Re: gEDA-user: 3 questions on a 4 layer board

[Dan McMahill <dan@xxxxxxxxxxxx>]

Rick Collins wrote:

>> Question 2: In this a reasonable assignment, or am I missing something?
>> That's the stackup I use.  In general, you want the ground plan near
>> the signal plane with the most signals.  For surface mount, that's the
>> top.  For through-hole it might be the bottom.
> 
> I took an excellent class in high speed digital design by Lee 
> Ritchey.  One of the things he covered was using ground planes and 
> controlling impedance.  Turns out that for the purpose of impedance 
> (which is why you want your traces close to the plane) a power plane 
> is exactly the same as a ground plane.  So it does not matter if your 
> signals are next to the power or the ground plane, they are both at AC ground.
 >
> Just thought I would chime in with my two cents worth.


If my traces were carrying ground referenced analog signals I wouldn't 
want the power plane being my "ground" plane.

And yes, I've seen a real life case with some rather frustrating 
behavior that the correct stackup totally fixed.  There was a moment of 
clarity when a bypass cap way on the other end of the board made a much 
larger than expected difference.

-Dan


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