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

Todd Westerhoff (toddw@cadence.com)
Sat, 11 Jul 1998 17:34:40 -0400

Impedance bumps? Antennas? At 100 MHz?

While I agree that the corner of the trace changes the cross sectional
width,

we're talking about a section of the trace that's how long? 4 mils?

Remember that transmission line effects become evident only when the
round trip

delay time of the net is long compared to the signal's rise time. So
what's the

round trip delay across the bend in the trace compared to the rise time?
Pretty

small, or so it seems to me.

As for antenna effects, remember that the signal's wavelength determines
how long

the antenna needs to be to begin radiating. For a sine wave, lamba is
about 3 feet

at 300 MHz. If we assume that most of the energy in a 100 MHz square
wave is contained

in the first few harmonics, we might consider lamda/4 for a 300 MHz sine
wave as a

reasonable length for a good antenna. That's 9 inches.

I'm the first to agree that I'm talking about a <bold>good</bold>
antenna, that radiates a <bold>lot</bold>,

and that we don't want boards to radiate <bold>at all</bold>. Still,
most microstrip traces sit

mere mils above a reference plane, and serpentines are rarely more than
half an

inch in length.

I suppose every little bit helps, but this one seems a stretch. Does
anyone know

of any empirical data available?

Todd.

At 01:17 PM 7/10/98 -0500, Mike Mayer wrote:

>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?

>

>--

>=============================================================================

>Mike Mayer Artesyn Communication Products,
Inc

> Madison, WI

> http://www.artesyn.com/cp

>=============================================================================

>

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