Why do people often lift a 50 Ω impedance when designing a PCB

Why do people often lift a 50 Ω impedance when designing a PCB
In PCB design, impedance usually refers to the characteristic impedance of the transmission line, which is the characteristic impedance when electromagnetic waves are transmitted in the wire, and is related to the geometric shape of the wire, the dielectric material, the surrounding environment of the wire and other factors.
For general high-speed digital signal transmission and RF circuits, 50 Ω is a commonly used impedance value.
Why is it 50 Ω? Can’t 30 Ω or 80 Ω be used? The default selection of 50 Ω impedance is determined by multiple factors such as history, PCB production process, circuit design, and cost.
1、 50 Ω impedance is an impedance standard established by the US military
During World War II, the selection of impedance was entirely dependent on the needs of use, without a standard value. However, with the advancement of technology, impedance standards need to be provided in order to achieve a balance between economy and convenience.
So, a joint organization of the US military, the Joint Army Navy, based on considerations such as impedance matching, signal transmission stability, and preventing signal reflection, ultimately chose 50 Ω impedance as the commonly used standard value.
Afterwards, the 50 Ω impedance gradually developed into the global default standard value.
2、 50 Ω impedance for maximum power transmission
From the perspective of PCB design, at an impedance of 50 Ω, signals can be transmitted at maximum power in the circuit, thereby reducing signal attenuation and reflection. In the field of wireless communication, the 50 Ω impedance is also the most commonly used antenna input impedance.
Generally, with low impedance, the performance of PCB wiring will be better. For a given line width transmission line, the closer it is to the plane, the corresponding EMI will decrease and crosstalk will also be reduced. However, from the perspective of the full path of the signal, the impedance cannot be too low – this involves the most critical factor, which is the driving ability of the chip.
In the early days, most chips were unable to drive transmission lines with impedance less than 50 Ω, while transmission lines with higher impedance were inconvenient to implement and did not perform as well, so using a 50 Ω impedance as a compromise was the optimal choice at the time.
3、 50 Ω for easy impedance matching
In PCB design, impedance matching is often required to reduce signal reflection and interference. When designing PCB wiring, we usually stack the items we want to design and calculate impedance based on information such as thickness, substrate, and number of layers.
The 50 Ω impedance is widely used in the field of electronic engineering, making it easy to perform impedance matching and reduce signal reflection and interference.
4、 Choosing a 50 Ω impedance makes PCB production easier
From the perspective of PCB production and processing technology, considering the equipment of most existing PCB manufacturers, it is relatively easy to produce 50 Ω impedance PCBs.
From the impedance calculation process, it can be seen that a low impedance requires a wider line width and a thin medium or a larger dielectric constant, which is relatively difficult to meet in space for current high-density boards. High impedance also requires a thinner line width and a thicker medium or a smaller dielectric constant, which is not conducive to EMI and crosstalk suppression. At the same time, for multi-layer boards and from a production perspective, the reliability of processing will be relatively poor.
Controlling the 50 ohm impedance in an environment where commonly used boards (such as FR4) and core boards are used, and producing commonly used board thickness products (such as 1mm, 1.2mm, etc.), common line widths (4-10mil) can be designed, making it very convenient for the board factory to process, and the requirements for the equipment used for processing are not very high.
5、 50 Ω impedance can improve compatibility between devices
Many standard and device circuit boards, connectors, and cables are designed for a 50 Ω impedance, so using a 50 Ω impedance can improve compatibility between devices.
6、 Using a 50 Ω impedance, PCB manufacturing costs are lower
Considering the balance between manufacturing cost and signal performance, a 50 Ω impedance is an economical and practical choice.
epilogue
Due to its relatively stable transmission characteristics and low signal distortion rate, the 50 Ω impedance is widely used in other fields, such as video signals and high-speed data communication. However, it should be noted that although the 50 Ω impedance is one of the most commonly used impedances in electronic engineering, in some applications, such as radio frequency, other impedance values may be required to meet specific needs. Therefore, in specific design, appropriate impedance values should be selected based on the actual situation.
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