The design of the printed circuit board starts from the determination of the size of the board. The size of the printed circuit board is limited by the size of the chassis shell. It should be placed in the shell. Secondly, the printed circuit board and external components (mainly Is a potentiometer, socket or another printed circuit board) connection method. The printed circuit board and external components are generally connected by plastic wires or metal isolation wires. But sometimes it is also designed as a socket. That is: install a plug-in printed circuit board in the device to leave the contact position as a socket. For larger components installed on the printed circuit board, metal accessories should be added to fix to improve vibration resistance and impact resistance.
The basic method of wiring diagram design first requires a complete understanding of the specifications, dimensions, and area of ​​the selected components and various sockets; reasonable and careful consideration of the location of each component, mainly from electromagnetic field compatibility, From the perspective of anti-interference, short wiring, less crossing, power supply, ground path and decoupling are considered. After the location of each component is determined, it is the wiring of each component, and the related pins are connected according to the circuit diagram. There are many methods to complete. The design of the printed circuit diagram has two methods of computer-aided design and manual design.
The most original is to arrange the layout manually. This is more time-consuming, and it is often repeated several times before it is finally completed. This can also be done when there is no other drawing equipment. This manual arrangement method is also very helpful for designers who have just learned printed board drawings. Computer-aided drawing, there are now a variety of drawing software, with different functions, but in general, drawing and modification are more convenient, and can be saved and printed.
Next, determine the required size of the printed circuit board, and determine the position of each component according to the schematic diagram, and then continue to adjust to make the layout more reasonable. The wiring arrangement between the components in the printed circuit board is as follows:
(1) No cross circuit is allowed in the printed circuit. For the lines that may cross, you can use two methods of "drilling" and "winding". That is, let a lead "drill" through the gap under other resistors, capacitors, transistors, or "wind" from one end of a lead that may cross, how to make the circuit very complicated under special circumstances, and to simplify the design It is allowed to use wire jumper to solve the cross circuit problem.
(2) There are "vertical" and "horizontal" installation methods for components such as resistors, diodes, and tubular capacitors. The vertical type refers to the component body being installed and welded perpendicular to the circuit board, which has the advantage of saving space, and the horizontal type refers to the component body being parallel and closely attached to the circuit board for installation and welding. For these two different mounting components, the component hole pitch on the printed circuit board is different.
(3) The grounding point of the circuit of the same level should be as close as possible, and the power supply filter capacitor of the circuit of this level should also be connected to the grounding point of this level. In particular, the grounding points of the base and emitter of the transistors in this stage should not be too far apart, otherwise the copper foil between the two grounding points will be too long, which will cause interference and self-excitation. Stable and not easily self-excited.
(4) The total ground wire must be arranged strictly in the order of high frequency-intermediate frequency-low frequency in the order of weak current to strong current, and it must not be randomly connected back and forth. Observe this regulation. In particular, the grounding wire arrangement requirements of the frequency conversion head, regeneration head, and frequency modulation head are more strict, and if improper, it will cause self-excitation to work. High-frequency circuits such as FM heads often use large-area surrounding ground wires to ensure a good shielding effect.
(5) Strong current leads (common ground, 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) Traces with high impedance should be as short as possible, traces with low impedance can be longer, because traces with high impedance tend to whistle and absorb signals, causing circuit instability. The power supply line, ground line, base trace without feedback element, emitter lead, etc. are all low-impedance traces. The base trace of the emitter follower and the ground of the two channels of the recorder must be separated, and they are all in one way. , Until the end of the function is put together again, such as the two-way ground wire is connected, it is easy to produce crosstalk, which reduces the separation.
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