Basic principles and points for attention in the d

2022-09-19
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Basic principles and points for attention in the design of printed circuit board diagram (I)

for an instrument with excellent performance, in addition to selecting high-quality components and reasonable circuits, the correct structural design of the component layout of printed circuit board and the direction of electrical connection is a key problem to determine whether the instrument can work reliably. For circuits with the same components and parameters, Then lock the black knob completely. Different results may be produced due to different component layout design and electrical connection direction, and the results may be very different. Therefore, it is necessary to consider how to correctly design the structure of the printed circuit board component layout and correctly select the wiring direction and the process structure of the overall instrument. A reasonable process structure can not only eliminate the noise interference caused by improper wiring, but also facilitate the installation, commissioning and maintenance in production

next, we will discuss the above problems. Because there is no strict "definition" and "mode" for the excellent "structure", the following discussion is only for reference. The structure of each instrument must adopt the corresponding structural design scheme according to the specific requirements (electrical performance, structural installation of the whole machine, panel layout, etc.), and compare and repeatedly modify several feasible design schemes

wiring structure selection of power supply and ground bus of printed board - system structure: simulation should be stopped for inspection; There are many similarities and differences between the circuit and digital circuit in the design of component layout and wiring methods when it needs to be checked in case of failure and failure in operation. In analog circuits, due to the existence of amplifiers, the extremely small noise voltage heating experiment generated by wiring can be completed, which will cause serious distortion of the output signal. In digital circuits, the TTL noise tolerance is 0.4V ~ 0.6V, and the CMOS noise tolerance is 0.3 ~ 0.45 times that of VCC. Therefore, digital circuits have strong anti-interference ability

the reasonable selection of good power supply and ground bus mode is an important guarantee for the reliable operation of the instrument. A considerable number of interference sources are generated through the power supply and ground bus, among which the noise interference caused by ground wire is the largest

I. The basic principle requirements of printed circuit board drawing design

1. The design of printed circuit board starts from determining the size of the board. The size of the printed circuit board is limited by the beautiful appearance of the chassis and the size of the shell, so it is appropriate to fit it into the shell. Secondly, the connection mode between the printed circuit board and external components (mainly potentiometers, sockets or other printed circuit boards) should be considered. Printed circuit boards and external components are generally connected through plastic wires or metal isolation wires. But sometimes it is also designed as a socket. That is, when installing a plug-in printed circuit board in the equipment, the contact position as a socket should be reserved

for larger components installed on the printed circuit board, metal accessories should be added to fix them to improve the vibration resistance and impact resistance

2. Basic method of wiring diagram design

first of all, you need to have a complete understanding of the specifications, dimensions, areas, etc. of the selected components and sockets; Reasonable and careful consideration shall be given to the location arrangement of each component, mainly from the perspective of electromagnetic field compatibility and anti-interference, short wiring, less crossing, power supply, ground path and decoupling. After the position of each component is determined, it is the online connection of each component. There are many methods to connect the relevant pins according to the circuit diagram. The design of printed circuit diagram has two methods: computer-aided design and manual design

the most original is to arrange the layout manually. This is quite troublesome, and it often takes several times to complete it. This can also be done when there is no other drawing equipment. This manual layout method is also very helpful for the newly learned printed board designer. Computer aided drawing, there are many kinds of drawing software with different functions, but in general, it is convenient to draw and modify, and it can be saved 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, and then make the layout more reasonable through continuous adjustment. The wiring arrangement between the components in the printed circuit board is as follows:

(1) there are no cross circuits in the printed circuit, and the possible cross lines can be solved by "drilling" and "winding". That is, let a lead "drill" through the gap at the foot of other resistors, capacitors, and triodes, or "wind" through one end of a lead that may cross. Under special circumstances, how to make the circuit is very complex. In order to simplify the design, wire bridging is also allowed to solve the problem of cross circuit

(2) resistance, diode, tubular capacitor and other components can be installed in "vertical" and "horizontal" modes. Vertical refers to the installation and welding of the component body perpendicular to the circuit board, which has the advantage of saving space. Horizontal refers to the installation and welding of the component body parallel and close to the circuit board, which has the advantage of good mechanical strength of component installation. For these two different mounting components, the component hole spacing 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 this level. In particular, the grounding points of the base and emitter of the transistor at this level cannot be too far away, otherwise the copper foil between the two grounding points will cause interference and self excitation. The circuit with such a "one point grounding method" works more stably and is not easy to self excitation

(4) the main ground wire must be arranged in strict accordance with the principle of high-frequency medium frequency low-frequency level in the order of weak current to strong current. It is not allowed to turn over and over randomly. It is better to have a longer connection between levels, but also to abide by this regulation. In particular, the requirements for the arrangement of grounding wires of frequency conversion head, regeneration head and frequency modulation head are more strict. If it is improper, it will produce self excitation and cannot work

high frequency circuits such as frequency modulation head often use large-area surrounding ground wire to ensure good shielding effect

(5) strong current leads (common ground wire, power amplifier power leads, etc.) should be as wide as possible to reduce wiring resistance and voltage drop, which can reduce self excitation caused by parasitic coupling

(6) the route with high impedance should be as short as possible, and the route with low impedance can be longer, because the route with high impedance is easy to whistle and absorb signals, causing circuit instability. The power line, ground wire, base wiring without feedback components, emitter lead wire, etc. are all low impedance wiring. The base wiring of emitter follower and the ground wires of the two sound channels of recorder must be separated and formed into one line until the end of the function. If the two ground wires are connected, it is very easy to produce crosstalk and reduce the degree of separation

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