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What is impedance? Why should impedance be controlled at 50 ohms in PCB design? Now let's learn about impedance. Impedance: in a circuit with resistance, inductance, and resistance, the resistance to the current in the circuit is called impedance. It is often referred to as Z, which is a complex number, the real part is called resistance, and the imaginary part is called reactance.
The unit of impedance is the ohm. In the process of PCB design, before wiring, we usually stack the items to be designed by ourselves, and calculate the impedance according to the information of thickness, base material and layer number. After calculation, the following figure can be obtained.
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First of all, the default is 50 ohms, and everyone in the industry accepts that, generally speaking, there has to be a standard set by some recognized institution, and people design according to the standard. A large part of the electronic technology comes from the military. First, the technology is used in the military, and then it is gradually transferred from the military to the civilian. In the early days of microwave applications, during world war ii, impedance selection was entirely dependent on usage, with no standard value. As technology advances, impedance standards need to be developed to strike a balance between economy and convenience.
In the United States, the most commonly used conduit is made up of an existing rod and water pipe. To address these issues for the joint army and navy, an organization called JAN (later DESC organization) was formed, specifically developed by the MIL, to take into account the overall selection of 50 ohms, from which the relevant conduit was manufactured and subsequently converted into a standard for various cables. European standard is 60 ohms, not long after, the dominant in the industry, such as hewlett-packard company, under the influence of europeans were changed, so 50 ohm inherited from the eventually become a standard in the industry, has become established, and a variety of cable connection of PCB, for the sake of impedance matching, finally also is according to 50 ohm impedance standards requirements.
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Secondly, the formulation of general standards is based on the comprehensive consideration of PCB production process, design performance and feasibility. From the perspective of PCB production and processing technology, it is relatively easy to produce PCB with impedance of 50 ohms considering the equipment of most PCB manufacturers. From the calculation process of impedance, it can be seen that too low impedance requires a wide line width and a thin medium or a large dielectric constant, which is difficult to meet in space for current high-density plates. Too high impedance requires a thin line width and a thick medium or a small dielectric constant, which is not conducive to the suppression of EMI and crosstalk, and at the same time, the processing reliability of multilayer plates and from the perspective of mass production is poor. Control 50 ohm impedance under the environment of common plate (FR4, etc.) and common core plate, the common plate thickness products (such as 1mm, 1.2mm, etc.) can be produced, and the common line width (4~10mil) can be designed. This model factory is very convenient for processing, and the equipment requirements for its processing are not very high.
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In terms of PCB design, 50 ohms is also the choice after comprehensive consideration. Generally speaking, low impedance is good for the performance of PCB wiring. For a transmission line with a given line width, the closer it is to the plane, the corresponding EMI will be reduced and crosstalk will be reduced accordingly. However, from the point of view of signal full path, the most critical factor needs to be considered, that is, the driving ability of chip. In the early stage, most chips could not drive the transmission line with the impedance less than 50 ohm, while the transmission line with the higher impedance was inconvenient to realize, so the compromise adopted 50 ohm impedance. Therefore, 50 ohm is generally selected as the default value of single-end signal control impedance in the normal time.
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