label: multilayer PCB,Printed-circuit board,PCB EMC technology
Electromagnetic compatibility
Electromagnetic compatibility (EMC) is a comprehensive discipline, and immunity to electromagnetic interference major research problems. Electromagnetic compatibility is an electronic device or system in the electromagnetic environment specified level, the no performance degradation due to electromagnetic interference, while they themselves produce electromagnetic radiation is not greater than the limit test level, without affecting the normal other electronic devices or systems run and reach interference between devices and equipment, systems and system requirements to work together reliably. Electromagnetic compatibility (EMC) includes two aspects: Electromagnetic radiation products and oral anti-electromagnetic interference and electronic products must be considered a good electromagnetic compatibility, electromagnetic radiation can neither interfere with other electronic devices, but also have lower electromagnetic sensitivity, resistance to electromagnetic interference requirements. Electromagnetic interference caused by electromagnetic disturbance is a consequence, it will make electronic equipment, transmission channel, and printed board assembly system performance degradation. Printing plates as basic components of electronic devices, the presence of electric and magnetic fields using the same, there is electricity, there is the presence of a magnetic field of electromagnetic compatibility problems, especially a lot of modern electronic devices using digital circuits, high-speed logic circuits, signal transmission speed is greatly improved, this also increases the factors causing electromagnetic radiation and electromagnetic interference, so consider the problem of electromagnetic compatibility is an important part of the printed circuit board design.
2 printed circuit board design for electromagnetic interference suppression
Due to the intensity of electronic devices and circuits on the PCB is growing, and the frequency of the signal is also rising , the problem will inevitably introduce E falsely (electromagnetic compatibility) and female I (electromagnetic interference).
External conducted interference and radiated interference almost no effect on the circuit on the PCB, in fact, take the right steps in the design can often also play a role in anti-interference and suppression of emission. In the design of printed circuit boards, the first based on actual needs, select the appropriate PCB type (sheet and plate layer), and then determine the position of components on the board, then turn layout, design ground, signal lines.
2.1 Selection of the printed circuit board
There are single-sided printed circuit boards, double-sided and multilayer points. Single-sided and double-sided generally used for low, low density integrated circuits and wiring circuits. Multilayer wiring for high density, high- speed digital integrated circuit chips. From the point of view of electromagnetic compatibility, electromagnetic radiation can be reduced multilayer circuit boards and circuit boards to improve anti-jamming capability. Because of the multilayer may be provided dedicated power and ground planes, the distance between the signal line and the ground line between the printed circuit board only in the interlayer distance. Such circuit board area of all signals can be minimized, thereby effectively reducing the differential -mode radiation.
Layout 2.2 components
Printed circuit board not only by simply connecting wires between the respective printed components, should be considered even more important is the characteristics and requirements of the circuit, the components are placed correctly.
l) by the circuit elements and components have interconnected relationships should be close to the layout. To reduce the length of the alignment between the components and connections, reducing radiation and interference.
2) by switching speed of the circuit or device operating frequency is relatively low partition layout, from I / 0 terminal to the remote circuit board in order to reduce the operating frequency distribution at higher frequencies in the region, should be close to the high-speed shock devices work / 0 end.
3) prone to electromagnetic radiation components (such as: clocks, oscillators, etc.) , away from electromagnetic sensitive devices or traces, taking electromagnetic shielding measures when necessary.
2.3 the layout of signal lines
The best signal lines arranged according to the order of signal flow arrangement , the signal on the circuit board to smooth. Signal lines should be mutually incompatible away , do not parallel lines . The signal line on the same level to keep a certain distance , preferably in the corresponding ground loop isolation to reduce crosstalk between lines . Distributed in the different layers of signal lines to be perpendicular to each other , thus reducing the electric and magnetic field lines interference fit between the disaster . High-speed signal loop area as small as possible in order to avoid radiation interference.
1) High-speed signal lines on the same layer as much as possible , do not change layer ;
2) smooth over the signal line , to avoid the signal reflected linewidth mutation ;
3) Do not part of the adjacent ground tracks go , part of the impedance caused by mutations not close ;
4) Do not leave the PCB signal line too close to the edge , otherwise it will lead to the characteristic impedance changes , and prone to the edge of the field , increasing the outward radiation ;
5) For the characteristic impedance of the wire demanding signal line wiring should be used in the form of stripline or microstrip line is conducive conductor width , thickness and the thickness of the insulating layer to adjust the characteristic impedance;
6) When the operating frequency of the circuit or device over SMHz edge data rates over sns, should be preferred plywood , help to reduce electromagnetic interference;
7) high-speed signal transmission lines as little as possible over the hole, and the aperture is small, beneficial reduce parasitic capacitance hole, usually a small diameter vias , buried or blind holes.
2.4 Ground Layout
1) The digital circuits and analog circuits separately. Existing high-speed logic circuit on the circuit board, there are linear circuits, so that they should try to separate the two should not be confused ground, connected to the power supply terminal ground respectively. To try to increase the contact area of linear circuits.
2) Select the correct single-point and multi- point grounding grounding. Low frequency circuit, the operating frequency of the signal is smaller than a positive z, its smaller inductance of the wiring between the device and, while the larger the circulation circuit is formed on a ground disturbance, which should point to ground. When the signal is greater than the operating frequency of 10MHz, ground impedance becomes large, you should minimize ground impedance, the nearest multi-point grounding should be used.
3) Try bold ground wire. If the ground is very small, the ground potential and the current changes with the change in the timing of the electronic device causes the signal level of instability, noise performance deteriorates. Should therefore be ground as bold, it three times through the printed circuit board to allow current.
4) to isolate the line should not be too close to the hole, the hole in the ring can not insulate too large, so as to prevent unintentional grooves on the impact layer impedance signal lines.
5) constitute a closed loop grounding wire. Only by a digital circuit design printed circuit board ground system, the ground wire made of closed loop can significantly improve noise immunity.
The width of the printed conductors 2.5
Wire width should meet the electrical performance yet easy to produce appropriate, its minimum to withstand the current may be, but not less than the minimum 0.Zmln, high-density , high-precision printed circuit, conductor width and spacing is generally desirable 0.3nnn; wire width at high currents should also take into account the temperature rise, single-panel test showed that when the foil thickness SOum, wire width of a one 1.smm, through current of 2A, the temperature rise is small.
In between DIP package traces feet when passing between two lines, the pad diameter can be set to 50mil, line width and line spacing are 10 anti- l; between his legs only when the wire through a pad diameter can be set to 64011, width and diameter are 12mil.
3 software anti-jamming measures
The watchdog circuit prevents the use of a hardware failure due to external interference and procedural error resulted in system downtime or interruption program enters an infinite loop; using software filtering method eliminate high-frequency pulse interference.
4 Summary
Printed circuit board electromagnetic compatibility problem is very complex, to take appropriate measures against specific issues. Good design, new design methods, learn from the advanced experience in the design, installation use sophisticated technology, effectively reduce electromagnetic interference. With the development of continuous improvement and learning PCB EMC technology, electromagnetic compatibility of its electronic products can also be significantly improved.
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