Basic Principles and Points for Attention in Printed Circuit Board Design (1)

In addition to selecting high-quality components and reasonable circuits for a high-performance instrument, the layout of printed circuit boards and the correct structural design of the electrical connection direction are the key issues that determine whether the instrument can work reliably. Circuits with parameters may produce different results due to different component layout designs and electrical connections. The results may vary greatly. Therefore, we must consider how to properly design the structure of printed circuit board assembly layout 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 circuit and digital circuit There are many similarities and differences in the layout and layout of the component layout. 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.

First, 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 generally 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 of all, it is necessary to have a complete understanding of the specifications, dimensions, and areas of the selected component devices and 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 fixing each component's position, it is the connection of each component. Connect the relevant pin according to the circuit diagram. There are many ways to accomplish this. There are two types of printed circuit diagram designs, 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) Components such as resistors, diodes, and tubular capacitors have two mounting methods, "vertical" and "horizontal". Vertical refers to the assembly body perpendicular to the circuit board installation, welding, its advantage is to save space, horizontal refers to the assembly body parallel and close to the circuit board installation, welding, its advantage is that the assembly of the mechanical strength is better. With these two different mounting components, the pitch of the 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 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, and emitter leads are all low-impedance traces. The base trace of the emitter follower and the two-channel grounds of the recorder 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.