@sukeshv - "How can that be related to PCB environment..??"
Good point - we do tend to get off-topic here. Betajet's explanation is good - PCB traces approximate to 50Ω and it is (happily) a commonly used impedance. But it will vary with the thickness of the board, track size, etc as Betajet points out. Also ECL - for years the predominant high-speed logic family - works at 50Ω as well, maybe that has something to do with it?
HI Glen. "The tale I heard is that a 1/4 wave vertical antenna on a right-angled ground plane is about 36 ohms, and a non-folded dipole above the ground effect is about 72 ohms. 50 ohms is a nice round number in the middle."
That's funny! SWRs don't worry about round numbers, and with most antennas you can get a really good SWR with 50Ω cable.
I think the natural resonance of a folded dipole is around 50 ohms, so that's why 50 ohms is so commonly used. But TV antennas seem to use 75Ω, as do satellite dishes.
I remember my comms textbooks had a lot to say on this subject, but they are back in Zimbabwe - possibly.......
The tale I heard is that a 1/4 wave vertical antenna on a right-angled ground plane is about 36 ohms, and a non-folded dipole above the ground effect is about 72 ohms. 50 ohms is a nice round number in the middle.
A 1/4 wave vertical antenna with a ground counterpoise angled downwards (such as might be found on the rooftop of a ham radio operator) is a very close match to 50 ohms. When properly tuned mine usually measured better than 1.1 to 1 standing wave ratio when fed with 50 ohm coax cable.
The trade-off is between dieletric breakdown and power delivered to the load...for a fixed inner conductor diameter one could increase outer diameter to increase breakdown voltage...but the characteristic impedance would increase and that would make power deliverable to the load smaller...so there is a well defined ratio of the of conductor diameters that maximizes power handling capability of the impedance of the coax
Here's my understanding, based on decades-old experience so the numbers may have changed over the years:
The impedance of a wire is determined by its dimensions, what other conductors are nearby, the distance to those conductors, and what material is used for insulation. On PC boards, a wire over a ground plane gives you 50 Ohms with a reasonable thickness of the insulating layer. You could increase the impedance by using a thicker insulating layer, but then you have a thicker board.
For USB and CAN, the dimensions of the twisted pair and insulation type and thickness give you the 90-120 Ohms.
Coaxial cable typically gives you 50-75 Ohms.
Being off by a few percent doesn't matter. An impedance mismatch gives you reflections, but if the mismatch is small the reflections are small compared to the signal.
Drones are, in essence, flying autonomous vehicles. Pros and cons surrounding drones today might well foreshadow the debate over the development of self-driving cars. In the context of a strongly regulated aviation industry, "self-flying" drones pose a fresh challenge. How safe is it to fly drones in different environments? Should drones be required for visual line of sight – as are piloted airplanes? Join EE Times' Junko Yoshida as she moderates a panel of drone experts.