In the PCB design process, before wiring, we usually stack the items we want to design and calculate the PCB impedance based on information such as thickness, substrate, number of layers, etc. After calculation, we can get the following picture.
Board impedance
As can be seen from the above figure, the single-ended network designed above is generally controlled at 50 ohms. Many people will ask why the control is based on 50 ohms instead of 25 ohms or 80 ohms?
First, the default selection is 50 ohms, and everyone in the industry accepts this value. Generally speaking, it is certain that recognized organizations have formulated certain standards, and we design according to the standards.
A large part of electronic technology is in the military field. First, the technology was used in the military, and gradually shifted from military to civilian use.
In the early days of microwave application, during World War II, the choice of impedance depended entirely on the needs of the application, and there was no standard value. As technology develops, impedance standards need to be given to balance economy and convenience.
In the United States, conduits are commonly connected by existing scale rods and water pipes. 51.5 ohms is very common, but the adapters and converters used are all 50-51.5 ohms. In order to solve these problems, mil specially developed an organization called Jan (later desc organization). After comprehensive considerations, 50 ohms was chosen, which is related to the fact that the conduits are manufactured and converted to standards for various cables.
At this time, the European standard is 60 ohms. Soon after, Europeans were forced to transform under the influence of industry giants such as Hewlett-Packard. Therefore, 50 ohm eventually became the industry standard and became the norm. In order to match the impedance, according to the 50 ohm impedance standard, the PCB with various cables needs to be connected.
Secondly, the formulation of general standards will be based on the comprehensive consideration of PCB production technology, design performance and feasibility.
From the perspective of PCB production and processing technology, considering the existing equipment of most PCB manufacturers, it is very easy to produce PCBs with an impedance of 50 ohms.
It can be seen from the impedance calculation process that low impedance requires a wider line width and a thinner medium or a larger dielectric constant, which is difficult to meet in space for current high-density boards; while high impedance requires Thinner line widths and thicker dielectrics or smaller dielectric constants are required, which is not conducive to suppressing EMI and crosstalk. At the same time, for multi-layer boards, from the perspective of mass production, the processing reliability is also very high and will be even worse.
Controlling 50 ohm impedance, in the environment of using ordinary boards (FR4, etc.) and ordinary core boards, it can be designed to produce ordinary board thicknesses (such as 1mm, 1.2mm, etc.) and ordinary line widths (4~10ml), which is very convenient for processing by board factories. , the requirements for processing equipment are not very high.
From the perspective of PCB design, 50 ohm is also a comprehensive choice. In terms of PCB wiring performance, low impedance is better. For a transmission line of a given line width, the closer it is to the plane, the corresponding EMI will be reduced, and the crosstalk will also be reduced.
However, looking at the entire signal path, we need to consider a key factor, which is the driving capability of the chip. Most early chips were unable to drive transmission lines with an impedance less than 50 ohms, and higher impedance transmission lines were not easy to implement, so an impedance of 50 ohms was used.
Therefore, 50 ohms is generally selected as the default value for single-ended signal control PCB impedance.