PCB News - Single-ended impedance control on PCB circuit board 50 ohm

Single-ended impedance control on PCB circuit board 50 ohm

When designing PCB circuit board drawings, many engineers will have this question for people who are new to PCB impedance. Why do common single-ended wiring in the circuit board board is controlled by 50 ohms by default instead of 40 ohms or 60 ohms ? This is a seemingly simple question that is not easy to answer. Before writing this article, we have also looked up a lot of information about the impedance of the circuit board. The most well-known one is Howard Johnson, PhD's answer to this question. I believe many people have read it.

Why is it difficult to answer? The signal integrity problem itself is a question of trade-offs, so the most famous sentence in the industry is: "Itdepends..." This is a problem where there is no standard answer.

1. 50 ohm PCB has a certain historical origin. This has to start with standard cables. We all know that a large part of modern electronic technology comes from the military, and slowly the military is converted to civilian use. In the initial stage of microwave application, the second During the World War, the choice of impedance was completely dependent on the needs of use. With the advancement of technology, impedance standards need to be given in order to strike a balance between economy and convenience. In the United States, the most commonly used conduit is connected by the existing rod and water pipe. 51.5 ohm is very common, but the adapter/converter used is 50 ohm to 51.5 ohm; it is solved for the joint army and navy. For these problems, an organization called JAN was established, which was later DESC, which was specially developed by MIL. After comprehensive consideration, 50 ohms was finally selected, and special conduits were manufactured and transformed into various cables. standard. At this time, the European standard was 60 ohms. Soon after, under the influence of companies that dominate the industry like Hewlett-Packard, Europeans were also forced to change, so 50 ohms eventually became a standard in the industry. It has become a convention, and the PCB connected to various cables is ultimately required to comply with the 50 ohm impedance standard for impedance matching.

2. From the point of view that circuit board production can be realized, 50 ohms is more convenient to realize. From the previous impedance calculation formula, it can be seen that a PCB circuit board impedance that is too low requires a wider line width and a thin dielectric (or a larger dielectric constant), which is more difficult to satisfy in terms of space for the current high-density board; too high impedance It also requires a thinner line width and a thicker dielectric (or a smaller dielectric constant), which is not conducive to the suppression of EMI and crosstalk. At the same time, the reliability of processing for multi-layer boards and from the perspective of mass production will be relatively poor ; And the ordinary line width and dielectric thickness (4mil-6mil) of 50 ohm in the environment of commonly used materials meet the design requirements. It is also convenient for processing, and it is not surprising that it has gradually become the default choice.

3. From the perspective of loss, according to basic physics, it can be proved that the skin effect loss of 50 ohm impedance is the smallest. Generally, the skin effect loss L (in decibels) of the cable is proportional to the total skin effect resistance R (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 a 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). Combining these factors, given d2 and the corresponding dielectric constant Er of the isolation material, the following formula can be used to minimize the skin effect loss.

Separate the constant term (/60)*(1/d2) from formula 3, and the effective term ((1+d2/d1)/ln(d2/d1)) to determine the minimum point. Look carefully at the minimum point of formula 3 only controlled by d2/d1, and has nothing to do with Er and the fixed value d2. Take d2/d1 as a parameter and draw a graph for L. When d2/d1=3.5911, the minimum value is obtained. Assuming that the dielectric constant of solid polyethylene is 2.25 and d2/d1=3.5911, the characteristic impedance is 51.1 ohms. A long time ago, radio engineers, for convenience, approximated this value to 50 ohms as the optimal value for coaxial cables. This proves that around 50 ohms, L is the smallest.

Finally, from the perspective of electrical performance, the advantage of 50 ohms is also a compromise after comprehensive consideration. Purely from the performance of PCB traces, low impedance is better. For a transmission line with a given line width, the closer the distance from the plane is, the corresponding EMI will be reduced, crosstalk will also be reduced, and it is not easy to be subjected to capacitive loads. Influence. However, from the perspective of the full path, one of the most critical factors needs to be considered, that is, the drive capability of the chip. In the early days, most chips could not drive the transmission line with the impedance of the PCB circuit board less than 50 ohms, and the transmission line with higher impedance was realized. It is inconvenient, so 50 ohm impedance is used as a compromise.

To sum up: 50 ohms as the default value of the industry has its inherent advantages, and it is also a compromise solution after comprehensive consideration, but it does not mean that 50 ohm PCB circuit board must be used. In many cases, it depends on the matching. Interfaces, such as 75 ohms, are still the standard for remote communication. Some cables and antennas use 75 ohms. At this time, a matching PCB line impedance is required. In addition, there are some special chips that reduce the impedance of the transmission line by improving the chip's driving ability to better suppress EMI and crosstalk. For example, most Intel chips require impedance control at 37 ohms, 42 ohms or even lower. I won't repeat them here.