From: Ron Miller (rmiller@Brocade.COM)
Date: Fri Dec 17 1999 - 15:47:31 PST
Chris Padilla wrote:
> 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.
Chris, if you are comparing differential microstrips vs differential
striplines it should be obvious to you that stripline, being surrounded
by grounds would radiate less.
If you are comparing single ended stripline with microstrip differential
you need to specify the coupling factor. That is for tightly coupled
microstrip with the impedance determined by the spacing of the traces there
could be little common mode ratiation, while loosly coupled microstrip
far apart would have rather high radiation..
> Ron Miller
> 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
> 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.)
> Thanks for your time.
> Chris Padilla C i s c o S y s t e m s
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