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PCB wiring is to lay the roads for energizing signals to connect various devices. This is like building roads and connecting cities to cars. It's exactly the same thing.
Understand PCB layout
Road construction requires two lines once and one back. PCB wiring is the same. A two-line loop needs to be formed. For low-frequency circuits, it is a loop. For high-speed electromagnetic fields, it is a transmission line. The most common one is a differential signal. String. Such as USB, network cable, etc. For the impedance characteristics of transmission lines, etc., this article will not explain further, please refer to the article "Understandable Electromagnetic Field Theory".
It can be said that the differential signal line is an ideal model for connecting device signals. The higher the signal requirements, the closer to the differential signal line.
Double-sided PCB board. Use the bottom layer as a common reference loop
When there are many components on a board, if they are all arranged according to differential lines, one is that the area of the PCB is too large, and the other is to lay out 2N lines, which is too much work and difficult, so people put forward a multilayer PCB for actual needs. The most typical concept is a double-sided PCB board. The bottom layer is used as a common reference circuit, so that only N+1 wires are needed for wiring, and the PCB layout is greatly reduced.
The common reference circuit, which is commonly referred to as the reference ground, for most embedded industries, because the signal quality is not very high after digitization, the use of a whole-layer reference ground can reduce the board size and improve efficiency., It greatly saves time, and is loved by everyone. In fact, reducing the board size means shortening the length of the signal line, which can also partially offset the signal quality degradation caused by the reference ground. Therefore, in practice, the PCB wiring effect of introducing the reference ground is basically close to the ideal model of the differential line. Today, we are all accustomed to this method, it seems that PCB wiring, is to have a layer of reference ground, there is no reason.
In the double-panel design, because there are often crossover wires, it is necessary to jumper the wire to the ground layer for crossover wire exchange. This needs to be pointed out that the jumper should not be too long. If it is too long, it is easy to divide the reference ground, especially for some For lines requiring high signal quality, the reference ground at the bottom cannot be divided. Otherwise, the signal circuit is completely destroyed, and the reference ground loses its meaning. Therefore, in general, the reference ground layer is only suitable for short jumpers of signal lines, and the signal lines should be placed on the top layer as much as possible, or more layers of PCB boards should be introduced.
If the road is too close to the road, it is easy to have an impact. For example, when taking a high-speed rail, you can feel the influence of the train from the opposite side on the train you are on. The same is true for the signal lines. They should not be too close. If the signal lines are parallel to the signal lines, a certain distance must be maintained. This is subject to experiment, and there must be a good reference ground at the bottom. Under low frequency and small signal, generally the influence is not very big, the high frequency strong signal needs attention.
For high-frequency, high-current PCB wiring, such as switching power supplies, the most taboo is that the driving signal is interfered by the output strong current and strong voltage. The driving signal of the MOS tube is easily affected by the strong output current. Keep a certain distance between the two and do not get too close. In the analog audio era, if the amplifier magnification is too high, the self-excitation effect will occur, and the reason is the same as that of the MOS tube.
The carrier of PCB wiring is the PCB board. Generally, the reference ground is 1mm away from the edge of the PCB board, and the signal line is approximately 1mm away from the edge of the reference ground. In this way, all signals are confined in the PCB board and EMC radiation can be reduced.
When there is no concept of PCB circuit board design, think more about the daily road, the two are completely the same.
