1、 Discussion on layout and structure design of printed circuit components
For an instrument with excellent performance, in addition to selecting high-quality components and reasonable circuits, the correct structural design of the component layout and electrical connection direction of the printed circuit board is a key issue to determine whether the instrument can work reliably. For circuits with the same component and parameter, different results will be produced due to the different component layout design and electrical connection direction, and the results may vary greatly. Therefore, it is necessary to consider how to correctly design the layout structure of printed circuit board components 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.
Let's discuss the above problems. Since there is no strict 'definition' and 'model' for good 'structure', the following discussion is only for reference. The structure of each instrument must adopt corresponding structural design scheme according to 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 printed board power supply and ground bus - system structure: analog circuit and digital circuit have many similarities and differences in the design and wiring method of component layout. In analog circuits, due to the existence of amplifiers, the very small noise voltage generated by wiring 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 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, and the noise interference caused by the ground wire is the largest.
2、 Basic principles and requirements of printed circuit board design
1. The design of the printed circuit board starts from the determination of the size of the board. Because the size of the printed circuit board is limited by the size of the chassis shell, 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 by 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 acting as a socket shall be reserved.
For larger components installed on the printed circuit board, metal accessories shall be added to fix them to improve vibration resistance and impact resistance.
2. Basic method of wiring diagram design
First of all, 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 shall be given to the position arrangement of each component, mainly from the perspective of electromagnetic compatibility, 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. The relevant pins are connected according to the circuit diagram. There are many ways to complete the design. There are two ways to design the printed circuit diagram: computer-aided design and manual design.
The most primitive is to arrange the layout manually. This is more time-consuming, and it often takes several times to finish. This can also be done when there is no other drawing equipment. This manual layout method is also very helpful for designers who have just learned the printed board drawing. There are many kinds of computer aided 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 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 components in the printed circuit board is as follows:
(1) Crossed circuits are not allowed in printed circuits. For lines that may cross, two methods can be used: 'drilling' and 'winding'. That is to say, 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 is the circuit very complex? In order to simplify the design, it is also allowed to use wire bridging to solve the problem of cross circuit.
(2) Resistors, diodes, tubular capacitors and other components can be installed in 'vertical' and 'horizontal' ways. Vertical refers to the installation and welding of the module body perpendicular to the circuit board. Its advantage is to save space. Horizontal refers to the installation and welding of the module body parallel and close to the circuit board. Its advantage is that the mechanical strength of the module installation is good. For these two different mounting components, the component hole spacing on the printed circuit board is different.
(3) The grounding point of the circuit at the same level should be as close as possible, and the power filter capacitor of the circuit at the same level should also be connected to the grounding point at that level. In particular, the grounding points of the base and emitter of the current level transistor should not be too far away, otherwise the copper foil between the two grounding points is too long, which will cause interference and self-excitation. The circuit using such 'one-point grounding method' will work stably and not be self-excited.
(4) The main ground wire must be arranged in strict accordance with the order of high frequency, medium frequency and low frequency, from weak current to strong current. It is not allowed to turn around and connect randomly. It is better to connect long lines between stages, but also to comply with this regulation. In particular, the grounding wire arrangement requirements of frequency conversion head, regenerative head and frequency modulation head are more strict. If it is improper, it will cause self-excitation and cannot work.
High-frequency circuits such as FM head often use large-area enclosed ground wire to ensure good shielding effect.
(5) Strong current leads (common ground wire, power amplifier power lead, etc.) should be as wide as possible to reduce wiring resistance and voltage drop, and reduce self-excitation caused by parasitic coupling.
(6) The wiring with high impedance should be as short as possible, and the wiring with low impedance can be longer, because the wiring with high impedance is easy to whistle and absorb signals, causing circuit instability. The power line, ground wire, base wire without feedback component, emitter wire, etc. are all low impedance wires. The base wire of emitter follower and the ground wire of the two channels of the radio recorder must be separated and formed into one path, until the end of the effect, and then combined. If the two ground wires are connected, it is easy to produce crosstalk and reduce the separation.
3、 The following points should be noted in the design of printed board drawings
1. Wiring direction: from the welding surface, the arrangement direction of the components should be consistent with the schematic diagram as much as possible, and the wiring direction should be consistent with the wiring direction of the circuit diagram. Since various parameters are usually required to be tested on the welding surface during the production process, this is convenient for inspection, debugging and maintenance in production (note: it refers to meeting the requirements of circuit performance and the installation of the complete machine and panel layout).
2. The arrangement and distribution of components should be reasonable and uniform, and strive to be neat, beautiful, and structurally rigorous.
3. The placement methods of resistors and diodes are divided into two types: horizontal placement and vertical placement:
(1) Lay flat: when the number of circuit components is small and the size of circuit board is large, lay flat is generally better; For resistance flat placement below 1/4W, the distance between two pads is generally 4/10 inch, and for resistance flat placement below 1/2W, the distance between two pads is generally 5/10 inch; When the diode is laid horizontally, 1N400X series rectifier tube is generally 3/10 inch; 1N540X series rectifier tube, generally 4~5/10 inches.
(2) Vertical placement: When the number of circuit components is large and the size of the circuit board is small, vertical placement is generally adopted. The spacing between the two pads is generally 1~2/10 inches.
4. Potentiometer: placement principle of IC base
(1) Potentiometer: It is used to adjust the output voltage in the regulator, so the design potentiometer should increase the output voltage when it is fully adjusted clockwise, and decrease the output voltage when it is adjusted counterclockwise; In the adjustable constant current charger, the potentiometer is used to adjust the charging current. When the potentiometer is designed, the current will increase when it is fully adjusted clockwise.
The potentiometer should be placed on the edge of the plate as much as possible, with the rotating handle facing outward.
(2) IC base: When designing the printed board drawing, when using the IC base, pay special attention to whether the orientation of the positioning slot on the IC base is correct, and pay attention to whether the position of each IC pin is correct. For example, the first pin can only be located at the lower right corner line or the upper left corner of the IC base, and close to the positioning slot (from the welding surface).
5. Layout of incoming and outgoing terminals
(1) The distance between the two associated lead ends should not be too large, generally about 2~3/10 inches is appropriate.
(2) The inlet and outlet ends should be concentrated on 1 to 2 sides as much as possible, and should not be too discrete.
6. When designing the wiring diagram, pay attention to the pin arrangement order and the component pin spacing should be reasonable.
7. On the premise of ensuring the circuit performance requirements, the design should strive to make the wiring reasonable, reduce the use of external jumper wire, and route the wiring according to a certain sequence requirements, and strive to be intuitive, convenient for installation, height and maintenance.
8. When designing the wiring diagram, the route shall be as few as possible to turn and the lines shall be simple and clear as possible.
9. The width of wiring strip and line spacing should be moderate, and the spacing between two pads of capacitor should be consistent with the spacing of capacitor lead pin as much as possible;
10. The design shall be carried out in a certain order, such as from left to right and from top to bottom.