What is cross talk & how to reduce the cross talk?

In this tutorial we are going to learn about what is cross talk & how to reduce the cross talk?

Crosstalk –

It is refers to unintended EM coupling between traces, wires, trace-to-wire, cable assemblies, components, & other electrical components subject to EM field disturbance. Crosstalk depends on th length of the parallel traces, the space between them & the rise & fall time of the signal. Some of the crosstalk occurrences are

  • Crosstalk between wires, cables & traces affects intersystem performance.
  • Crosstalk is considered a functionality concern (signal quality).
  •  Design techniques to prevent crosstalk.

        First, note the following observations:

  • Decreasing the trace separation increases the mutual capacitance Cm & the crosstalk.
  •  With parallel traces, longer parallel lengths increase the mutual inductance Lm & the crosstalk.
  •  Decreasing the rise time of the signal, increases the cross-talk.

Some of the measures to be taken to reduce crosstalk are:

1. Minimize physical distance between components during placement.

2. Minimize parallel routed trace lengths.

3. Group logic families according to functionality. Keep bus structure tightly controlled.

4. Provide proper termination on impedance-controlled traces, or traces rich in RF harmonic energy.

5. Locate components away from Input/Output interconnects & other areas susceptible to data corruption & coupling.

6. Reduce trace impedance and signal drive level.

7. Reduce signal-to-ground reference distance separation.

8. Route adjacent layers (microstrip/stripline) orthogonal. This prevents capacitive coupling between adjacent layers

9. Avoid routing of traces parallel to each other. Provide sufficient separation between traces to minimize inductive coupling(the 3 W Rule: ) (the 3 W Rule: The distance of separation between traces must be 3 times the width of the traces, measured center-line to center-line – This rule for trace separation will reduce the crosstalk flux by approximately 70%. (For a 98% reduction, change the 3 to 10.)

NOTE: Note that the traces near the edge of the plane need to be > 1W from the edge!) or use guard traces

10. Provide a band-limiting filter on specific TM lines to prevent RF from coupling between source & victim traces. This filter consists of a simple RLC resonant shunt circuit in series between the source trace & 0V-reference.

11. Isolate signal layers that must be routed in the same axis by a solid planar structure (typical of backplane stack up assignments).

13. Partition or isolate high noise emitters (clock, I/O, high-speed interconnects, etc.) onto Different layers within the stack up assignment.

13. Route signal on adjacent layers perpendicular to each other wherever possible. (Especially keep analog and digital signals are routed together).

14. Keep the clock lines away from the I/O signals lines or have a good clock shielding to prevent coupling.

15. Keep spacing between the adjacent active traces greater than trace width.

16. Keep clock & other HF signals grouped together & separated from connectors, other low speed & sensitive traces such as interrupt or reset lines.

17. Use narrow traces (8 mils or less) to increase HF dumping & reduce capacitive coupling.

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