How the anti-interference design is carried out in EMC testing

In EMC testing, the basic task of anti-interference customization is that the system or equipment will not operate or fail due to the influence of external interference signals, nor will it push too much noise interference to the outside world, so as to avoid the normal operation of other software or equipment. Therefore, improving the comprehensive anti-interference ability is also a key step in the system design.

Summarize the circuit anti-interference design principle:

1. Power cord design

(1) Select appropriate power supply;

(2) Enlarge the power line as much as possible;

(3) Ensure that the direction of the power line and the bottom line is consistent with the direction of data transmission;

(4) Apply anti-interference elements;

(5) Add decoupling capacitance (10)~100uf) to the power channel.

2. Ground wire design

(1) Try to choose single point grounding;

(2) Connect the sensitive circuit to a stable grounding reference source;

(3) Separation of analog and digital;

(4) Enlarge the ground wire as much as possible;

(5) Partition the pcb board, separate the high-bandwidth noise circuit from the low-frequency circuit;

(6) Try to avoid the area of the grounding loop (the channel formed by returning all equipment to the power supply after grounding is called the area of the grounding loop ("ground loop").

3. Component configuration

(1) The parallel power line should not be too long;

(2) Ensure that the input terminals of the pcb clock generator, crystal oscillator and cpu clock are as close as possible to avoid other low-frequency devices;

(3) Components shall be equipped around key components to avoid the length of conductor as much as possible;

(4) Partition the pcb board;

(5) Consider the position and direction of the pcb board in the chassis;

(6) Reduce wires between high-frequency components.

4. Configuration of decoupling capacitor

(1) One charging and discharging capacitor (10) uf) shall be added for every 10 integrated circuits;

(2) Low frequency of conductor capacitance and high frequency of chip capacitance;

(3) Each integrated chip shall be equipped with a 00.1uf ceramic capacitor;

(4) High frequency decoupling capacitance shall be added for devices with weak anti-noise capability and large power supply change during shutdown;

(5) There is no need to use the same through hole in the middle of the capacitor;

(6) The decoupling capacitor wire should not be too long.

5. Noise and interference signal reduction standards

(1) Try to choose 45 ° curve instead of 90 ° curve (try to avoid sending high-frequency signal and lotus root);

(2) Reduce the jumping speed of the circuit signal edge through series resistance;

(3) Quartz crystal oscillator housing shall be grounded;

(4) No need to hang idle door circuit;

(5) The impact is small when the clock is perpendicular to the IO line;

(6) Try to make the electric trend around the clock close to zero;

(7) IO drive circuit shall be close to the edge of pcb as far as possible;

(8) All signals need not generate loops;

(9) For high frequency board, the distributed capacitance of capacitance and inductance can not be ignored;

(10) The general power line and AC line shall be on a board different from the power line as far as possible.

6. Other design principles

(1) The unused pins of CMOS should be grounded or powered by resistance;

(2) RC circuit is used to absorb the original discharge current such as solenoid valve;

(3) Adding 10k pull-up and pull-down resistance to the bus is beneficial to anti-interference;

(4) Choosing full decoding has better anti-interference;

(5) Components do not need pins to be connected to the power supply with 10k resistance;

(6) The bus shall be as short as possible, and the same length shall be kept as far as possible;

(7) The double-layer line shall be vertical as far as possible;

(8) The thermal element bypasses the sensitive element;

(9) Horizontal wiring on the front and vertical wiring on the back. As long as the space allows, the thicker the wiring, the better (only for ground wire and power cable);

(10) In order to have a good stratum line, try to wire from the front and use the back as the stratum line;

(11) Keep a good distance, such as filter output, optocoupler output, AC power line, weak power line, etc;

(12) Long line plus low filter. Wire as much as possible, and the long line that must be connected should be inserted into the reasonable position of C.RC. or LC low-pass filter;

(13) Except for ground wire, fine wire can be used instead of thick wire.

7. Total width and current

(1) The general overall width is not less than 00.2 mm (8 mil);

(2) The spacing and line spacing of high-density and high-precision pcb are generally 0.3mm (12mil);

(3) When the thickness of copper conductor is 50 um, the total width of conductor is 1~1.5 mm (60 mil)=2A;

(4) Generally, 800mil is used in public places, and more attention should be paid to the application of microcontrollers.

8. Power cord

The power line should be as short as possible, straight, preferably tree-shaped, not looped.

9. Layout

First, understand the size of PCB. When the PCB specification is too large, the printed wire frame is long, the impedance is increased, the noise resistance is reduced, and the cost is increased; If it is too small, the heat removal is not good, and the adjacent wire frame is easily affected.

After confirming the PCB specification, the position of the unique parts shall be determined. Finally, according to the functional modules of the circuit, all components of the circuit are arranged.

The following criteria should be followed when identifying unique components:

(1) The connection between high-frequency components shall be minimized, and their distributed parameters and mutual interference signals shall be minimized. The easily affected components should not be too close, and the input and output components should be avoided as far as possible.

(2) There may be a high potential difference between some components or wires, so the distance between them should be increased to prevent accidental short circuit caused by discharge. High-voltage components shall be arranged in areas that are not easy to touch during adjustment.

(3) Components with weight more than 15g shall be fixed by bracket and then welded. These are big, heavy and heavy Parts with high calorific value should not be installed on the printed board, but on the bottom of the whole machine, and attention should be paid to heat removal. Heating elements shall be avoided for thermal elements.

(4) For the layout of adjustable elements such as resistors, adjustable inductance coils, variable capacitors and microswitches, attention should be paid to the structural regulations of the whole machine. If the fuselage is adjusted, it should be placed in an area convenient for adjustment on the printed board; If the host machine is adjusted, its part should be consistent with the position of the adjustment knob on the chassis panel.

(5) The location hole of the printing plate and the part occupied by the support frame should be empty.

When the circuit components are arranged according to the circuit function module, the following standards shall be met:

(1) Allocate the components of each functional circuit unit according to the circuit flow to make the layout convenient for signal flow and keep the signal direction as consistent as possible.

(2) Take the key components of each functional circuit as the core and lay out around them. The components shall be symmetrical Neat and closely arrange PCB. Try to avoid and reduce wires and connections between components.

(3) For circuits operating at high frequencies, it is necessary to understand the distribution parameters between components. General circuits shall be arranged in parallel as far as possible. In this way, it is not only beautiful, but also easy to assemble and weld, and convenient for batch production.

(4) The part located at the edge of the circuit board is generally not less than 2mm from the edge of the circuit board. The best appearance of the circuit board is square. The aspect ratio is 3:204:3. When the circuit surface specification exceeds 2000x150mm, attention should be paid to the impact toughness of the circuit board.

10. Wiring

The wiring principle is as follows:

(1) The wires used at the output end shall be parallel to each other as far as possible. It is better to add ground wire between wires to avoid feedback coupling.

(2) The minimum width of printed wire is mainly determined by the adhesion strength between the wire and the insulating substrate and the current through it. When the thickness of copper pool is 0.05 mm and the total width is 1~15 mm, the temperature is not higher than 3 ℃ according to 2A current, so the total width of conductor is 1.5 mm, which can meet the requirements.

For integrated circuits, especially data circuits, generally 0.02~0.3mm wire is always wide. Naturally, as long as allowed, or try to use wide wires, especially power lines and ground wires. The minimum spacing of conductors is mainly determined by the line indirect ground resistance and breakdown field strength in the worst case. For integrated circuits, especially data circuits, the interval can be as small as 5~8mm only if the process allows.

(3) The turning of the printed wire is generally arc, and the right angle or intersection angle in the high-frequency circuit will affect the electrical performance. In addition, the application of large-scale copper pool should be minimized, otherwise, the copper pool is easy to expand and fall off when it is hot for a long time. When a large copper pool must be used, it is better to use grid grid. This is beneficial to remove the volatile gas generated by the heating of the adhesive between the copper pool and the substrate.

11. Welding layer

The center hole of the welding layer will be slightly larger than the wire hole of the equipment. If the welding layer is too large, it is easy to produce empty welding. The diameter D of the welding layer is generally not less than D (d1.2) mm, and D is the diameter of the wire. For high-density digital circuits, the minimum aperture of the solder layer can be (d1.0) mm.

12. PCB and circuit anti-interference measures

Anti-interference design of printed circuit board is closely related to the actual circuit. Here are only a few common anti-interference customized countermeasures for a PCB.

13. Power cord design

According to the current of the printed circuit board, the total width of the power line should be widened as much as possible to reduce the loop resistance. At the same time, make the direction of power line and ground line consistent with the direction of data transmission, which will help improve the anti-noise ability.

14. Ground wire design

The principle of ground wire design is:

(1) Digital and analog separation. If there are both logic circuits and linear circuits on the circuit board, they should be separated as far as possible. The ground of low-frequency circuit shall be spotting and grounding as far as possible. When the specific wiring is difficult, it can be partially connected and then connected to the ground. Multi-point series grounding shall be adopted for high frequency circuit, and the grounding wire shall be short and thick, and grid large-scale grounding foil shall be used around high frequency components as far as possible.

(2) The grounding wire should be thickened as much as possible. If the grounding wire is very thin, the line impedance will change greatly. When a large current passes through, the grounding potential will change greatly, reducing the anti-noise characteristics. Therefore, the grounding wire should be thickened first so that it can pass three times the allowable current in the printed board. If possible, the grounding wire should be more than 2~3mm.

(3) The ground wire forms a closed loop. Most printed boards composed only of digital circuits can improve the anti-noise ability of their grounding circuits.

15. Equipped with decoupling capacitor

One of the basic methods of PCB design is to equip the key parts of the printed board with appropriate lotus capacitor.

Generally, the principle of decoupling capacitor is:

(1) The input end of power supply is connected with 100~100uf electrolytic capacitor. If possible, connect 100. More than uF.

(2) In principle, each integrated circuit chip should be equipped with a 0.01pF ceramic tile capacitor.

(3) The anti-noise ability is weak. For devices with large power changes, such as those with large power changes when turned off, the RAM. ROM memory device should immediately connect the decoupling capacitor between the power line and the ground wire of the chip.

(4) The capacitance wire should not be too long, especially the high-frequency bypass capacitor should not have wires.