In the learning process of high-speed PCB design, crosstalk is an important concept that needs to be mastered. It is the main way for the propagation of electromagnetic interference. Asynchronous signal lines, control lines, and I\O ports are routed. Crosstalk can cause abnormal functions of circuits or components.
Crosstalk
Refers to the undesired voltage noise interference of adjacent transmission lines due to electromagnetic coupling when the signal propagates on the transmission line. This interference is caused by the mutual inductance and mutual capacitance between the transmission lines. The parameters of the PCB layer, the signal line spacing, the electrical characteristics of the driving end and the receiving end, and the line termination method all have a certain impact on the crosstalk.
The main measures to overcome crosstalk are:
Increase the spacing of parallel wiring and follow the 3W rule;
Insert a grounded isolation wire between the parallel wires;
Reduce the distance between the wiring layer and the ground plane.
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 kept without mutual interference, which is called the 3W rule. If you want to achieve 98% of the electric field without interfering with each other, you can use a 10W spacing.
Note: In actual PCB design, the 3W rule cannot fully meet the requirements for avoiding crosstalk.
Ways to avoid crosstalk in PCB
In order to avoid crosstalk in the PCB, engineers can consider from the aspects of PCB design and layout, such as:
1. Classify logic device series according to function and keep the bus structure under strict control.
2. Minimize the physical distance between components.
3. High-speed signal lines and components (such as crystal oscillators) should be far away from the I/() interconnection interface and other areas susceptible to data interference and coupling.
4. Provide the correct termination for the high-speed line.
5. Avoid long-distance traces that are parallel to each other and provide sufficient spacing between traces to minimize inductive coupling.
6. The wiring on adjacent layers (microstrip or stripline) should be perpendicular to each other to prevent capacitive coupling between layers.
7. Reduce the distance between the signal and the ground plane.
8. Segmentation and isolation of high-noise emission sources (clock, I/O, high-speed interconnection), and different signals are distributed in different layers.
9. Increase the distance between the signal lines as much as possible, which can effectively reduce capacitive crosstalk.
10. Reduce the lead inductance, avoid using very high impedance loads and very low impedance loads in the circuit, and try to stabilize the load impedance of the analog circuit between loQ and lokQ. Because the high impedance load will increase the capacitive crosstalk, when using very high impedance load, due to the higher operating voltage, the capacitive crosstalk will increase, and when using very low impedance load, due to the large operating current, the inductive Crosstalk will increase.
11. Arrange the high-speed periodic signal on the inner layer of the PCB.
12. Use impedance matching technology to ensure the integrity of the BT certificate signal and prevent overshoot.
13. Note that for signals with fast rising edges (tr≤3ns), carry out anti-crosstalk processing such as wrapping ground, and arrange some signal lines that are interfered by EFT1B or ESD and have not been filtered on the edge of the PCB.
14. Use a ground plane as much as possible. The signal line that uses the ground plane will get 15-20dB attenuation compared to the signal line that does not use the ground plane.
15. Signal high-frequency signals and sensitive signals are processed with ground, and the use of ground technology in the double panel will achieve 10-15dB attenuation.
16. Use balanced wires, shielded wires or coaxial wires.
17. Filter the harassment signal lines and sensitive lines.
18. Set the layers and wiring reasonably, set the wiring layer and wiring spacing reasonably, reduce the length of parallel signals, shorten the distance between the signal layer and the plane layer, increase the spacing of signal lines, and reduce the length of parallel signal lines (within the critical length range) , These measures can effectively reduce crosstalk.