[SI-LIST] Re: Return current crosstalk?
- From: Nelson Don <dhwn@xxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 1 Apr 2004 16:06:22 -0500
Patrick,
I believe that you'll be fine routing this way; the return currents
will be confined to the surfaces of the plane, which is likely several
skin-depths thick. The time you'd want to worry about two horizontal
layers would be in a dual stripline configuration, where you have two
routing layers between two ground planes. Single striplines are quite
forgiving!
Just remember that there are other ways for crosstalk to creep into
your system... For instance: if you drop a via to another routing layer
that is not referenced to the same ground plane, the return currents
will need to seek a path from one plane to the other, either through an
inter-plane via or bypass capacitor. If you don't have such a path
near this kind of layer change, you can introduce crosstalk even though
the victim and aggressor signal traces are nowhere near each other
because their return currents could interact.
To be safe, either transition only between routing layers that share
a single reference plane (in which case, the return current snakes
through the via anti-pad from one surface to the other) or, if you must
go to another routing layer with a different reference plane, add an
interplane via (if ground to ground) or a decoupling cap (if ground to
power). This will keep your return currents nicely controlled.
hope this helps,
-don
--
Don Nelson
"The Constitution only gives people the right to pursue happiness. You
have to catch it yourself." --Ben Franklin
On Apr 1, 2004, at 3:35 PM, Scott McMorrow wrote:
> Patrick
>
> at high frequencies there is no problem. The skin penetration depth of
> the field at high frequencies is quite small, at least an order of
> magnitude less than the thickness of the copper plane. This provides
> ample isolation of layers separated by planes.
>
> The only time there would be a concern is for low frequency signals, in
> the range below about 20 MHz. At these low frequencies, fields can
> penetrate the planes. However, even then, the level of crosstalk is
> quite low and only a problem for analog noise isolation, not digital.
>
> regards,
>
> scott
>
> --
>
> Scott McMorrow
> Teraspeed Consulting Group LLC
> 2926 SE Yamhill St.
> Portland, OR 97214
> (503) 239-5536
> http://www.teraspeed.com
>
> Teraspeed(SM) is the service mark of Teraspeed Consulting Group LLC
>
>
> Patrick Jabbaz wrote:
>
>> All,
>> I have a Board stack-up that currently has a reference plane in the
>> middle,=20
>> Between two signal Layers
>>
>> L3 Int-sig1 Hor
>> L4 Plane
>> L5 Int-Sig2 Hor
>>
>> I normally would run adjacent signal layers at 90 degrees from each
>> other, not in this case.
>> Let's assume that I have signals running horizontal left to right on
>> Int
>> Signal layer 3
>> I also have signals running horizontal (left to right) on Int Signal
>> layer 5
>> The signal traces lined up perfectly over each other.
>> With that in mind, my question is the following
>> Would the return currents cause cross-talk? Or would the return
>> current
>> be=20
>> Riding only on the top and bottom surface of the reference plane
>> without
>>
>> Interacting with one another?
>>
>> Inkra Networks
>> Patrick Jabbaz CID
>> Sr Board Layout Eng.
>> 40971 Encyclopedia Circle=20
>> Fremont, CA 94538
>> Work (510) 249-4835
>> Mobile (408) 621-6533
>> patrick@xxxxxxxxx
>>
>>
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