Re: [SI-LIST] : Design Note on Right Angle Bends

Gerald Johnson (jjohnson@minn.teradyne.com)
Fri, 10 Jul 1998 13:47:57 -0500

Mike,

I believe we're once again seeing a potential misapplication
of rules of scale. That is, a rule like this might have
value and accuracy in certain applications, but is easily
applied to areas where it isn't true. For a long time
people working with digital signals with transition times
over 1 ns were told not to use right angle corners due to the
impedance bump. In reality that bump isn't easily measured with
20 to 30 ps transition time TDRs for common PCB layups and
trace widths. So, a true statement misapplied. Right angle
corners do cause impedance bumps, just not anywhere near the
scale of vias, parts packages, surface mount pad patterns for
termination resistors, etc.

In this case it may be perfectly true that for some circumstances,
the right angle PCB trace corners act as antennas and cause
radiation. However, if we need to eliminate right angle corners,
how about vias, part leads, routes on the silicon, and all the
other right angle corners. Does make one wonder doesn't it?

Like you, I've never seen any rigor applied to these statements.
Perhaps someone on the list can enlighten us both. Not to if
right angle corners *can* act as antennas, but as to the effects
significance for what range of geometries, frequencies, etc.

Jerry

> From Mike.Mayer@heurikon.com Fri Jul 10 13:32:02 1998
> Date: Fri, 10 Jul 1998 13:17:15 -0500 (CDT)
> From: Mike Mayer <Mike.Mayer@heurikon.com>
> To: si-list@silab.Eng.Sun.COM
> Subject: [SI-LIST] : Design Note on Right Angle Bends
>
> I am puzzled by a design note on the Ultracad web site:
>
> http://www.ultracad.com/tech.htm
>
> It states that:
>
> THE REASON YOU DON'T USE RIGHT ANGLE TURNS ON CIRCUIT BOARDS IS
> THAT RIGHT ANGLE CORNERS BEGIN TO LOOK LIKE ANTENNAS.
>
> There are no references given or any further explanation. Can anyone
> explain what they mean?
>
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