Printed Circuit Board (PCB) design is essential to the work of electric engineers and it's not apparently easy to design a perfect PCB. A perfect PCB derives from not only its rationality of component selection and distribution but its high signal conductivity as well. In this thesis, the knowledge on routing techniques on PCB high-speed signal circuit design will be introduced and displayed in order to provide some help for your engineering work.
PCB Routing based on Multi-layer Board
When designing PCB, most engineers would like to complete the high-signal routing by using multi-layer boards. Besides its role as the core of PCB, this type of multi-layer board is also capable of decreasing the circuit interference, which is a main method for engineers confronted with such a problem. When designing the high-speed signal circuit on PCB based on the utilization of multi-layer board, engineers need to shrink the board size by rationally determining the number of layer, to make the best use of the middle layer for shield setting to realize the nearby ground, all of which can effectively decrease the parasitic inductance, shrink the signal transmission length, reduce the cross interference between signals and so on. All of these methods are quite beneficial for the reliability of high-speed signal circuit.
Apart from the methods above of increasing the reliability of PCB signal transmission with the help of multi-layer board, some authority data shows that when the same material is used, the noise generated by four-layer board is 20dB lower than that by 2-layer board. For the lead bending, the fewer the bending appears, the better it'll be. It's best to use the whole line and when bending is required, a 45 degree line or an Arc line can be used so that the emission towards outside will be cut down from high-speed signal and mutual coupling and both the radiation and reflection will be decreased as well.
Making the Lead Pin between Components in High-speed Circuit As Short As Possible
In the process of PCB high-speed signal circuit design and routing, engineers need to make the lead pin between components in high-speed circuit as short as possible. Because the longer the lead is, the larger both the distributed inductance and distributed capacitor is, which will lead to the reflection and oscillation in the high-speed circuit.
Besides the shortening of the lead pin between components in high-speed circuit, the lead interlayer alternation between lead pins of components on each high-speed circuit should be shortened in the process of PCB routing, which means that the through-holes in the process of component connection should be as few as possible. Generally, a through hole can bring about 0.5pF distributed capacitance, which will obviously lead to the increasing of circuit delay. Meanwhile, in the process of high-speed circuit routing, the cross interference inducted by signal line short-range parallel routing should be taken into full consideration. If the parallel distribution cannot be bypassed, a large-scale ground can be set at the back of the parallel signal lines to cut down the interference. In the neighboring two layers, the routing direction must be vertical.
Ground Surrounding on the Especially Important Signal Lines or Local Parts
In the process of PCB routing design, ground surrounding is suggested being utilized on the especially important signal lines or local parts by engineers. Routing is being carried out to less susceptible to be interfered signals such as clock signal and high-speed analog signal at the time ground wire for protection is added to peripherals, with the signal lines to be protected in the middle. This is because all types of signal routing can't form loops, neither is the ground wire. However, if loop routing circuit comes into being, great interference will take place in the system. The advantage of routing with ground wire surrounding signal lines leads to the effective avoidance of loops in the process of routing. It's suggested that one or some high-frequency decoupling capacitors be set near each integrated circuit block. When analog ground wire or digital wire is connected to the public ground wire, high-frequency choke link should be taken into use. Some high-speed signal lines require special treatment. For example, differential signal is required on the same layer and to be as close to parallel routing as possible. Any signal cannot be inserted between differential signal lines and each should have the same length.
Besides the methods mentioned above, while designing PCB signal routing, engineers should try to avoid high-speed signal wiring branch or forming stub. Since relatively big electromagnetic radiation can be generated when high-frequency signal wires are set on the surface layer, high-frequency signal wires should be set between power and ground wires so that the generated radiation will be decreased at a high level because of the electromagnetic absorption from power and the bottom layer.
Of course, in the practical project, theory never comes before practice. I'd like to share some of my experience in terms of PCB routing design. First, if you are not the only routing designer of a PCB, then spare enough time to check the design of routers. The little precaution is much better than much remediation. It's a silly idea to expect routers to understand what you think. Your advice and instructions are the most important in the primary stage of routing design. The more information you can provide and the more you get involved in the design, the better PCBs you'll obtain. Here is a good method: you can set a tentative completion point for the PCB design engineer so that the routing procedure will take off strictly according to your steps. This method is like a closed loop in that the routing won't be off the track so that the possibility of reworking can be decreased to the minimum.
Then the instructions you should provide to your routing engineers include: the short description of circuit function; the PCB sketch with labels of input and output places; PCB layer information such as the thickness, the number of layers, the detailed information of each signal layer and ground plate; the type of signal each layer requires; the requirement in terms of the place of important components; the specific places of bypass components; the significance of printed lines; the significance of circuits in need of impedance controlling printed lines; the circuits requiring the mapping length; the components' size; the printed lines, circuits or components requiring distance or closeness; the type of components put on the top or bottom.
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