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3W rules to prevent crosstalk in PCB design
2019-09-24
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Author:ipcb      Share

 3W rules to prevent crosstalk in PCB design


CrossTalk refers to the mutual interference between different networks on PCB caused by long parallel wiring, mainly due to distributed capacitance and inductance between parallel lines.The main measures to overcome crosstalk are:
Increase the spacing of parallel wiring, follow the 3W rule.
Insert grounded isolators between parallel lines.
Reduce the distance between the wiring layer and the ground plane.


3 W Rules

In order to reduce crosstalk between lines, the line spacing should be large enough. When the line center spacing is not less than 3 times the line width, 70% of the electric field can be maintained without mutual interference, which is called 3W rule.To achieve a 98% electric field that does not interfere with each other, a spacing of 10W can be used.


In actual PCB design, the 3W rule cannot completely meet the requirement of avoiding crosstalk.


According to practical experience, if there is no shielding ground, printed above is greater than the LCM distance between signal lines to prevent crosstalk, well so in PCB circuit wiring, requires the noise signals (such as the clock line) and the noise signal, and EFTlB, ESD interference such as the "dirty" and "clean" need to be protected in line, not only to be forced to use 3 w rules, but also for shielding ground packet processing, in order to prevent the happening of the crosstalk.


In addition, to avoid crosstalk in PCB, PCB design and layout should be considered, for example:


1. Classify logic device series according to function, keep bus structure strictly controlled.
2. Minimize the physical distance between components.
3. High-speed signal lines and components (such as crystal oscillator) should be kept away from I/() interconnection interfaces and other areas susceptible to data interference and coupling.
4. Provide correct terminals for high-speed lines.
5. Avoid long distance parallel wiring, and provide sufficient spacing between wiring to minimize inductive coupling.
6. Wiring on adjacent layers (microstrip or strip) should be perpendicular to each other to prevent capacitance coupling between layers.
7. Reduce the distance between the signal and the ground plane.
8. Separate and isolate high-noise emission sources (clock, I/O, high-speed interconnection), and different signals are distributed in different layers.
9. Increase the distance between signal lines as much as possible, which can effectively reduce capacitive crosstalk.
10. Reduce the lead inductance, avoid using load with very high impedance and load with very low impedance in the circuit, try to keep the load impedance of the analog circuit stable between loQ ~ lokQ.Because high impedance load will increase capacitive crosstalk,when very high impedance load is used, capacitive crosstalk will increase due to high operating voltage, while when very low impedance load is used, inductive crosstalk will increase due to high operating current.
11. Lay high-speed periodic signals on the inner layer of PCB.
12. Use impedance matching technology to ensure the integrity of BT signal and prevent overshoot.
13. Pay attention to the signal with fast rising edge (tr≤3ns), carry out ground anti-crosstalk processing, and arrange some signal lines disturbed by EFTlB or ESD and not filtered on the edge of PCB.
14. Use the ground plane as much as possible. The signal line using the ground plane will get 15 ~ 20dB attenuation compared with the signal line not using the ground plane.
15. Signal high frequency signal and sensitive signal are included in the ground processing, and the attenuation of 10 ~ 15dB will be obtained by using the ground covering technology in the double panel.
16. Use a balance line, shield line, or coaxial line.
17. Filter harassment signal line and sensitive line.
18. Reasonable setting of layers and wiring, reasonable setting of wiring layers and wiring spacing, reduction of the length of parallel signals, reduction of the spacing between signal layers and plane layers, increase of the spacing between signal lines, and reduction of the length of parallel signal lines (within the key length range) can effectively reduce crosstalk.