At present, printed circuit boards are still the main assembly method for various electronic devices and systems used in electronic equipment. Practice has proven that even if the circuit schematic is designed correctly and the printed circuit board is not designed properly, it can have adverse effects on the reliability of electronic devices. For example, if two thin parallel lines on a printed circuit board are placed very close, it will cause a delay in the signal waveform, resulting in reflected noise at the end of the transmission line. Therefore, when designing printed circuit boards, attention should be paid to using the correct methods.

1、 Ground wire design

Grounding is an important method for controlling interference in electronic devices. If grounding and shielding can be combined correctly, most interference problems can be solved. The grounding structure in electronic devices generally includes systematic grounding, chassis grounding (shielding grounding), digital grounding (logic grounding), and analog grounding. In ground wire design, the following points should be noted:

1. Correct selection of single point grounding and multi-point grounding

In low-frequency circuits, the operating frequency of a signal is less than 1MHz, and its wiring and inductance between devices have a relatively small impact. However, the circulating current formed by a grounded circuit has a significant impact on interference, so a single point grounding should be used. When the operating frequency of the signal is greater than 10MHz, the impedance of the ground wire becomes large. At this time, the impedance of the ground wire should be reduced as much as possible, and nearby multi-point grounding should be used. When the working frequency is between 1-10MHz, if a single point grounding is used, the length of the ground wire should not exceed 1/20 of the wavelength. Otherwise, the multi-point grounding method should be used.

2. Separate digital circuits from analog circuits

There are both high-speed logic circuits and linear circuits on the circuit board, and they should be separated as much as possible. The ground wires of the two should not be mixed and should be connected to the ground wires of the power supply end separately. Try to increase the grounding area of linear circuits as much as possible.

3. Try to thicken the grounding wire as much as possible

If the grounding wire is very thin, the grounding potential will change with the change of current, causing the timing signal level of electronic devices to be unstable and the anti noise performance to deteriorate. Therefore, the grounding wire should be thickened as much as possible, so that it can pass through the allowable current of the three positions on the printed circuit board. If possible, the width of the grounding wire should be greater than 3mm.

4. Form a closed loop with the grounding wire

When designing a ground wire system for printed circuit boards composed solely of digital circuits, making the ground wire a closed loop can significantly improve noise resistance. The reason is that there are many integrated circuit components on the printed circuit board, especially when encountering high power consuming components. Due to the limitation of the thickness of the grounding wire, a large potential difference will be generated on the ground junction, causing a decrease in noise resistance. If the grounding structure is formed into a loop, the potential difference will be reduced, and the noise resistance of electronic devices will be improved.

2、 Electromagnetic compatibility design

Electromagnetic compatibility refers to the ability of electronic devices to work in a coordinated and effective manner in various electromagnetic environments. The purpose of electromagnetic compatibility design is to enable electronic devices to suppress various external interferences, enabling them to operate normally in specific electromagnetic environments, while also reducing the electromagnetic interference of electronic devices themselves on other electronic devices.

1. Choose a reasonable wire width. The impact interference generated by transient current on printed lines is mainly caused by the inductance component of printed wires, so the inductance of printed wires should be minimized as much as possible. The inductance of printed wires is directly proportional to their length and inversely proportional to their width, so short and precise wires are beneficial for suppressing interference. The signal lines of clock leads, row drivers, or bus drivers often carry large transient currents, and printed wires should be as short as possible. For discrete component circuits, a printed wire width of around 1.5mm can fully meet the requirements; For integrated circuits, the printed wire width can be selected between 0.2 and 1.0mm.

2. Adopting the correct wiring strategy and using equal routing can reduce wire inductance, but the mutual inductance and distributed capacitance between wires increase. If the layout allows, it is best to use a grid like wiring structure. The specific method is to wire one side of the printed circuit board horizontally and the other side vertically, and then connect them with metalized holes at the cross holes. In order to suppress crosstalk between printed circuit board wires, it is advisable to avoid long-distance equal routing when designing wiring, and to widen the distance between wires as much as possible. Signal wires, ground wires, and power wires should not cross as much as possible. Setting a grounded printed wire between signal lines that are highly sensitive to interference can effectively suppress crosstalk.

In order to avoid electromagnetic radiation generated by high-frequency signals passing through printed wires, the following points should also be noted when wiring printed circuit boards:

● Minimize the discontinuity of printed wires as much as possible, such as avoiding sudden changes in wire width, and prohibiting circular routing with wire corners greater than 90 degrees.

The clock signal lead is most prone to electromagnetic radiation interference. When routing, it should be close to the ground circuit, and the driver should be close to the connector.

The bus driver should be adjacent to the bus it wants to drive. For those leads that leave the printed circuit board, the driver should be tightly adjacent to the connector.

The wiring of the data bus should have a signal ground wire sandwiched between every two signal wires. It is best to place the ground circuit closely adjacent to the least important address lead, as the latter often carries high-frequency current.

When arranging high-speed, medium speed, and low-speed logic circuits on a printed circuit board, the components should be arranged in the manner shown in Figure 1.

3. Suppression of Reflection Interference In order to suppress reflection interference that occurs at the end of printed lines, except for special needs, the length of printed lines should be shortened as much as possible and slow circuits should be used. If necessary, terminal matching can be added, that is, a matching resistor with the same resistance value can be added at the end of the transmission line to ground and the power supply end. Based on experience, for TTL circuits with generally fast speeds, terminal matching measures should be adopted when the printed lines are longer than 10cm. The resistance value of the matching resistor should be determined based on the maximum output driving current and absorption current of the integrated circuit.

3、 Decoupling capacitor configuration

In a DC power supply circuit, changes in load can cause power noise. For example, in digital circuits, when a circuit transitions from one state to another, a large spike current is generated on the power line, forming a transient noise voltage. Configuring decoupling capacitors can suppress noise caused by load changes and is a common practice in the reliability design of printed circuit boards. The configuration principles are as follows:

The power input terminal is connected to a 10~100uF electrolytic capacitor. If the position of the printed circuit board allows, using electrolytic capacitors with a voltage of 100uF or higher will have better anti-interference effect.

Configure a 0.01uF ceramic capacitor for each integrated circuit chip. If the printed circuit board space is small and cannot be installed, a 1-10uF tantalum electrolytic capacitor can be configured for every 4-10 chips. This type of device has a particularly small high-frequency impedance, with an impedance of less than 1 Ω in the range of 500kHz to 20MHz, and a very small leakage current (below 0.5uA).

For devices and ROMs with weak noise capability and significant current changes during shutdown RAM and other storage devices should be directly connected with decoupling capacitors between the power line (Vcc) and ground wire (GND) of the chip.

The leads of decoupling capacitors cannot be too long, especially high-frequency bypass capacitors cannot have leads.

4、 Dimensions of printed circuit boards and arrangement of components

The size of the printed circuit board should be moderate. If it is too large, the printed lines will be long and the impedance will increase, which not only reduces the noise resistance but also increases the cost; If it is too small, it will not dissipate heat well and is also susceptible to interference from adjacent lines.

In terms of device layout, like other logic circuits, interrelated devices should be placed as close as possible to achieve better noise resistance. As shown in Figure 2. The clock inputs of the clock generator, crystal oscillator, and CPU are prone to noise and should be placed closer to each other. Devices, low current circuits, and high current circuits that are prone to generating noise should be kept as far away from logic circuits as possible. If possible, a separate circuit board should be made, which is very important

5、 Thermal design

From the perspective of facilitating heat dissipation, it is best for printed boards to be installed upright, and the distance between boards should generally not be less than 2cm. Moreover, the arrangement of devices on printed boards should follow certain rules:

·For equipment that uses free convection air cooling, it is best to arrange integrated circuits (or other devices) in a longitudinal manner, as shown in Figure 3; For equipment that uses forced air cooling, it is best to arrange the integrated circuits (or other devices) in a horizontal length pattern, as shown in Figure 4.

·Devices on the same printed circuit board should be arranged in zones according to their heat generation and heat dissipation degree as much as possible. Devices with low heat generation or poor heat resistance (such as small signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) should be placed at the top (inlet) of the cooling airflow, while devices with high heat generation or good heat resistance (such as power transistors, large-scale integrated circuits, etc.) should be placed at the downstream of the cooling airflow.

·In the horizontal direction, high-power devices should be arranged as close as possible to the edge of the printed circuit board to shorten the heat transfer path; In the vertical direction, high-power devices should be arranged as close as possible to the top of the printed circuit board to reduce the impact of these devices on the temperature of other devices during operation.

·It is best to place temperature sensitive devices in the lowest temperature area (such as the bottom of the equipment), and never place them directly above the heating device. Multiple devices should be staggered on a horizontal plane.

·The heat dissipation of the printed circuit board inside the equipment mainly relies on air flow, so in the design, it is necessary to study the air flow path and configure the components or printed circuit board reasonably. When air flows, it always tends to flow in areas with low resistance, so when configuring components on printed circuit boards, it is important to avoid leaving large voids in certain areas. The configuration of multiple printed circuit boards in the entire machine should also pay attention to the same issues.

A large amount of practical experience has shown that using a reasonable arrangement of devices can effectively reduce the temperature rise of printed circuits, thereby significantly reducing the failure rate of devices and equipment.

The above are only some general principles for the reliability design of printed circuit boards. The reliability of printed circuit boards is closely related to specific circuits, and it is not necessary to deal with specific circuits in the design to ensure the reliability of printed circuit boards to the greatest extent possible.

6、 Suppression plan for product harassment

1 Grounding 1.1 Equipment Signal Grounding

Purpose: To provide a common reference potential for any signal in the device.

Method: The signal grounding system of the equipment can be a metal plate.

1.2 Basic signal grounding methods

There are three basic signal grounding methods: floating ground, single point grounding, and multi-point grounding.

1.2.1 Floating Ground Purpose: To isolate circuits or equipment from common conductors that may cause circulating currents, and floating ground also makes it easier to coordinate circuits with different potentials. Disadvantage: It is easy to accumulate static electricity and cause strong electrostatic discharge. Compromise solution: Connect a discharge resistor.

1.2.2 Single point grounding method: Only one physical point in the circuit is defined as the grounding reference point, and any grounding required is connected to this point. Disadvantage: Not suitable for high-frequency applications.

1.2.3 Multi point grounding method: All points that need to be grounded are directly connected to the nearest grounding plane, in order to minimize the length of the grounding wire. Disadvantage: Maintenance is cumbersome.

1.2.4 Mixed grounding: Select single point and multi-point grounding as needed.

1.3 Processing of Signal Grounding Wire (Lapping)

Lapping is the process of establishing a low impedance path between two metal points.

Divided into direct overlap and indirect overlap methods.

Regardless of the overlap method, the most important thing is to emphasize good overlap.

1.4 Equipment grounding (grounding)

The equipment is connected to the earth, with the earth as the reference point, for the purpose of:

1) Implement safe grounding of equipment

2) Release the accumulated charge on the chassis to avoid internal discharge of the device.

3) To ensure the stability of equipment operation and avoid changes in the potential of the equipment to the ground under the influence of external electromagnetic environment.

1.5 Method of pulling the earth and grounding resistance grounding rod.

1.6 Grounding of Electrical Equipment

Example 2 Shielding 2.1 Electric Field Shielding 2.1.1 Mechanism Distribution of Electric Field Shielding Coupling between Capacitors

Handling method:

1) Increase the distance between A and B.

2) Try to get as close to the grounding plate as possible.

3) Insert a metal shielding plate between A and B.

2.1.2 Key points of electric field shielding design:

1) Shielding board programmable protected object; The grounding of the shielding board must be good.

2) Pay attention to the shape of the shielding board.

3) The shielding board should have good conductors, no thickness requirements, and sufficient strength.

2.2 Magnetic field shielding

2.2.1 Mechanism of magnetic field shielding

The low magnetic resistance of high permeability magnetic materials creates a magnetic shunt effect, greatly reducing the magnetic field inside the shielding body.

2.2.2 Key points of magnetic field shielding design

1) Select high permeability materials.

2) Increase the wall thickness of the shielding body.

3) Do not place the shielded object close to the shielding body.

4) Pay attention to structural design.

5) Double layer magnetic shielding is used for strong force.

2.3 Mechanism of electromagnetic field shielding

1) Surface reflection.

2) Absorption inside the shielding body.

2.3.2 Effect of materials on electromagnetic shielding

2.4 Actual electromagnetic shielding body

7、 Internal electromagnetic compatibility design of the product

Electromagnetic compatibility in printed circuit board design

1.1 Common impedance coupling problem in printed circuit boards: The digital ground is separated from the analog ground, and the ground wire is widened.

1.2 Layout of printed circuit boards

When mixing high, medium, and low speeds, pay attention to different layout areas.

※ Low analog circuits and digital logic should be separated.

1.3 Wiring of printed circuit boards (single or double-sided boards)

※ Dedicated zero voltage line, with a wiring width of ≥ 1mm for the power line.

※ The power cord and ground wire should be as close as possible, and the power and ground on the entire printed board should be distributed in a "well" shape to achieve balanced current distribution.

※ A zero voltage line should be provided specifically for analog circuits.

To reduce line to line crosstalk, it is necessary to increase the distance between printed lines and insert some zero voltage lines as line to line isolation.

※ The plugs of printed circuits should also be arranged with more zero voltage lines as line to line isolation.

Pay special attention to the size of the wire loop in the current flow.

If possible, add R-C decoupling at the entrance of the control line (on the printed board) to eliminate possible interference factors during transmission.

※ The line width on the printing arc should not change abruptly, and the wire should not suddenly corner (≥ 90 degrees).

1.4 Beneficial suggestions for using logic circuits on printed circuit boards

※ Anything that does not require high-speed logic circuits is not necessary.

※ Add a decoupling capacitor between the power supply and ground.

※ Pay attention to waveform distortion during long-distance transmission.

Use R-S trigger as a buffer between the button and the electronic circuit.

1.4.1 Power line interference and suppression methods introduced during logic circuit operation

1.4.2 Distortion issues in the transmission of logic circuit output waveforms

1.4.3 Coordination issues between button operation and electronic circuit operation

The interconnection of 1.5 printed circuit boards is mainly caused by line to line crosstalk, and the influencing factors are:

※ Right angle wiring

※ Shielded wire

※ Impedance matching

※ Long line drive

Electromagnetic compatibility in the design of 2-switch power supplies

2.1 Disturbance and suppression of power grid conduction by switching power supplies

Harassment source:

① Nonlinear flow.

② The conducted common mode noise generated by the radiative coupling between the power transistor housing and the heat sink in the primary circuit at the power input end.

Suppression method:

① Trim the switch voltage waveform.

② Install an insulation gasket with a shielding layer between the transistor and the heat sink.

③ Add a power filter to the mains input circuit.

2.2 Radiation Disturbance and Suppression of Switching Power Supplies

Pay attention to radiation disturbance and suppression

Suppression method:

① Minimize the loop area as much as possible.

② The layout of positive and negative current carrying conductors on printed circuit boards.

③ Use soft recovery diodes or parallel polyester film capacitors on the diodes in the secondary rectification circuit.

④ Trim the transistor switch waveform.

2.3 Reduction of output noise

The reason is the sudden change in reverse current of the diode and the distribution of inductance in the circuit. The diode junction capacitance forms high-frequency attenuation oscillation, and the equivalent series inductance of the filtering capacitor weakens the filtering effect. Therefore, the solution to peak interference in output waveform modification is to reduce the inductance and high-frequency capacitance.

Internal wiring of 3 devices

3.1 Electromagnetic coupling phenomenon between lines and suppression methods

Coupling of magnetic fields:

① The best way to reduce interference and the loop area of sensitive circuits is to use twisted pair and shielded wires.

② Increase the distance between lines (to reduce mutual inductance).

③ Try to route the interference source line at a right angle to the induced line as much as possible.

For capacitive coupling:

① Increase the distance between lines.

② Grounding of shielding layer.

③ Reduce the input impedance of sensitive circuits.

④ If possible, use balanced circuits as inputs in sensitive circuits and utilize the inherent common mode suppression ability of balanced circuits to overcome interference from sources on sensitive circuits.

3.2 General wiring methods:

According to power classification, wires of different classifications should be bundled separately, and the distance between separately laid wire harnesses should be 50-75mm.

Grounding of shielded cables

4.1 Common cables

Twisted pair cables are very effective when used below 100KHz, but are limited at high frequencies due to uneven characteristic impedance and resulting waveform reflections.

A shielded twisted pair cable allows signal current to flow on two inner wires, while noise current flows within the shielding layer, eliminating the coupling of common impedance. Any interference will be induced on both wires simultaneously, causing the noise to dissipate.

The ability of unshielded twisted pair to resist electrostatic coupling is relatively poor. But it still has a good effect on preventing magnetic field induction. The shielding effect of unshielded twisted pair is directly proportional to the number of twists per unit length of wire.

Coaxial cables have a relatively uniform characteristic impedance and low losses, making them have good characteristics from true current to very high frequency.

Unshielded ribbon cable.

The best wiring method is to connect the signal to the ground wire, and the less common method is to connect one ground wire, two signals to the ground wire, and so on, or to use a dedicated grounding plate.

4.2 Grounding of cable shielding layer

In short, grounding the load directly is not appropriate because the shielding layers grounded at both ends provide a shunt for the magnetic induced ground loop current, resulting in a decrease in magnetic field shielding performance.

4.3 Termination methods for cable lines

In high demand situations, a complete 360 ° wrapping should be provided for the inner conductor, and coaxial joints should be used to ensure the integrity of the electric field shielding.

5 pairs of electrostatic protection

Electrostatic discharge can enter electronic circuits through three methods: direct conduction, capacitive coupling, and inductive coupling.

Direct electrostatic discharge of a circuit often causes damage to the circuit, while discharge of adjacent objects through capacitance or inductance coupling can affect the stability of the circuit's operation.

Protection methods:

① Establish a complete shielding structure, with a grounded metal shielding shell that can release discharge current to the ground.

② Grounding the metal casing can limit the increase in casing potential, causing discharge between the internal circuit and the casing.

③ If the internal circuit needs to be connected to a metal casing, a single point grounding should be used to prevent discharge current from flowing through the internal circuit.

④ Add protective devices at the cable inlet.

⑤ Add a protective ring at the entrance of the printed board (connected to the grounding terminal).

Handling of internal switch contacts in equipment

6.1 Transient interference formation during switch disconnection process

6.2 Interference suppression measures

6.2.1 Handling of switched inductive loads

6.2.2 Handling of switch contacts

8、 How to improve the anti-interference ability and electromagnetic compatibility of electronic products

How to improve anti-interference ability and electromagnetic compatibility when developing electronic products with processors?

1. The following systems need to pay special attention to anti electromagnetic interference:

(1) A system with a microcontroller clock frequency that is particularly high and a bus cycle that is particularly fast.

(2) The system contains high-power, high current drive circuits, such as spark generating relays, high current switches, etc.

(3) A system that includes weak analog signal circuits and high-precision A/D conversion circuits.

2. To increase the system's ability to resist electromagnetic interference, the following measures are taken:

(1) Choosing a microcontroller with low frequency: Choosing a microcontroller with a low external clock frequency can effectively reduce noise and improve the system's anti-interference ability. Square waves and sine waves of the same frequency have much more high-frequency components than sine waves. Although the amplitude of the high-frequency component of a square wave is smaller than that of the fundamental wave, the higher the frequency, the more likely it is to be emitted as a noise source. The most influential high-frequency noise generated by microcontrollers is approximately three times the clock frequency.

(2) Reduce distortion in signal transmission: Microcontrollers are mainly manufactured using high-speed CMOS technology. The static input current of the signal input terminal is about 1mA, the input capacitance is about 10PF, and the input impedance is quite high. The output terminals of high-speed CMOS circuits have considerable load capacity, that is, a considerable output value. When the output terminal of a gate is led to the input terminal with a relatively high input impedance through a long line, the reflection problem is very serious, which can cause signal distortion and increase system noise. When Tcd>Tr, it becomes a transmission line problem and must consider issues such as signal reflection and impedance matching.

The delay time of the signal on the printed circuit board is related to the characteristic impedance of the leads, that is, to the dielectric constant of the printed circuit board material. It can be roughly assumed that the transmission speed of signals through the leads of printed circuit boards is between 1/3 and 1/2 of the speed of light. The Tr (standard delay time) of commonly used logic telephone components in systems composed of microcontrollers is between 3 and 18ns.

On a printed circuit board, the signal passes through a 7W resistor and a 25cm long lead, with a delay time of approximately 4-20ns on the line. That is to say, the shorter the lead of the signal on the printed circuit, the better, and the longest should not exceed 25cm. And the number of through holes should also be as small as possible, preferably not more than 2.

When the rise time of the signal is faster than the delay time of the signal, it should be processed according to fast electronics. At this point, impedance matching of the transmission line should be considered. For signal transmission between integrated blocks on a printed circuit board, it is necessary to avoid situations where Td>Trd. The larger the printed circuit board, the slower the system speed. A rule for designing printed circuit boards can be summarized as follows: when a signal is transmitted on a printed circuit board, its delay time should not exceed the nominal delay time of the device used.

(3) Reduce cross interference between signal lines: A step signal with a rise time of Tr is transmitted to the B end through lead AB at point A. The delay time of the signal on the AB line is Td. At point D, due to the forward transmission of the signal at point A, the signal reflection after reaching point B, and the delay of the AB line, After Td time, a page pulse signal with a width of Tr will be sensed. At point C, due to the transmission and reflection of the signal on AB, a positive pulse signal of 2Td with a width twice the delay time of the signal on AB line will be induced. This is the cross interference between signals. The strength of the interference signal is related to the di/at of the C-point signal and the distance between lines. When the two signal lines are not very long, What is actually seen on AB is the superposition of two pulses.

The micro control manufactured by CMOS technology has high input impedance, high noise, and high noise tolerance. The digital circuit is stacked with 100-200mV noise, which does not affect its operation. If line AB in the figure is an analog signal, such interference will become intolerable. If a printed circuit board is a four layer board, with one layer having a large area of ground or a double-sided board, and the back of the signal line having a large area of ground, the cross interference between these signals will be reduced. The reason is that the characteristic impedance of the signal line is greatly reduced, and the reflection of the signal at the D end is greatly reduced. The characteristic impedance is inversely proportional to the square of the dielectric constant of the medium between the signal line and ground, and directly proportional to the natural logarithm of the thickness of the medium. If the AB line is an analog signal, the interference of the digital circuit signal line CD to AB should be avoided, There should be a large area of land below the AB line, The distance between AB line and CD line should be 2-3 times greater than the distance between AB line and ground. Local shielding can be used, and ground wires can be laid on the left and right sides of the lead with a junction.

(4) Reduce noise from the power supply: While providing energy to the system, the power supply also adds its noise to the power supply it is supplying. The reset line, interrupt line, and other control lines of the microcontroller in the circuit are most susceptible to external noise interference. The strong interference on the power grid enters the circuit through the power supply, and even in a battery powered system, the battery itself has high-frequency noise. Analog signals in analog circuits cannot withstand interference from power sources.

(5) Pay attention to the high-frequency characteristics of printed circuit boards and components: In high-frequency situations, the distribution of leads, vias, resistors, capacitors, and connectors on the printed circuit board, such as inductance and capacitance, cannot be ignored. The distribution of capacitance and inductance cannot be ignored. Resistance generates reflection of high-frequency signals, and the distributed capacitance of the leads plays a role. When the length is greater than 1/20 of the corresponding wavelength of the noise frequency, an antenna effect is generated, and noise is emitted outward through the leads. The via of the printed circuit board causes a capacitance of approximately 0.6pf. The packaging material of an integrated circuit itself introduces 2-6pf capacitors. A connector on a circuit board with a distributed inductance of 520nH. A 24 pin integrated circuit socket with dual row straight pins, introducing a distributed inductance of 4-18nH. These small distribution parameters can be ignored in the microcontroller system at lower frequencies; Special attention must be paid to high-speed systems.

(6) The layout of components should be reasonably partitioned: The placement of components on the printed circuit board should fully consider the issue of electromagnetic interference resistance, and one principle is to minimize the lead wires between each component. In terms of layout, the analog signal section, high-speed digital circuit section, and noise source section (such as relays, high current switches, etc.) should be reasonably separated to minimize signal coupling between them. G   Handle the grounding wire properly on the printed circuit board, and the power and ground wires are the most important. The main way to overcome electromagnetic interference is through grounding.

For double-sided boards, the grounding arrangement is particularly particular. By using a single point grounding method, the power and ground are connected from both ends of the power supply to the printed circuit board, with one contact for the power supply and one contact for the ground. On a printed circuit board, there should be multiple return ground wires that converge to the contact point of the return power supply, which is called single point grounding. The so-called analog ground, digital ground, and high-power device ground separation refers to the separation of wiring, and ultimately all converge to this grounding point. When connecting to signals outside of the printed circuit board, shielded cables are usually used. For high-frequency and digital signals, both ends of the shielded cable are grounded. A shielded cable for low-frequency analog signals should be grounded at one end.

Circuits that are highly sensitive to noise and interference or circuits with particularly severe high-frequency noise should be shielded with metal covers.

(7) Make good use of decoupling capacitors: Good high-frequency decoupling capacitors can remove high-frequency components up to 1GHz. Ceramic capacitors or multilayer ceramic capacitors have better high-frequency characteristics. When designing printed circuit boards, a decoupling capacitor should be added between the power supply and ground of each integrated circuit. Decoupling capacitors have two functions: on the one hand, they are the energy storage capacitors of this integrated circuit, providing and absorbing the charging and discharging energy of the integrated circuit at the moment of opening and closing the door; On the other hand, bypass the high-frequency noise of the device. A typical decoupling capacitor in digital circuits is a 0.1uF decoupling capacitor with a 5nH distributed inductance. Its parallel resonance frequency is about 7MHz, which means it has a good decoupling effect on noise below 10MHz and almost no effect on noise above 40MHz.

A 1uf, 10uf capacitor with a parallel resonance frequency above 20MHz provides better removal of high-frequency noise. It is often advantageous to have a 1uf or 10uf high-frequency capacitor at the point where the power enters the printed board, even in battery powered systems. Every 10 or so integrated circuits need to be equipped with a charging and discharging capacitor, also known as a storage and discharging capacitor, with an optional size of 10uf. It is best not to use electrolytic capacitors. Electrolytic capacitors are rolled up with two layers of thin films, which act as inductors at high frequencies. It is best to use capacitor or polycarbonate capacitor.

The selection of decoupling capacitance value is not strict and can be calculated as C=1/f; For a system composed of microcontrollers, a frequency range of 0.1-0.01uF can be set for 10MHz.

3. Experience in reducing noise and electromagnetic interference.

(1) If you can use low-speed chips, you don't need high-speed ones. High speed chips are used in critical areas.

(2) A resistor can be connected in series to reduce the speed of the upper and lower edge jumps in the control circuit.

(3) Try to provide some form of damping for relays, etc.

(4) Use the minimum frequency clock that meets system requirements.

(5) The clock generator should be as close as possible to the device using the clock. The quartz crystal oscillator shell needs to be grounded

(6) Circle the clock area with a ground wire and keep the clock wire as short as possible.

(7) The I/O driver circuit should be placed as close as possible to the edge of the printed board to allow it to leave as soon as possible. The signal entering the printed circuit board should be filtered, and the signal from the high noise area should also be filtered. At the same time, a series terminal resistor method should be used to reduce signal reflection.

(8) The useless end of MCD should be connected high, or grounded, or defined as the output end. The end that should be grounded to the power source on the integrated circuit should be connected without being suspended.

(9) Unused gate circuit inputs should not be suspended, and unused operational amplifiers should have the positive input grounded and the negative input connected to the output.

(10) Try to use 45 line wiring instead of 90 line wiring for printed circuit boards to reduce the external transmission and coupling of high-frequency signals.

(11) Printed circuit boards are divided according to frequency and current switching characteristics, and noise and non noise components should be further away.

(12) Single panel and double-sided boards should use single point power supply and single point grounding, and the power and ground wires should be as thick as possible. If it is affordable, multi-layer boards should be used to reduce the capacitance inductance of the power and ground.

(13) Clock, bus, and chip selection signals should be kept away from I/O lines and connectors.

(14) Analog voltage input lines and reference voltage terminals should be kept as far away from digital circuit signal lines as possible, especially clocks.

(15) For A/D devices, it is better to unify the digital and analog parts rather than cross them.

(16) The clock line has less interference perpendicular to the I/O line than parallel I/O lines, and the clock component pins are far away from the I/O cable.

(17) The component pins should be as short as possible, and the decoupling capacitor pins should be as short as possible.

(18) The key lines should be as thick as possible and protective areas should be added on both sides. The high-speed line should be short and straight.

(19) Lines sensitive to noise should not be parallel to high current or high-speed switching lines.

(20) Do not wire under quartz crystals or noise sensitive devices.

(21) Weak signal circuits, do not form current loops around low-frequency circuits.

(22) Do not form a loop for any signal. If unavoidable, keep the loop area as small as possible.

(23) One decoupling capacitor per integrated circuit. A small high-frequency bypass capacitor should be added to the edge of each electrolytic capacitor.

(24) Use large capacity tantalum capacitors or polycarbonate capacitors instead of electrolytic capacitors as circuit charging and discharging energy storage capacitors. When using tubular capacitors, the casing should be grounded.