Why PCB designs generally control 50 ohm impedance?

In the process of PCB design development, before the line, we generally need to layer the project management of the teaching design of the students themselves, 1oz vs 2oz analyze and calculate the impedance according to different thickness, substrate, layer number and other information technology, and generally get the following figure after calculation.

Single-ended networks are designed to be controlled at 50 ohms, so many people ask, why is it regulated at 50 ohms and not 25 or 80 ohms?

First of all, the default is 50 ohms, which is accepted by everyone in the industry. Generally speaking, a standard must be developed by a recognized body, and everyone will design according to the standard.

A large part of electronic technology originated from the military, how to test a circuit board with a multimeter the first technology was used in the military, and slowly from the military to civilian.

In the early days of microwave applications, during World War II, the choice of impedance was entirely dependent on the needs of use, and there was no standard value. As technology evolves, impedance standards need to be given in order to strike a balance between economy and convenience.

Multilayer impedance board

In the United States, the catheter that can be used at most is connected by the existing rod and water pipe, 51.5 ohms technology is very common, but the adapters and converters seen and used are also 50-51.5 ohms; In order to cooperate with the Chinese army and navy to solve our social problems, an organization called JAN was formed (later DESC), which was developed by MIL in particular, and after comprehensive analysis and consideration, finally selected 50 ohms, from which the relevant conduit was manufactured and thus transformed into a standard for various kinds of cables for students.

The European standard at the time was 60 ohms, and not long ago, under the influence of major companies like HP, the Europeans were forced to change, so 50 ohms eventually became the industry standard, became a common practice, and pcbs were connected to various cables, and finally, the 50 ohms impedance standard was also required for impedance matching.

Secondly, the development of a common standard will be based on a comprehensive consideration of the PCB production process, design performance and feasibility.

From the PCB manufacturing process point of view, considering the equipment of most existing PCB manufacturers, it is easy to produce a PCB with an impedance of 50 ohms.

From the process of impedance calculation, it can be seen that too low impedance requires a wide line width, a thin medium or a large dielectric constant, which is difficult to meet the space requirements for the current high-density plate. Excessive impedance requires thinner line widths, thicker media or smaller permittivity, which is not conducive to EMI and crosstalk suppression, and the reliability of processing is poor for multi-layer plates and from a mass production point of view.

Control 50 ohms contact impedance In the use of some common material plate (FR4, etc.), commonly used core plate environment, production enterprises commonly used plate thickness products (such as 1mm, 1.2mm, etc.), you can design a common line width (4~10mil), the prototype factory processing development is very simple and convenient, The equipment management requirements for its processing technology use are not very high.

From the perspective of PCB design, 50 ohms is also the choice after comprehensive consideration. From the PCB wiring performance, the general low impedance is better, for a given line width of the transmission line, and the closer the plane distance, the corresponding EMI will be reduced, crosstalk will also be reduced.

But from the perspective of the entire signal path, the most critical factor to consider is the drive capability of the chip. Most of the early chips could not drive a transmission line with an impedance of less than 50 ohms, and a transmission line with a higher impedance was not convenient to achieve, so 50 ohms was used as a compromise.

Therefore, 50 ohms is generally chosen as the default value for single-ended signal control impedance.