I like Rick. At least they got that it was the original coax cables that drove it. Then once you have lots of measurement equipment and cables, it's hard to change (even SMA is 50ohm).
You can still get 75ohm BNC cables (close to 77) and 93ohms (and I think some 150/300ohm antenna cables?). Once you get over a few GHz all the inputs are balanced differential anyway and you have T networks to optimize power reflection.
It doesn't mention at all that free space impedance that matters a lot to actual aerial antenna design is 377ohms. I don't know enough about it, but assume they're using transformers to balance loads (about 300ohms?).
Many antennas are designed for a given gain and polarization, and you get the impedance you get from the materials and geometry used. Almost all of them have a matching network at the entrance to the antenna, and usually one set up based on actual S-parameter measurements. If you're really fancy, you can get custom coax cables made to match your antenna at any arbitrary impedance, and then put a much better matching circuit in your rack instead of using one on the antenna.
High-speed digital signals haven't entirely settled on 50 ohm (100 ohm differential) impedance, though. SSTL, HSTL, and POD (used by different types of memory and high-speed DACs/ADCs) often use 35-40 ohm impedance instead of 50. It's also not uncommon to see drivers having lower impedance and receivers having higher impedance (eg having a 32 ohm driver and a 48 ohm receiver on a 40 ohm wire) because a little bit of reflection can actually help signal integrity here.
Long digital wires like twinax Ethernet are all 100 ohm differential signals, though, since reflection is not beneficial there at all.
All the medium speed stuff I see (not 64Gb Serdes or even Thunderbolt) is differential current drive/receive though they may be voltage limited on the output. Those still seem to be designed as 50ohm, but they have some fairly high resistance connection's. Even at 1Gb getting a clean eye requires lots of equalization and careful routing, if you want to go 10+ inches.
> usually one set up based on actual S-parameter measurements
My brother-in-law made this funny remark couple years back. He's into mechanical engineering. "Having a simulator is awesome, but sometimes it's just faster to let nature do the calculations for you."
It's funny how you can't even measure things at high frequencies without parasitics screwing everything up.
You can still get 75ohm BNC cables (close to 77) and 93ohms (and I think some 150/300ohm antenna cables?). Once you get over a few GHz all the inputs are balanced differential anyway and you have T networks to optimize power reflection.
It doesn't mention at all that free space impedance that matters a lot to actual aerial antenna design is 377ohms. I don't know enough about it, but assume they're using transformers to balance loads (about 300ohms?).