50 ohm why

These cables would also span long distances, reaching hundreds of miles. This means the cables need to be designed with highest power transfer, highest voltage, and lowest attenuation. Which impedance should be used to satisfy all three objectives? The graph below shows the tradeoff between losses and power. The file below is provided by Wikimedia, but you can find similar graphs from many other references. The power calculation requires using the full solution for the fundamental propagating mode and the characteristic impedance.

One point to understand about the above graph is that dielectric dispersion is generally not included and will affect the results at higher frequencies. Dispersion both Dk value and loss tangent are taken to have flat dispersion when calculating these curves, which may not match reality within your frequency range. However, the curve gives us a good idea as to why there is the focus on 50 Ohm impedance.

The quick answer to this question is that 50 Ohms is the least bad compromise between the impedance corresponding to minimum loss, maximum power, and maximum voltage. However, one might notice that the impedance with minimum loss in a PTFE-filled coaxial cable is just about 50 Ohms, so this seems like another natural explanation!

Low-cost coaxial cables with air or low-Dk dielectric filler can shoot for 77 Ohm impedance for long cables runs, but the reason for rounding off to 75 Ohms instead of using 77 Ohms is still a mystery to me. In coaxial cables with steel core, the diameter is just slightly oversized to give some extra flexibility, so the impedance would come out to 75 Ohms.

When working with high-speed or high-frequency channels, we generally use S-parameter measurements as important signal integrity metrics. If you have a measured S-parameter matrix for an interconnect on your PCB, you can transform it to a new S-parameter matrix with the following transformation:.

This is useful for understanding how your S-parameters might change when you switch your reference media e. We have only scratched the surface of what is possible to do with Altium Designer on Altium You can check the product page for a more in-depth feature description or one of the On-Demand Webinars. Zachariah Peterson has an extensive technical background in academia and industry. He currently provides research, design, and marketing services to companies in the electronics industry.

In the early days of microwaves - around World War II, impedances were chosen depending on the application. In those days, there were no flexible cables, at least for higher frequencies, only rigid tubes with air dielectric. Semi-rigid cable came about in the early 50s, while real microwave flex cable was approximately 10 years later.

Somewhere along the way it was decided to standardize on a given impedance so that economy and convenience could be brought into the equation. In reality, in the U. Eventually, 50 won out, and special tubing was created or maybe the plumbers allowed their pipes to change dimension slightly. There are two factors that drive 50 Ohm impedance: losses and power. If we can plot transmission line loss versus characteristic impedance. It turns out that insertion loss is minimum at around 77 ohms.