In PCB design, impedance usually refers to the characteristic impedance of transmission lines. This is the impedance that electromagnetic waves encounter when propagating through a conductor, and it is influenced by various factors such as the geometric shape of the conductor, the dielectric material, and the surrounding environment.
For general high-speed digital signal transmission and RF circuits, 50Ω is a commonly used impedance value.
But why 50Ω? Can’t we use 30Ω or 80Ω instead? The default selection of 50Ω impedance is determined by multiple factors, including history, PCB manufacturing technology, circuit design, and cost.
① 50Ω impedance standard was established by the US military
During World War II, the choice of impedance was entirely based on the specific needs of each application, without any standardized value. However, as technology advanced, there was a need to establish impedance standards that balanced both economic and practical considerations.
As a result, a joint organization of the US military, the Joint Army Navy (JAN) organization, based on considerations of impedance matching, signal transmission stability, and prevention of signal reflection, ultimately selected 50Ω impedance as a commonly used standard value.
Subsequently, the 50Ω impedance gradually developed into the default global standard value.
② 50Ω impedance enables maximum power transfer
From a PCB design perspective, at 50Ω impedance, signals can be transmitted at maximum power in the circuit, thereby reducing signal attenuation and reflection. In wireless communication, 50Ω impedance is also the most commonly used antenna input impedance.
Generally, lower impedance results in better performance of PCB traces. For a given line width of a transmission line, the closer it is to the plane, the lower the corresponding EMI and crosstalk will be. However, from the perspective of the entire signal path, the impedance cannot be too low, which involves the most critical factor, the chip’s driving capability.
In the early days, most chips could not drive transmission lines with an impedance of less than 50Ω, while higher impedance transmission lines were inconvenient to implement and did not perform as well. Therefore, adopting 50Ω impedance was the optimal choice at that time.
③ 50Ω impedance is easy to match
In PCB design, impedance matching is often necessary to reduce signal reflection and interference. When designing PCB traces, we usually stack the layers and calculate the impedance based on information such as thickness, substrate, and number of layers. The calculation typically results in a value close to 50Ω impedance, which is widely used in electronic engineering and makes it easy to achieve impedance matching and reduce signal reflection and interference.
④ Choosing 50Ω impedance makes PCB production easier
From the perspective of the PCB manufacturing process, it is relatively easy to produce PCBs with 50Ω impedance using the equipment of most PCB manufacturers.
As can be seen from the impedance calculation process, low impedance requires wider trace widths and thinner dielectrics or higher dielectric constants, which are difficult to meet in high-density boards. On the other hand, high impedance requires narrower trace widths and thicker dielectrics or lower dielectric constants, which is not conducive to EMI and crosstalk suppression and may reduce the reliability of manufacturing for multilayer boards and mass production.
By controlling the 50Ω impedance for commonly used materials (such as FR4), common core board environments, and commonly used board thicknesses (such as 1mm and 1.2mm), and designing common trace widths (4-10mil), PCB manufacturers can easily process the boards and do not require high-end equipment.
⑤ Using 50Ω impedance can improve compatibility between devices in electronic systems
Many standards and devices’ circuit boards, connectors, and cables are designed for 50Ω impedance. Therefore, using 50Ω impedance can improve the compatibility between devices.
⑥ Using 50Ω impedance can lower the PCB manufacturing cost
Considering the balance between manufacturing cost and signal performance, 50Ω impedance is an economical and practical choice.
Words in the end
In conclusion, due to its relatively stable transmission characteristics and low signal distortion rate, 50Ω impedance is widely used in various fields, such as video signals and high-speed data communication. However, it should be noted that although 50Ω impedance is one of the most commonly used impedances in electronic engineering, other impedance values may be required in certain applications, such as RF, to meet specific requirements. Therefore, in specific designs, the appropriate impedance value should be selected based on the actual situation.