Why PCB design generally controls 50 ohm impedance?

A large portion of electronics technology originated in the military, first the technology was used in the military and slowly moved from military to civilian use.

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

In the United States, the most used conduits were made from existing scale rods and water pipes connected together, 51.5 ohms was very common, but the adapters and converters seen and used were again 50-51.5 ohms; to unite the Army and Navy in solving these problems, an organization called JAN was formed (later the DESC organization) developed specifically by MIL, and the combination of considerations eventually led to the selection of 50 ohms,. From this the relevant conduits were manufactured and from this translated into standards for various cables.

At this time, the European standard is 60 ohms, not long after, like Hewlett-Packard such as dominant companies in the industry under the influence of the Europeans were forced to change, so the 50 ohms eventually became a standard in the industry down, it also became a common practice, and a variety of cables connected to the PCB, in order to match the impedance, and ultimately in accordance with the 50 ohms impedance standards. requirements.

Secondly, the development of general standards is based on the PCB production process and design performance, feasibility of the comprehensive consideration.

From the point of view of PCB production and processing technology, most of the existing PCB manufacturers to consider the equipment, the production of 50 ohm impedance PCB is relatively easy to realize.

From the impedance calculation process can be seen, too low impedance requires a wider line width and thin dielectric or larger dielectric constant, which for the current high density board is difficult to meet the space; too high an impedance and the need for thinner line width and thicker dielectric or smaller dielectric constant, is not conducive to EMI and crosstalk suppression, and at the same time for the multilayer boards and from the point of view of the volume of the processing of the reliability of the poorer.

Control 50 ohm impedance in the use of commonly used plates (FR4, etc.), commonly used core board environment, the production of commonly used board thickness of the product (such as 1mm, 1.2mm, etc.), can be designed for common line width (4 ~ 10mil), so that the board factory is very convenient to process up to the processing of the equipment used in the process is not very high requirements.

From the PCB design considerations, 50 ohms is also a comprehensive consideration after the selection. From the performance of the PCB alignment, 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.

However, from the perspective of the full path of the signal, but also need to consider the most critical factor, that is, the chip’s ability to drive, in the early days most of the chip can not drive the impedance of less than 50 ohms of the transmission line, and higher impedance transmission line due to the inconvenience of realizing it, so a compromise with the 50-ohm impedance.

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