In the field of wireless communications, various RF devices, connectors, coaxial cablecables, and even some RF PCB printed wires are standardised with 50Ω impedance. Why is this?
1, from the PCB design considerations
50Ω is also a comprehensive consideration after the selection. From the impedance calculation process can be seen, for the width of the alignment, the thinner the plate medium, the lower the impedance. 3 main factors will affect the PCB alignment impedance.
Firstly, the EMI (Electromagnetic Interference) of the near field of the PCB alignment and the height of this alignment from the reference plane (i.e., the distance of the microstrip line to the reference ground) are proportional to a certain degree, the lower the height means that the radiation is smaller.
Secondly, crosstalk with the height of the alignment will have a significant change, the height is reduced by half, crosstalk will be reduced to nearly a quarter.
Finally, the lower the height, the smaller the impedance, less susceptible to capacitive loads.
2, from the signal all path considerations
Design also need to consider the most critical factor, that is, the chip's ability to drive, most of the chip in the early drive impedance less than 50Ω transmission line, and higher impedance transmission line due to the inconvenience of implementation, so a compromise using 50Ω impedance.
3, from the perspective of electrical performance
Here again from the perspective of loss. In the high-frequency high-speed line there is a skin effect. The industry has proved that 50Ω for the skin effect, it is the smallest loss. Usually the skin effect of the cable loss L (in decibels as a unit) and the total skin effect resistance R (unit length) divided by the characteristic impedance Z0 is proportional to.
The total skin effect resistance R is the sum of the shield and the intermediate conductor resistance. The skin effect resistance of the shield 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).
Taking these factors together, given d2 and the corresponding dielectric constant Er of the isolation material, the ratio d2/d1 can be calculated in the case where skin effect losses are minimised. Assuming that the dielectric constant of solid polyethylene is 2.25, the skin effect loss is minimised, d2/d1 = 3.5911 resulting in a characteristic impedance of exactly 50Ω.
4, from the perspective of the production process
From the point of view of PCB production and processing technology, most of the existing PCB manufacturers to consider the equipment, the production of 50Ω impedance PCB is relatively easy to achieve.
From the impedance calculation process can be seen, too low impedance requires a wider line width and thin dielectric or larger dielectric constant, which for the current high density board space is more difficult to meet.
Too high an impedance and the need for thinner line width and thicker dielectric or smaller dielectric constant, not conducive to EMI and crosstalk suppression, and for multilayer boards and from the point of view of mass production in terms of processing reliability will be worse.
Control 50Ω impedance in the use of commonly used plates (FR4, RO4350B, etc.), commonly used core board environment, the production of commonly used board thickness of the product (such as 0.508mm, 1mm, 1.2mm, etc.), can be designed to common line width (4 ~ 10mil), so that the board factory is very convenient to process, and the processing of the equipment used in the requirements are not very high.
In summary, for RF applications kind of 50Ω impedance standard, is the industry after a long period of practice to unify down, from manufacturing, from the design, electrical performance, historical factors are a compromise choice.
