Electronic equipment used in all kinds of electronic equipment and systems are still in printed circuit board as the main assembly way.Practice has proved that even if the circuit principle diagram design is correct,Printed circuit board design undeserved, also can produce adverse effect on the reliability of electronic equipment.For example, if the two parallel lines in close proximity, PCB will form the signal delay, reflection noise in transmission line terminal formation.Therefore, in the design of the printed circuit board, should pay attention to the right way.
1.ground design in electronic equipment, earthing is an important way to control interference
If you can combine grounding and shielding correct use, can solve most of the interference problem.Electronic equipment in the ground structure roughly systematically, casing (block), digital earth (logically) and simulation, etc.The ground should be paid attention to in the design of the following:
(1)Correct selection of single point grounding and multi-point earthing in low frequency circuit, the signal of the working frequency is less than 1 MHZ, little impact on its inductance between wiring and devices,The grounding circuit had a greater influence on the formation of the circulation of interference, thus should use one-point earthing.When the working frequency of the signal is more than 10 MHZ, ground impedance becomes large, at this point should try to reduce the ground impedance, nearby multipoint earthing should be adopted.When the working frequency in 1 ~ 10 MHZ, if the grounding, the ground wire length should not exceed 1/20 of the wavelength, otherwise the multipoint earthing method should be adopted.
(2)Separate digital circuit and analog circuit.Circuit board has both high speed logic circuit, and linear circuit, should make them apart as far as possible, and both ground not mixed, respectively connected to the power end ground.Try to increase the grounding area of linear circuit.
(3)Bold grounding line as far as possible if the ground wire is very thin, grounding potential changes over electric current, the timing signal level instability of the electronic equipment, noise resistance worse.Therefore earth wire should be bold as far as possible, make it can pass three allows current in printed circuit board.If possible, the width of the ground wire should be greater than 3 mm.
(4)Earth wire will constitute a closed loop road design only composed of a digital circuit PCB ground system, to make the grounding line closed loop road can obviously improve the ability to resist noise.The reason for this is that there are many integrated circuit components on printed circuit board, especially in case of more components, power consumption due to restrictions of the grounding line thickness, can produce large potential difference on the knot, cause anti-noise ability to drop, if the grounding structure into the loop, would be to reduce the potential difference value, improve the ability to resist noise electronic equipment.
2.the electromagnetic compatibility design
Electromagnetic compatibility is refers to the electronic equipment in various kinds of electromagnetic environment is still able to coordination, ability to work effectively.Electromagnetic compatibility design of the purpose is to make electronic devices can inhibit a variety of external interference, make the electronic equipment can work normally in the specific electromagnetic environment, and also can reduce the electronic device itself electromagnetic interference to other electronic equipment.
（1）Choose the reasonable width of wire.Due to transient current impact on the printed line produced by the interference is mainly caused by printed conductor inductance components, therefore should minimize the inductance of printed wiring.Printed conductor inductance is proportional to its length, its width is inversely proportional to, so the short and fine wires is favorable to suppress interference.Clock, lead line drive or bus driver signal often contains large transient current, printed wiring to short as possible.For discrete component circuit, printed wiring width at around 1.5 mm, can fully meet the requirements;For integrated circuits, printed wiring width can choose between 0.2 ~ 1.0 mm.
（2）Adopt the strategy of the correct wiring.Using equal linear conductor inductance can be reduced, but the mutual inductance between conductor and distributed capacitance increases, if allow layout, had better use well glyph structure, network cabling concrete side lateral approach is printed circuit board wiring, the other side vertical wiring, then in the cross hole is connected with metallized hole.In order to prevent the PCB cross talk between wires, when designing the wiring should try to avoid long distance equal walk the line, the distance between the lines to open and as far as possible, signal wire and ground wire and the power cord as far as possible, don't cross.In some very sensitive to interference signal lines between a grounded printing line, can effectively restrain cross talk.
In order to avoid the high frequency signal through printed wiring of electromagnetic radiation, the printed circuit board wiring, also should pay attention to the following:
●Try to reduce the discontinuity of printed wiring, such as wire width don't mutation, the wire should be greater than 90 degrees of corner ban ring line etc.
●Clock signal leads the most susceptible to electromagnetic radiation interference, go line should close, with different ground loop drive should be close to the connector.
●The bus driver should be close to its desire to drive the bus.For those who leave the printed circuit board, and lead the drive should be tightly close to the connector.
●Data bus wiring should be every two signal lines between a signal ground.Best is tightly close to the least important address fuses placed ground loops, which often contains high frequency current.
●In PCB layout, high speed, medium speed and low speed logic circuit should be arranged device
（3）Reflection interference suppression.In order to prevent the reflection interference appears in the printed line terminal, in addition to the special needs, should as far as possible, shorten the length of the printed line and adopting slow circuit.Add terminal matching when necessary, that is, at the end of the transmission line to ground and added a different power supply side resistance matching resistance.According to the experience, generally faster TTL circuit, when the printed line longer than 10 cm above the terminal matching measures should be adopted.Matching resistance value should be based on the output of the integrated circuit drive current and the maximum absorption current to decide.
3.decoupling capacitor configuration
In dc power supply circuit, the change of the load will cause the power supply noise.As in digital circuits, when the circuit from one state into another state, will be on the power line to produce a large peak currents, the formation of the noise of the transient voltage.Configuration decoupling capacitor can restrain the noise due to load changes, is the reliability of the printed circuit board design of a conventional practice, configuration principle is as follows:
●Power input end of the jumper a 10 to 100 uf electrolytic capacitor, the position of the printed circuit board permitting, adoption of more than 100 uf electrolytic capacitor anti-interference effect will be better.
●For each of the integrated circuit chip configuration a 0.01 uF ceramic capacitor.If printed circuit board and a small space, can every 4 ~ 10 chip configuration a 1 ~ 10 uf tantalum electrolytic capacitor, this particularly small high frequency impedance of the device in 500 KHZ ~ 20 MHZ range impedance is less than 1 Ω, and leakage current is very small (below 0.5 uA).
●For weak noise, shut off the current change device and ROM, RAM, storage devices, such as should be the power cord of the chip (Vcc) and ground (GND) direct access to the decoupling capacitor.
●Decoupling capacitor lead cannot too long, especially high frequency bypass capacitors can't lead
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