[SI-LIST] Re: 90 degree bend
- From: Ray Anderson <reanderson@xxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Mon, 30 Aug 2004 09:32:40 -0700
Kanakaraj wrote:
>Hi,
>The simplest thing is that when you make 90 degree turns on the pcb track,
>the electron moving in it cannot make sudden turns..( remember inertia of
>direction in physics and a bus making a 90 degree turn at full speed)... so
>some of the electrons will jump out of the pcb track and will cause emi
>issues..
>
>and Andy, Andrew, Ray anderson and the seniors will give you more inputs on
>that.
>Well I am too like you. a beginner :-)
>Kanak
>
>
>
Nice story, unfortunately not a very good analogy. The crux of the
problem (if you consider it a problem in your application) is that a
sharp right angle bend will introduce an impedance discontinuity in the
trace due to a bit of excess C at the bend. You can miter the bend by 45
degrees to mitigate the excess C. (the microwave guys have developed
what is called an 'optimal' miter [whose details I don't have handy at
the moment] that is even better than a simple 45 degree miter at dealing
with the problem. The problem of excess C at the turn isn't very
problematic until you get to really high frequencies or very fast
risetimes. As far as electrons missing the turn and hurtling off the
road............
This story reminds me of the one that related by a US Navy instructor to
a class of new aviation electronics techs at the Naval Air Technical
Training Command at NAS Millington back in the early 70's. He explained
(with a straight face) that one could understand how a capacitor worked
if you looked at the schematic symbol. He explained that it blocked DC
because there was a gap between the parallel lines in the symbol, and
that it passed AC because the sine wave would "go from the connecting
wire, up over the first line, down through the gap, around the second
line and eventually reach the second wire"
-Ray
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