[SI-LIST] : Re: [SI-LIST] - Design Note

david (david@innosys.com)
10 Jul 1998 12:34:04 -0800

Reply to: RE>>[SI-LIST] : Design Note on Right Angle Bends

There's a brief reference to right angle bends in Howard Johnson's "High Speed
Digital Design," p. 174. Basically says that the effect is insignificant
until you get below 100 ps rise times using very wide lines.

David Miller
InnoSys

>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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