run into the same thing in Altium’s impedance calculator, I’d like to hear how you got around it. Get in touch.