# Re: [SI-LIST] : Inner vs. Outer Layer Routing

From: Jim Freeman ([email protected])
Date: Fri Dec 17 1999 - 14:52:23 PST

Hi Chris,
two points,
1. I'm not sure what you mean by balanced differential lines, but my
interpretation would be that lengths,Z0,number of vias and their
placement would all be exactly balanced for the two lines. Other points
would be the use of broadside coupling to reduce the near field,ensure
that the current flows are always equal and opposite(no skews in the
values of current), etc.
2. The dispersion is generally thought to be a property of the
material that the wave is propagating through and more particularly, the
variation in dielectric strngth and magnetic permeability of the
material over frequency. This would not seem to be a function of
stripline of microstrip construction except for the variablilty of these
values in air vs. frequency.

Jim Freeman

> More specifically, this is in regards to EMI and the use of
> hi-speed differential signaling.
>
> By hi-speed, I would say sub-nanosecond rise/fall times.
>
> In terms of the likelihood to radiate (or be susceptable to),
> it seems logical to me that microstrips would tend to radiate
> more than (balanced) striplines. Looking at this single-ended,
> it would seem true.
>
> So the next question would be how does a differentially routed
> signal help the cause to "justify" (to me) routing hi-speed
> signals on outer layers rather than inner layers.
>
> (Lets try and put off the SI implications of my thoughts here. I
> know that signals travel faster on outer layers. I also know that
> there is greater dispersion of the signal when placed on outer
> layers. Lengths of traces are also important here, terminations,
> vias, layer jumping, decoupling, etc., etc.) [Hmmm, now that I
> write all those down, they may well be important!]
>
> This leads me to ask exactly how much of a signal traveling down
> this transmission line is differential-mode and how much is
> common-mode? We know that radiation is often of the common-mode
> variety.
>
> Putting some numbers to this, let's assume 5-mil traces (w),
> 10-mil separation (s), and 4-mil dielectric height(s) (h). Is
> trace height (t) an important factor here?
>
> //////////////////////////////////////// Ground for stripline case
> 5 10 5
> ____ ____
> |____|<---s---->|____| t
> h h
> //////////////////////////////////////// Ground plane (infinite)
>
> I have plans to run some very basic cases using Ansoft's HFSS--mostly
> because that is what I am familiar with AND have access to. I am
> posting this on the SI-LIST to gather any insightful thoughts on this
> whole idea.
>
> I have a feeling the overall answer to my question is: It depends!
> It depends on components near the traces, vias, layer jumps, length
> of the lines, environment these traces will be placed in (outside, in
> a chassis, on a PC, etc.)
>
>
> Chris Padilla C i s c o S y s t e m s
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