[SI-LIST] Re: Crosstalk mechanism

Shawn, it takes db/dt to magnetically induce an EMF in the victim.  
Higher db/dt, means higher induced EMF.  Shrinking dt surely gets you there.

Steve.
Shawn Zheng wrote:
> 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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