[SI-LIST] Crosstalk mechanism

Hi,
I am a little confused about the impact of return current on xtalk. Common
sense is that the higher frequency signal causes more xtalk. High frequency
return current of signal tend to concentrated on the ground under the signal
trace so as to minimize the inductance. So the current density falls off
with the distance. By contrast, at relatively low frequency the return
current tends to spread across the ground plane. So there can be a large
amount of current induced (due to return currents from many other
conductors) underneath a conductor. I wonder why we dont care about low
frequency but widely spreaded return current. On many production boards, i
think there could be quite a lot of current (from DC to very high frequency)
that flows above or underneath signal traces. The magnetic field may be weak
due to low fequency loop current. but it is getting closer to victim than
the concentrated high frequency return current. right?
of course my intuition tells me that i dont need worry about them if two
conductors stay at large distance.

Shawn


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