Re: [SI-LIST] : What's your favorite Screwy SI Concept?

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From: Ron Miller (rmiller@Brocade.COM)
Date: Thu Jan 13 2000 - 18:15:29 PST


It has been a long time but I remember making measurements on microstrip
about 30 years ago with square corners and cutting the corners/mitreing them.

The effect I noticed was an increase in return loss. Recognizing return loss as
a reflective source, when many are added together the circulating currents
between reflections causes more radiation. Insertion loss(thru-loss) will have
hardly any effect. It is only when looking at reflections that you will see it. Accordingly
in the time domain a TDR trace will also show it.

Ron Miller

> I had run 3-D simulations using a certain length of trace, first running it
> straight, and then giving it several 90 degree bends. I did not observe
> any significant change in emissions up to a frequency of 2 GHz. In our
> board designs, I freely allow use of 90 degree bends.
>
> Regards, Ravinder
> PCB Development and Design Department
> IBM Corporation - Storage Systems Division
> Email: ajmani@us.ibm.com
> ***************************************************************************
> Always do right. This will gratify some people and astonish the rest.
> .... Mark Twain
>
> Doug McKean <dmckean@corp.auspex.com>@silab.eng.sun.com on 01/13/2000
> 03:33:57 PM
>
> Please respond to si-list@silab.eng.sun.com
>
> Sent by: owner-si-list@silab.eng.sun.com
>
> To: si-list@silab.eng.sun.com
> cc:
> Subject: Re: [SI-LIST] : What's your favourite Screwy SI Concept?
>
> Well, to continue, here's another one that has
> me scratching my head frankly.
>
> SI Concept: Avoid Right Angle Corners in Traces
>
> Underlying Concept: Right Angle Corners in traces
> decrease signal integrity/
> increase emi ...
>
> Why It's Used: There may be some truth to it.
>
> Now, as far as Maxwell goes, we should see an
> increase in radiation. But as far as I've read
> and experimented, I just did a little mockup in
> the chamber moments ago, the effect is minor.
> It may in fact signify nothing.
>
> For instance, I take a six inch piece of wire,
> stick it in one end of a barrel connector and
> stick the other end of the barrel connector
> to a coaxial cable which is connected to the
> tracking generator of a Spectrum Analyzer.
> This is my output.
>
> From 0 to 1.3 GHz I see a particular maximized
> profile on the SA at 3 meters. I then force a
> sharp right angle in the wire and maximize again.
> The profile at 3 meters has changed but slightly.
> but, this is far field.
>
> I disconnect the antenna, disable the antenna
> factors, connect directly to the SA a near
> field probe and measure along the wire. At the
> bend of the wire when compared to a straight
> geometry, there is approx (very hard to tell)
> a +3 dB increase. And there could be a host
> of errors here. But I would expect some sort
> of change.
>
> Now this might not say anything for emissions.
> As far as a tdr in concerned, I doubt it would
> show a significant change in impedance to worry.
> BUT, the change in near field amplitude makes me
> suspect a corresponding increase in crosstalk
> (perhaps minor) say in a bus architecture. A
> tight high speed bus architecture.
>
> The above experiment strictly an observation.
>
> Comments? - Doug McKean
>
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--
Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017
     ==========0000-(_)0000===========
Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA  95131
rmiller@brocade.com,  rbmiller@sjm.infi.net

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