PCB News - Why choose 50 ohm impedance for PCB wiring

Why do many engineers use 50Ohm PCB transmission lines? Sometimes this becomes the default setting for PCB wiring. Why not 60 Ohm or 70 Ohm? When the line width is fixed, there are three main factors that affect the impedance of the PCB board.

1, the influence of the electromagnetic interference layer closest to the PCB board transmission line is proportional to the distance from the PCB transmission line to the nearest reference plane. The smaller the distance, the smaller the radiation.

2, the crosstalk also changes significantly with the thickness of the transmission line, and reducing the thickness of the transmission line by half will reduce the crosstalk of the transmission line.
Third, the smaller the distance, the smaller the impedance, which helps to reduce the influence of capacitive load. All three factors encourage designers to design transmission lines closer to the reference plane. The main reason for preventing the thickness of the transmission line from dropping to zero is that most chips cannot drive transmission lines less than 50 Ohm well, except for Rambus 27Ω and the old National BTL Alliance 17Ω.

Not all impedance control with 50Ω PCB is the best. For example, when the old NMOS8080 processor works at 100Khz, there is no electromagnetic interference, crosstalk, and capacitive load. At the same time, it is impossible to drive a 50Ω transmission line at any time. As far as this processor is concerned, since high impedance will reduce the driving power, we must use the thinnest and highest impedance transmission line we can make.

At the same time, we must also consider mechanical issues (process issues). For example, in the high-density multi-layer board high-voltage interlayer space, 70Ω transmission lines are difficult to manufacture under the current micro printing technology. In this case, you may switch to 50Ω PCB transmission lines. 50Ω transmission lines allow Use a line width wider than 70Ω to make the circuit board manufacturable.

What about the impedance of the coaxial cable? In the world of radio frequency, the problem to be considered is different from that of the printed circuit board. So far, the radio frequency industry still pays attention to coaxial cables with similar impedances, according to the International Electrotechnical Association Publication 78 (1967), 75Ω is a very easy to accept coaxial impedance standard, because you can more easily match several popular antenna structures. It also defines the structure of a 50Ω solid polyethylene wire, which gives a fixed outer shielding layer diameter and a fixed dielectric constant of 2.2. Because 50Ω can reduce the skin effect during transmission.

At the same time, you can prove the superiority of 50Ω coaxial cable line impedance from the theorem of physics. The cable skin effect loss is L (per unit length) proportional to the skin effect resistance R (per unit length) divided by the characteristic impedance Z of the cable. The total skin resistance of the cable is the sum of the resistance of the outer shielding layer plus the resistance of the inner transmission line. At high frequencies, the series skin resistance of the shielding layer is inversely proportional to its diameter d2. The series skin resistance of the inner transmission line is inversely proportional to its diameter d1. The total series resistance is proportional to (1/d2+1/d1). Combining the above factors, the mutual permittivity ER, the shielding layer diameter d2 is given The fixed value of, you can minimize the loss of skin effect with the following formula,

In any elementary electromagnetic field electromagnetic wave textbook, you can find the following formula: Z0 is expressed as the formula of d2, d1, and ER:

Substituting formula 2 into formula 1, formula 3 can be obtained as follows

Separate the constant term ((√ER/60 )(1/d2)) from the formula three, and the variable ((1+d2/d1)/ln(d2/d1)) determines the point where the skin loss is minimized. Check formula three carefully, It is found that the point of minimum loss is only related to the ratio of d2/d1, and has nothing to do with ER and fixed d2.

Taking L as a function of the independent variable d2/d1, an operable structure shows that the minimum loss point is d2/d1=53.5911. Assuming that the dielectric constant of a solid polyethylene insulator is 2.25, which is equivalent to the propagation speed of light 66%. d2/d1=53.5911. The characteristic impedance of the transmission line is 51.1Ω when used in formula 2. A long time ago, radio engineers resolutely only made the impedance of the coaxial line reach a more convenient 50Ω. This does not mean you have to use 50Ω. For example, if you design a 75Ω transmission line, this line has the same outer shielding layer diameter and dielectric constant, and its skin loss is only increased by 12%. Different dielectric constant materials can optimize the value of d2/d1, Resulting in optimized impedance.

The above is the introduction of 50 ohm impedance for routing. IPCB is also provided to PCB manufacturer and PCB manufacturing technology.