Factors affecting the magnitude of crosstalk due to the mixed effect of capacitive coupling and inductive coupling
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Factors affecting the magnitude of crosstalk due to the mixed effect of capacitive coupling and inductive coupling

Posted Date: 2024-01-19

Crosstalk

Crosstalk is the generation of undesired voltage and current noise on adjacent transmission lines due to electromagnetic coupling when signals propagate on transmission lines. The fringe field effect of signal lines is the fundamental cause of crosstalk.

When a signal propagates along a transmission line, turns of electric power lines and magnetic field lines will be generated in the signal path and return path, and the electromagnetic field will extend to the surrounding space. These extended fields are called fringe fields.

The crosstalk at the end of a static network close to the interference source is called near-end crosstalk (backward crosstalk), and the crosstalk at the end far away from the interference source is called far-end crosstalk (forward crosstalk). According to the different causes of crosstalk, crosstalk is divided into: capacitive coupling crosstalk and inductive coupling crosstalk.

1. Capacitive coupling crosstalk:

"Capacitive coupling" means that when there is signal transmission on the interference line, due to the change in signal edge voltage, the distributed capacitance on the interference line will induce a time-varying electric field in the area near the signal edge, and the victim line is in this electric field. , so the changing electric field will produce an induced current on the victim line.

2. Inductive coupling crosstalk:

"Inductive Coupling" When a signal propagates on an interference line, due to changes in signal current, a time-varying magnetic field will be generated in the area near the signal transition through the action of distributed inductance. The changing magnetic field will induce noise on the victim line. voltage, and then form an inductive coupling current, which propagates to the near end and the far end respectively.

3. The mixed effect of capacitive coupling and inductive coupling:

Since the current flow direction is opposite to that of the far-end capacitive coupling, the coupling current reaching the far end of the victim line is the difference between the two. Generally, on a complete reference ground plane, the crosstalk voltages generated by capacitive coupling and inductive coupling are equal in magnitude. Therefore, the total noise of far-end crosstalk is due to the opposite polarity of the current generated by capacitive coupling and inductive coupling, and the magnetic fields cancel each other out.

For strip lines, it can show a good balance between the two, and its far-end coupling coefficient is extremely small. For microstrip lines, since most of the electric fields related to crosstalk pass through the air rather than other insulating materials, Therefore, capacitive coupling is smaller than inductive coupling, causing its far-end crosstalk to be a negative number.

4. Factors affecting the size of crosstalk:

4.1. Effect of coupling length on crosstalk:

For far-end crosstalk, the peak value is proportional to the coupling length. The longer the coupling length, the greater the crosstalk. For near-end crosstalk, when the coupling length is less than the saturation length, the crosstalk will increase as the coupling length increases, but when the coupling length is greater than the saturation length, the crosstalk will increase. , the near-end crosstalk is a stable value.

4.2. Effect of line spacing on crosstalk:

The size of crosstalk is inversely proportional to the line spacing. As the line spacing increases, both near-end crosstalk and far-end crosstalk will decrease. When the line spacing ≧3 times the line width, the crosstalk is already very small.

4.3. The impact of signal edge flip speed on crosstalk:

The magnitude of crosstalk is proportional to the edge flip speed of the signal, that is, the faster (the steeper) the rising edge/falling edge of the signal, the greater the crosstalk. A high signal frequency does not mean a faster rising edge/falling edge. Similarly, a low signal frequency does not mean a slower rising edge/falling edge.

4.4. Other factors affecting crosstalk:

The size of crosstalk is also affected by factors such as media thickness, current direction, and load size.

4.5. How to suppress crosstalk:

üIncrease PCB wiring spacing.

ü Reduce the parallel length of PCB wiring, or use vertical cross wiring.

ü Slow down the rising and falling edges of the signal.

üPCB routing is close to the reference ground plane or power layer.

üAdd ground wire shielding between PCB wiring to reduce crosstalk.

ü Through termination, signal reflection is suppressed and crosstalk is reduced.

ü Reduce signal drive current and operating voltage.

ü Use stripline to suppress crosstalk.

üAdopt 3W PCB wiring principle.

üUse laminates with lower dielectric constants.

üDo not share return pins in packages and connectors.

üChange load size.

üAdopt differential signal wiring method.

Source: This content comes from the WeChat public account "Fenglingdukouhua EMC". Special thanks!

Review Editor: Tang Zihong


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