> Point 6. Has anybody measured a difference in radiation using 90 degree
> angles? Or is this based on heresay (an electronics "Urban Myth")?


>> 6. Eliminate ALL 90 degree angles on traces, and planes.  All 90 degree 
>>    angles radiate, it is important to run your clean up utilities if you
>>    haven't already.  Also consider the planes as capable of radiation
>>    especially at higher frequecies, so 45 all the corners.


I am not a Phd, but have taken some classes and hope my response is adequate.  


The radiation at a 90 degree bend is based on theory.  I know of two sources
which discusses the importance of chamfering corners.  How much the right 
angle bends radiate is a good question, and I will try to uncover some 
tests or discuss with people who may have some greater information on how 
much does the 90 degree bend radiate.   


Please reference:   High-Speed Digital Design 
                    by Howard W. Johnson & Martin Graham
                     section 4.4.4 Right-Angle Bends

"At the right angle bend the effective transmission line width increases.
An increase in width contributes extra unwanted parasitic capacitance.  The
right-angle bend looks like a capacitive load attached to the transmission
line."

The wider line also effects impedance and this causes reflections. The 
reflection causes the signal to bounce back along the transmission line 
toward where it came from.  Once reflections are introduced the trace on the 
board will seem longer because the signal is bouncing around.  Designing
traces to be short will make a difference.  However we can not always make 
all traces short in length, sometimes it is physically impossible, which 
is why I fall back on good design practice.    
  
Some suggested reading  "Field and Waves in Communication Electronics"
by Simon Ramo, John R. Whinnery, and Theodore Van Duzer.  Specificly chapter
5 on transmission lines.

In general, I found having some education in communications, and transmission 
line theory a tremendous help when it come to high speed design. I would 
recommend PC designers look into these coarses.  To me understanding is better 
then just following some rules, and understanding can come from college 
coarses, or experience, or testing.  I think my point is, evan though 
theory tells us something and testing provide us with more information, it 
seems to me that if the design rules cause little additional effort on the 
part of the pc designers then why not just make it a standard practice.  If 
there is something we can do a little differently and it causes no more work, 
then why not just do it.  


Although we are discussing design practices for pc designers to use on 
circuit boards, my opinion is most of the issues related to EMI radiation 
has to do with the enclosure and how well the enclosure acts as a shield.  
Unfortunately my field is pcb design and not enclosure design.

 
joez
-----------------------------------------------------------------
 Joseph Zdybowicz                      FORE Systems
 CAD Engineer                          174 Thorn Hill Road
 Email:   [log in to unmask]                Warrendale, PA 15086-7586
 Direct:  412-772-6552                
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