In high-speed circuit design, the inductance and capacitance on the circuit board circuit make the wires equivalent to a transmission line. The incorrect layout of termination components or incorrect wiring of high-speed signals can cause transmission line effects, resulting in incorrect data output, abnormal circuit operation, or even complete failure of the system. Based on the transmission line model, transmission lines can bring adverse effects such as signal reflection, crosstalk, electromagnetic interference, power, and ground noise to circuit design. To design a high-speed PCB circuit board that can work reliably, it is necessary to fully and meticulously consider the design, solve some unreliable problems that may arise during layout and wiring, shorten the product development cycle, and improve market competitiveness.
The design of the printed circuit board starts with determining the size of the board. The size of the printed circuit board is limited by the size of the chassis shell, so it is appropriate to fit it into the shell. Secondly, the connection mode between the printed circuit board and external components (mainly Potentiometer, socket, or other printed circuit board) should be considered. Printed circuit boards are generally connected to external components through plastic wires or metal isolation wires. But sometimes it is also designed in the form of a socket. For larger components installed on printed circuit boards, metal accessories should be added for fixation to improve vibration and impact resistance. PROTEL PCB
The basic method of wiring diagram design:
Firstly, it is necessary to have a complete understanding of the specifications, dimensions, and areas of the selected components and various sockets; Reasonable and careful consideration should be given to the arrangement of the positions of each component, mainly from the perspective of electromagnetic compatibility, anti-interference, short wiring, less crossing, power supply, ground path, and decoupling. After the location of each component is determined, it is the connection of each component. Connect the relevant pins according to the circuit diagram. There are many ways to complete the connection. The design of the printed circuit diagram includes Computer-aided design and manual design.
The most primitive method is to arrange the layout manually. This is quite cumbersome and often requires several iterations before completion, which can also be achieved without other drawing equipment. This manual arrangement method of layout is also very helpful for designers who are just learning to print board drawings. Computer-assisted drawing, there are various drawing software available with different functions, but overall, drawing and modification are more convenient and can be saved, stored, and printed. Next, determine the required size of the printed circuit board and preliminarily determine the position of each component according to the schematic diagram. After continuous adjustment, the layout is more reasonable. The wiring arrangement between each component in the printed circuit board is as follows:
(1) Cross circuits are not allowed in printed circuits. For lines that may cross, two methods can be used: “drilling” and “winding”. That is, to allow a certain lead to “drill” through the gap at the foot of other resistors, capacitors, or transistors, or to “wind” through one end of a possible cross lead. In special cases, the circuit is very complex, and to simplify the design, wire bridging is also allowed to solve the problem of cross circuits.
(2) There are two installation methods for components such as resistors, diodes, and tubular capacitors: “vertical” and “horizontal”. Vertical refers to the installation and welding of component bodies perpendicular to the circuit board, which has the advantage of saving space. Horizontal refers to the installation and welding of component bodies parallel and tightly attached to the circuit board, which has the advantage of good mechanical strength during component installation. The hole spacing of these two different installation components on the printed circuit board is different.
(3) The grounding point of the same level circuit should be as close as possible, and the power filter capacitor of the current level circuit should also be connected to the grounding point of that level. Especially, the grounding points of the base and emitter of this level transistor should not be too far away, otherwise, the copper foil between the two grounding points is too long, which can cause interference and self-excitation. Using this “one-point grounding method” in the circuit is more stable and less prone to self-excitation.
(4) The main ground wire must be strictly arranged in the order of high frequency, medium frequency, and low frequency in the order of weak current to strong current. It is not allowed to switch and connect randomly, and it is better to have longer connections between stages, but also to comply with this regulation. Especially, the grounding wire arrangement requirements for frequency conversion heads, regeneration heads, and frequency modulation heads are more strict. If not properly arranged, it will cause self-excitation and make it unable to work. High-frequency circuits such as frequency modulation heads often use large area enclosed ground wires to ensure a good shielding effect.
(5) Strong current leads (common ground wires, power amplifier power leads, etc.) should be as wide as possible to reduce wiring resistance and voltage drop and to reduce self-excitation caused by parasitic coupling.
(6) High-impedance wiring should be as short as possible, while low-impedance wiring can be longer because high-impedance wiring is prone to sounding and absorbing signals, causing circuit instability. The power line, ground wire, base wire without feedback components, emitter wire, etc. are all low-impedance wires. The base wire of the emitter follower and the ground wire of the two channels of the receiver must be separated and formed into one path until the end of the function is combined again. If the two ground wires are connected back and forth, it is easy to produce crosstalk and reduce separation.
While ensuring internal quality and reliability, taking into account overall aesthetics, reasonable circuit board planning should be carried out. Components should be parallel or perpendicular to the board surface and parallel or perpendicular to the main board edges. The distribution of components on the board should be as uniform as possible, with a consistent density. In this way, it is not only aesthetically pleasing but also easy to assemble and weld, making it easy for mass production.