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Why PCB Impedance Control Is 50 OHM Often?

Many newhands are curious Why the common single impedance controlled by 50 ohms instead of 40 ohms or 60 ohms?

First of all, 50 ohm has a certain historical origin, it must be mentioned from the standard cable. As we all know, a great part of modern electronic technology is derived from the army and gradually converted to civilian use. In the early days of microwave application, the choice of impedance was entirely dependent on the need for use during the Second World War.As technology advances, impedance standards need to be given to balance the economy and convenience. In the United States, the most used catheters are connected by existing rod and water pipes, 51.5 ohms are very common, but 50 ohms to 51.5 ohm to see and used in the adapter and converter.As the joint army and navy to solve these problems, an organization called JAN was established, which later became the DESC, especially by MIL development, considering the final choice of 50 ohms, and the catheter was created, and thus transformed into a variety of cable standard. This European standard is 60 ohms, soon after, in effect such as Hewlett-Packard in the industry of dominant firms, Europeans are forced to change, so the 50 ohm will eventually become a standard industry inherited, also become established, and all kinds of cable connections and PCB for impedance matching. The final is in accordance with the requirements of the standard impedance of 50 ohms.

Secondly, from the point of view of PCB fabrication, 50 ohm is more convenient to implement. The calculation formula of impedance from the front, low impedance to wide linewidth and thin dielectric (or larger dielectric constant), the present high density board space is difficult to meet the high impedance and linewidth; thin and medium thick (dielectric constant or smaller), is not conducive to the inhibition of EMI and crosstalk, and the reliability and production from the perspective of plate processing will be relatively poor; and 50 ohms in commonly used materials under the environment of ordinary width and medium thickness (4~6mil) that meets the design requirements (Figure 1, impedance calculation and convenient processing), slowly become the default option is be not at all surprising.

Third, from the point of view of loss, according to the basic physics, it can be proved that the 50 ohm impedance has the least skin effect and the least loss (from Howard Johnson, PhD’s reply). In general, the skin effect of the cable, the loss of L (in decibels), and the total skin effect resistance, R (unit length), are directly proportional to the characteristic impedance Z0. The total skin effect resistance R is the sum of the resistance of the shield layer and the intermediate conductor. The skin effect resistance of the shield is inversely proportional to its diameter D2 at high frequencies. The skin effect resistance of the internal 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 the dielectric constant Er of D2 and the corresponding isolation material, the skin loss effect can be minimized by using the following formula.

 impedance formula 1      formula 1

In any basic book on electromagnetic fields and microwaves, you can find that Z0 is a function of D2, D1, and Er.

 impedance formula 2      formula 2

Substituting formula 2 in Formula 1, the numerator denominator is multiplied by D2 at the same time

 impedance formula 3formula 3 

A constant term (/60) * (1/d2), valid item ((1+d2/d1) /ln (d2/d1)) is determined from the formula 3 to determine the minimum point. Check that the minimum point of formula 3 is controlled only by d2/d1, independent of Er and fixed value d2. Take d2/d1 as the parameter, do the graph for L, and display the minimum value when d2/d1=3.5911. Assuming that the dielectric constant of solid polyethylene is 2.25, the characteristic impedance of d2/d1=3.5911 is 51.1 ohms. A long time ago, radio engineers, for convenience, approximated the value to 50 ohms as the optimum value for coaxial cables. This proves that L is the smallest in the vicinity of 50 ohms.

Finally, from the point of view of electrical performance, the advantage of 50 ohms is considered as a compromise. For the pure performance from PCB line, low impedance transmission line is better, for a given width, and the closer the corresponding EMI will decrease, crosstalk will therefore decrease, but also less susceptible to capacitive load effects. But from the perspective of the path, but also need to consider a key factor driving ability, that is the chip, most of the early chip driving transmission lines not less than 50 ohm impedance, and high impedance transmission line due to achieve so inconvenience, using a 50 ohm impedance compromise.

To sum up: 50 ohms as the default value of the industry has its inherent advantages, but also the comprehensive consideration of the compromise, but not to say that we must use 50 ohm impedance, often depends on the matching interface, such as 75 ohm is still remote communication standard, some cable and antenna is the use of 75 ohms, then you need to match the impedance of PCB line. There are also some special chip chip driven by improving the ability to reduce the impedance of the transmission line, in order to get a better suppression of EMI and crosstalk effects, such as most of the requirements of Intel chip impedance control in 37 ohm, 42 ohm or less, and will not go.

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