Why Most PCB Transmission Line is 50 Ohms

Dated:2018-01-30      Popularity:1375


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Why do so many engineers use 50 ohm PCB transmission lines, sometimes this becomes the default PCB wiring layout. Why not 60 ohms or 70 ohms? 

In the case of a fixed line width, there are three main factors that affect the impedance of the PCB.

First, the impact of the nearest electromagnetic interference layer to the PCB transmission line is proportional to the distance from the PCB transmission line to the nearest reference plane, and the smaller the distance, the smaller the radiation.

Second, crosstalk varies significantly with the thickness of the transmission line. Reducing the thickness of the transmission line by half will reduce crosstalk in the transmission line.

Third, the smaller the distance, the smaller the impedance, which helps reduce the effects of capacitive loads.

All three factors encourage designers to design transmission lines closer to the reference plane. The main reason blocking the transmission line thickness to zero is that most chips will not be able to drive transmission lines smaller than 50 ohms, except for the Rambus 27 ohms and the old National BTL Alliance 17 ohms.

Not all PCB impedance control should use 50 ohms. Such as: if the old NMOS 8080 processor operates at 100Khz frequency, then there’s no electromagnetic interference and crosstalk and capacitive load problems, and at any time it is also impossible to drive a 50 ohms transmission line.

At the same time we have to consider the mechanical problems or process problems. For example, a 70 ohms transmission line can be very difficult to make under today's tiny printing technologies in a high-pressure multi-layer sandwich space, in which case you may instead use a 50 ohms transmission line that is wider than 70ohms line width, so that printed circuit board can be manufactured.


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