RE: [SI-LIST] : Bends in PCB traces (was: MECL System Design Hand book)

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From: Ingraham, Andrew ([email protected])
Date: Wed Jun 06 2001 - 10:53:18 PDT

I have pretty much stayed out of this thread, but want to throw my hat into
the ring on this one.

Scott McMorrow wrote:

> With the impedance of a trace held as a constant, the discontinuity seen
> by an
> edge passing a corner should remain consistent across a very wide range
> of trace widths. Why? Because the important dimension W/h remains
> constant with size scaling. Since W/h is constant (to achieve the same
> impedance)
> Eeff is also constant.
The reasoning seems backwards to me. Eeff is constant because you use
similar board dielectric materials. W/h remains constant because (1) Eeff
was constant (not the other way around), and (2) you want the same
characteristic impedance.

> With Eeff constant for varying trace widths (and associated
> adjustment of the trace height above the plane for impedance) the trace
> velocity will be
> identical.
> Since the trace velocity is identical, the distributed L and C is exactly
> the
> same for any length of trace or size of feature. This means that as the
> trace width, corner, and trace height above the plane are scaled
> geometrically.
> the effect of the corner discontinuity remains the same.
Not necessarily, I think. If part of the corner effect is due to excess
capacitance of the corner, then smaller trace widths result in there being
less extra copper at that corner. With C per unit length unchanged, the
effect of that excess C would indeed scale with trace width.

> Now, there are limits to this analysis.
> 1) TEM mode propagation is assumed.
What if it isn't TEM mode? In fact it can't be strictly TEM mode at the
corner. Maybe part of the corner effect, even if you mitre the corner so
there is no excess C, is because of that. I don't know, just conjecturing.


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