Crosstalk

PCB designers are often asked to fit a design in as small space as possible. To accomplish the challenge, PCB traces have to be routed as close as possible. Sometimes these traces are at a minimum distance allowed by the PCB manufacturer (typically 4 mils). Too close traces create electric and magnetic coupling. Changing electric or magnetic field causes undesired signal in the nearby victim trace and can be cause of failure. PCB designers must be aware of the nets that can fail because of the cross talk.

Inductive Coupling or Crosstalk

If one trace is very close to another, changing magnetic field in one trace will induce electric voltage in another. This creates inductive coupling. Inductive coupling is predominant for traces on the same layer running close to each other for large length of routing.

How to avoid Inductive Crosstalk

Use the following methods to reduce the inductive crosstalk in adjacent traces.

1. Identify the critical nets susceptible to crosstalk. This includes high speed clock nets and nets with high edge rate and/or high frequency.
2. Route the critical nets father apart from one another. 15 mils separation is a good number for digital boards.
3. Keep the layer stack up such that the separation between the signal layer and the ground layer is as small as possible. 5 mils separation is a good number.
4. Use stripline to route all the critical traces running close by. Minimize the length of the traces running in parallel on the outer layers
5. If adjacent traces can not be spaced apart, try to minimize the length for which they run in parallel. However, once they cross the critical length, the length of parallelism does not matter for near end cross talk.


Previous                    Next