[SI-LIST] Re: Crosstalk between different layers

Roberto,

That formula is the crosstalk limit for two adjacent microstrips.  H is the 
height from signal to the nearest reflection plane.

Four layer boards present a compromise between signal layer to plane 
height, and spacing requirements to meet crosstalk.  In most cases, you 
will want to put the two planes in the middle, and keep them as close 
together as possible.  For a 31 mil stack up, and 3 mils plane to plane, 
this will yield approximately 14 mils from signal to plane.  So by the 
formula, the worst case center to center spacing required would be about 50 
mils.  But this only applies for lines that are a significant portion of 
the rise time.  If you have really fast signals, consider going to six 
layers or a faster board.  If you move the planes away from each other, the 
inductance rises quickly and you will be far more susceptible to 
simultaneous switching noise.

Regards,


Steve.
At 03:46 PM 6/21/01 +0200, you wrote:

>Hi all,
>
>I'm investigating this configuration in order to estimate the crosstalk
>between  L2 and L3.
>
>L1       -------------      Plane (GND)
>L2               ---             signal
>L3               ---             signal
>L4       -------------      Plane (Power)
>
>My target is to obtain a minimum spacing between the two layers that allow 
>a maximum
>crosstalk level (for example < 5%).
>I'm wondering if I can use the H.Johnson book formula :
>
>crosstalk < 1/[1+(D/H)^2].
>
>If yes, assuming a ground-signal distance of 5 mils and a signal-signal 
>distance of 20 mils,
>which is the value I have to use for H?
>
>And if one signal layer is 90 degree routed, there is any difference?
>
>In H.Johnson book section 5.8.5 (ten-layer stack) there is a similar 
>configuration:
>the crosstalk between layers 4 and 6 is determined by the ratio of 
>separation to ground plane
>height. But I don't understood how the geometric average height of 0.007 
>inch is calculated.
>
>Thanks in advance.
>
>Roberto Carretta.
>
>
>
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