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Date: | Tue, 24 Jun 1997 18:58:46 -0400 |
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Jack Olson wrote:
<snipped material>
> Many of the RF designs here were done in AutoCAD, and the only reason I
> have been given for this was that it was easier to chamfer the corners
> of the delay lines (fairly wide 50 ohm traces on 20mil teflon). I may be
> wrongly assuming that the reason for the chamfer was to minimize the
> effect of the impedance change "around the corner", but my new CAD
> package can do curved traces which suggests NO mismatch around bends...
> constant width everywhere, right?
> Unfortunately, no one here has any data to support the need for
> chamfers. (I have to painstakingly enter precise polygons if chamfers
> are really required, so I would rather avoid it).
> We are working with 1.9GHz, and are moving to Rogers 4003 material.
>
> Can anyone with RF experience comment on this?
Hi Jack,
The issue of corners comes up every now and then.
Part of the issue is that as frequency goes up,
the turn of the trace is part of a turn for a loop.
i.e. inductor.
The other part I have always used I will try to
address in the following.
A side trace-end-on point of view, current
distribution in the ground plane will look
like the following...
______
Trace | |
|______|
........
. .
. .
. .
. .
Ground Plane .____________________________.
Strongest underneath, dropping off exponentially
as you move away from the trace to some other point
in the ground plane. Put two traces very close to
one another and their respective current distributions
in the ground plane will conflict with one another.
Now, let's add a right angle turn to your trace
and you get more of a mess. The current distribution
on the outside bend drops off much more rapidly than
the above. Why? It's spread across a 270 degree angle.
The current on the inside of the bend does not drop
off so rapidly. Why? It's compressed into a 90 degree
angle. This spreading and compression causes an
"impedence bump". The current is not evenly distributed
as before. The only way this can be explained is if there
is a discontinuity. In this case, it's a discontinuity
in the current that "appears" as a discontinuity in
impedence. Put another trace close to your first and
it just adds to the mess.
90 degree corners modified to two 45 degree
corners or a corner with a radius helps.
And keep your traces fairly spaced from one another.
Hope this helped...
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