will the person who wrote this response please contact me? I need info 
     on RF also.(P.S. Jack, sounds like the new job is a challenge.)
     
     Pete Waddell 
     Printed Circuit Design Mag
     voice: 770- 952-1303
     fax:   770-952-6461
     e: [log in to unmask]


______________________________ Reply Separator _________________________________
Subject: Re: RF Design - Plating and Routing
Author:  [log in to unmask] at Internet
Date:    6/24/97 5:45 PM


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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