Talking about the Single End Impedance Control of 50 Ohms on PCB_Shenzhen StarLight Industrial Development Co., Ltd.

Talking about the Single End Impedance Control of 50 Ohms on PCB

source:News release time:2022-12-05 Hits: Popular:PCB Assembly company: Share:

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　　When many engineers are designing PCB drawings, they will have this question for people who are new to PCB impedance. Why is it that common single end wiring in a PCB is controlled according to the default requirement of 50 ohms instead of 40 ohms or 60 ohms? This is a seemingly simple but difficult question to answer. Before writing this article, we also looked up a lot of impedance data about circuit boards, among which Howard Johnson is the most famous. I believe many people have read the answer of PhD to this question.

　　Why can't you answer? The question of signal integrity itself is a matter of trade-offs, so the most famous sentence in the industry is: "It depends..." This is a question that has no standard answer, and different people have different opinions. Today, Mr. Gao has also summarized various answers to this question. This is also a good place to start. I hope more people can summarize more relevant factors from their own perspectives.

　　1、 50 ohm has a certain historical origin, starting from the standard cable. We all know that a large part of modern electronic technology came from the military, and gradually the military was converted to civilian use. In the early days of microwave application, during World War II, the selection of impedance completely depended on the needs of use. With the development of technology, it is necessary to provide impedance standards in order to balance the economy and convenience. In the United States, the most commonly used conduit is connected by the existing scale rod and water pipe, 51.5 ohms is very common, but the adapters/converters used are 50 ohms to 51.5 ohms; In order to solve these problems, an organization named JAN was established, which was later DESC developed by MIL. After comprehensive consideration, 50 ohm was finally selected, and special conduits were manufactured, which were converted into various cable standards. At this time, the European standard was 60 ohms. Soon after, under the influence of companies such as Hewlett Packard, which were dominant in the industry, Europeans were forced to change. So 50 ohms eventually became a standard in the industry and became a convention. For the sake of impedance matching, PCBs connected with various types of cables were finally required to follow the 50 ohm impedance standard.

　　2、 From the point of view that the circuit board can be made, 50 ohm is relatively convenient. From the impedance calculation formula above, it can be seen that the low PCB impedance requires a wide line width and a thin medium (or a large dielectric constant), which is difficult to meet in space for high-density boards at present; High impedance requires thin line width and thick medium (or small dielectric constant), which is not conducive to EMI and crosstalk suppression. At the same time, the processing reliability of multilayer boards and mass production is poor; In the environment of common materials, the common line width and dielectric thickness (4mil-6mil) of 50 ohm not only meet the design requirements (as shown in the figure below - impedance calculation), but also facilitate processing. It is not surprising that it gradually becomes the default choice.

　　3、 From the point of view of loss, it can be proved that 50 ohm impedance skin effect has the lowest loss according to basic physics (from Howard Johnson, PhD's reply). In general, the skin effect loss L (in decibels) of the cable is proportional to the total skin effect resistance R (in unit length) divided by the characteristic impedance Z0. The total skin effect resistance R is the sum of the resistance of the shielding layer and the intermediate conductor. The skin effect resistance of the shielding layer is inversely proportional to its diameter d2 at high frequencies. The skin effect resistance of the inner conductor of the coaxial cable is inversely proportional to its diameter d1 at high frequencies. The total series resistance R is therefore proportional to (1/d2+1/d1). Considering these factors, given the dielectric constant Er of d2 and the corresponding isolation material, the following formula can be used to minimize the skin effect loss.

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