First, the layout of the printed circuit components layout design discussion

In addition to selecting high-quality components and reasonable circuits for a high-performance instrument, the layout of components of the printed circuit board and the correct structural design of the electrical connection direction are the key issues that determine whether the instrument can work reliably. The components and parameters of the circuit, due to the layout of the components and the direction of the electrical connection will produce different results, the results may be very different. Therefore, we must consider how to properly design the layout of printed circuit board components and the correct selection of the wiring direction and the overall instrument process structure. The reasonable process structure can eliminate the noise interference caused by improper wiring, and at the same time, it is convenient. Production installation, commissioning and overhaul.

Below we discuss the above issues. Since there is no strict "definition" and "model" for good "structure", the following discussion will only serve as a reference for reference. The structure of each instrument must be based on specific requirements (electrical performance, overall machine structure installation, and panel layout, etc.), and a corresponding structural design scheme must be adopted, and several feasible design schemes must be compared and repeatedly modified.

Printed Circuit Board Power Supply, Ground Bus Wiring Structure Selection ---- System Structure: Analog Circuits and Digital Circuits There are many similarities and differences in the layout and layout of component layouts. In the analog circuit, due to the existence of the amplifier, a very small noise voltage generated by the wiring will cause serious distortion of the output signal. In the digital circuit, the TTL noise margin is 0.4V-0.6V, and the CMOS noise margin is 0.3 for Vcc. ~0.45 times, so the digital circuit has a strong anti-jamming capability. Reasonable choice 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 caused by the ground line is the largest.

Second, the basic principles of printed circuit board design requirements

1. The design of the printed circuit board starts with the determination of the size of the board. The size of the printed circuit board is limited by the size of the chassis casing, so that it can be properly placed into the housing. Second, the printed circuit board and external components should be considered (mainly It is the connection of potentiometer, socket or other printed circuit board). Printed circuit boards and external components are usually connected by plastic wires or metal wire. But sometimes it is designed as a socket. That is, installing a plug-in PCB in the device leaves the contact position as a socket.

For larger components mounted on printed circuit boards, add metal attachments to improve vibration and shock resistance.

2. The basic method of wiring diagram design

First, it is necessary to have a complete understanding of the specifications, dimensions, and areas of the selected component devices and various sockets; reasonable and careful consideration should be given to the arrangement of each component, mainly from the perspective of electromagnetic field compatibility and anti-jamming. Short lines, few crossings, power supply, ground path and decoupling etc. After the position of each component is fixed, it is the wiring of each component. According to the circuit diagram, the relevant pins are connected. There are many methods to complete. There are two types of designs for the printed wiring diagram: computer-aided design and manual design.

The most primitive is the manual layout. This is more time consuming and often takes a few iterations to complete. This can be done without other drawing equipment. This hand-arranging method is also very helpful for designers who are just learning to print the board. Computer-aided drawing, there are a variety of graphics software, features vary, but in general, drawing, modification is more convenient, and can be stored and printed.

Then, the required size of the printed circuit board is determined, and the position of each component is preliminarily determined according to the schematic diagram, and then the adjustment is continuously made to make the layout more reasonable. The wiring arrangement among the components in the printed circuit board is as follows:

(1) Cross circuits are not allowed in printed circuits. For lines that may intersect, "drilling" and "winding" can be used. That is, if a certain lead is “drilled” from the gaps under the feet of other resistors, capacitors, and triodes, or “wraps around” from one end of one of the leads that may cross. In a special case, how the circuit is complicated is to simplify the design. Allows cross-connections to solve cross-circuit problems.

(2) Resistors, diodes, tubular capacitors, and other components are available in "vertical" and "horizontal" mounting styles. Vertical refers to the component body perpendicular to the circuit board installation, welding, its advantage is to save space, horizontal refers to the component body parallel and close to the circuit board installation, welding, its advantage is that the component installation of mechanical strength is better. The two different mounting components, the printed circuit board component pitch is not the same.

(3) The grounding point of the same level circuit should be as close as possible, and the power supply filter capacitor of this stage circuit should also be connected to the grounding point of this level. In particular, the grounding point of the transistor's base and emitter cannot be too far apart. Otherwise, the copper foil between the two grounding points will cause interference and self-excitation, and the “one-point grounding method” circuit will be used. Stable, not easy to self-excited.

(4) The total ground line must be in strict accordance with the order of the weak current to the strong current in the high-frequency, intermediate frequency, and low-frequency levels, and must not be changed over and over again, but it is better to connect long-distance between stages and grades. Obey this rule. In particular, the arrangement of the grounding wires for the frequency conversion head, the regeneration head, and the frequency modulation head is more stringent, and if it is improper, it will cause self-excitation and it will not work. Frequently, high-frequency circuits such as frequency modulation heads use large-area enclosed ground lines to ensure good shielding effects.

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

(6) The trace with high impedance should be as short as possible, and the trace with low impedance can be longer, because the trace with high impedance is easy to flute and absorb the signal, causing circuit instability. Power supply lines, ground lines, base traces with no feedback components, emitter leads, etc. are all low impedance traces. The base traces of the emitter followers and the ground traces of the recorder's two channels must be separated. Until the end of efficiency is combined again, if the two ground lines are connected, the crosstalk can easily occur and the degree of separation can be reduced.

(To be continued) Third, the following points should be noted in the design of printed boards

1. Wiring direction: From the view of the soldering surface, the orientation of the components should be kept consistent with the schematic diagram as much as possible, and the wiring direction should be consistent with the wiring direction of the circuit diagram. As the production process usually requires testing of various parameters at the soldering surface, This facilitates inspections, commissioning, and inspections during production (Note: This refers to the premise of satisfying circuit performance and overall machine installation and panel layout requirements).

2. Arrangement of the components, the distribution should be reasonable and uniform, strive for orderliness, appearance, strict structural requirements.

3. Resistance, diode placement: divided into two kinds of flat and vertical:

(1) Flat laying: When the number of circuit components is small, and the circuit board size is large, it is generally better to use flat placement; for resistances below 1/4W, the distance between two pads is generally Take 4/10 inch, 1/2W resistance when laying flat, the distance between two pads is generally 5/10 inch; diode flat, 1N400X series rectifier, generally take 3/10 inch; N540X series rectifier, generally Take 4 to 5/10 inches.

(2) Vertical placement: When the number of circuit components is large and the circuit board size is not large, vertical placement is generally adopted. When the vertical placement is performed, the distance between two pads is generally 1 to 2/10 inches.

4. Potentiometer: Placement principle of IC holder

(1) Potentiometer: used to adjust the output voltage in the regulator, so the design potentiometer should be adjusted clockwise when the output voltage rises, anti-clock regulator output voltage decreases;
In the adjustable constant current charger, the potentiometer is used to adjust the charging current fold size. When the potentiometer is designed to be adjusted clockwise clockwise, the current increases. Potentiometer mounting position Xuan should be full of the structure of the whole machine installation and panel layout requirements, so should put Xuan on the edge of the board, rotating handle outward.

(2) IC Holder: When designing a printed board drawing, when using an IC Holder, be sure to pay special attention to whether the orientation of the positioning groove on the IC Holder is correct, and pay attention to whether each IC pin is correct. For example, pin 1 can only be Located in the lower right corner or upper left corner of the IC seat, and close to the positioning slot (see from the welding surface).

5. Access terminal layout

(1) The associated two lead terminals should not be too far away, generally about 2 to 3/10 inches.

(2) Inlet and outlet ends should be concentrated on one or two sides as much as possible. Do not be too discrete.

6. When designing the wiring diagram, pay attention to the order of the pins, and the distance between the components should be reasonable.

7. Under the premise of guaranteeing the circuit performance requirements, the design should be based on the principle of seeking reasonable cabling, using less external crossover wires, and routing the cabling according to certain requirements, and strive to be intuitive, easy to install, and maintain and maintain.

8. When designing the wiring diagram, the wiring should be as small as possible, and the lines should be simple and straightforward.

9. The width of the wiring strip and the spacing between the lines must be moderate, and the distance between the two pads of the capacitor should be as close as possible to the pitch of the capacitor lead.

10. The design should be carried out in a certain order, for example from left to right and from top to bottom.