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Engineers conclude: PCB design goes from shallow to deep
Posted: 02:44 PM April 04, 2018 Updated: 02:44 PM April 04, 2018

First, most PCB designers are proficient in the working principle of electronic components, know their mutual influence, and understand the various data transmission standards that constitute the input and output of the circuit board. 

An excellent electronic product requires not only excellent schematics, but also PCB layout and routing. The latter plays a crucial role in the success or failure of the final circuit board. However, the more schematic designers know more about good layout technologies, the more opportunities there are to avoid major problems.

Second, the treatment of noise problems. 

As design PCB speeds increase, concurrent switching noise, concurrent switch output, ringing, cross-talk ground bounce, and power supply noise appear. To solve these problems, we must also address the problem:

A. Ringing and crosstalk. 

For the key signal lines, we must pay attention to the crosstalk problem. The commonly used method is to use differential signals. The traces are routed using differential pairs. This can fundamentally eliminate the influence of induction and help reduce the “bounce” induced current in the return path. noise.

B. Pay attention to impedance matching. 

I have done antenna matching circuits and impedance matching plays a vital role. The 100Ω characteristic impedance has now become the industry standard for differential connections. The 100Ω differential line can be made with two 50Ω single-ended lines of equal length. Since the two traces are close to each other, the field coupling between the lines will reduce the differential mode impedance of the line. In order to maintain the impedance of 100Ω, the width of the trace must be reduced a little. As a result, the common-mode impedance of each wire in a 100Ω differential pair will be slightly higher than 50Ω. If you really do not want to toss, when doing PCB, with the manufacturer to determine what kind of impedance required to trace.

C. Use of decoupling and bypass capacitors. 

In general, decoupling capacitors help reduce the inductance between the power supply of the PCB and the ground plane, and control the impedance of signals and ICs everywhere on the PCB. The bypass capacitor provides a clean power supply (a charge reservoir is provided). Usually we need to place decoupling capacitors anywhere in the PCB layout. For the use of capacitors, it should be noted that the routing of the decoupling capacitors should be as short as possible.

Third, the layout issue. 

We all know that the most critical connection design in PCB design is the shortest and most direct path, so that the simplest method can be used to obtain the best results. In this way, why not?

Fourth, the clock signal processing. 

It is believed that PCB design is subject to or prepared to suffer from clock signal interference. Because the clock line is too long or too long or passes through the signal line, it will amplify jitter and offset for the downstream, especially when the clock speed increases. First of all, the PCB design should avoid using multiple layers to transmit the clock, and do not have vias on the clock line because the vias will increase the impedance changes and signal reflections of the traces. Second, if you must use the inner layer to lay the clock, then the upper and lower layers should use the ground plane to reduce the delay. Again, if unfortunately introducing clock noise on the power plane increases PLL jitter, a "power island" can be created when modifying the PCB design. This technique can use thicker etch in the metal plane to implement PLL analog power and digital power. isolation.

Fifth, the reference design plan. 

Now any MCU will give its corresponding reference design, although these circuit boards are usually designed for a variety of purposes, not necessarily exactly match the design requirements you do. However, they can still serve as a starting point for creating a solution. From this we can see the alignment and placement of the key components. This is a great improvement in the success rate of the design.

The above is a summary of some of my experiences and lessons in PCB design. I hope to help everyone. It is hoped that everyone will discuss the PCB design technologies together.

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