From: Chris Rokusek ([email protected])
Date: Tue May 09 2000 - 08:44:02 PDT
My interest in the 20H rule is to understand the mechanisms involved. I
would like to quantify how well the 20H rule is able to reduce surface
currents. I would like to quantify the magnitude of these surface currents
with and without the 20H rule and compare. I would like to know what
magnitude source is required to generate what magnitude surface currents. I
would like understand the 20H rule's effects with and without other
conductors on the board besides the planes. I would like to know whether
the coupling to surface parts is capacitive or inductive or both. I would
like to compare different stackups...
Answers to these questions may allow better design techniques. Better
decoupling can limit the source across the planes to safe level. Knowing
the distribution of surface current can aid placement/routing. Knowing the
coupling mechanisms can aid in placement/routiing. How can a board designer
make tradeoffs among all the various design techniques (Timing/EMI/SI/etc.)
without knowing the limits of each?
Honestly I don't expect the answers to these tough questions to be posted on
this list (great surprise if they would), but perhaps these SI-List threads
will encourage a thesis or two and we can all catch the results at the next
Knowledge is ultimate. Keep it coming!
> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]]On Behalf Of [email protected]
> Sent: Monday, May 08, 2000 4:52 PM
> To: [email protected]
> Subject: Re: [SI-LIST] : May 9th Presentation: "Radiation from Edge
> Effects in Printe...
> Chris R.:
> One exemplary case I recently came across had a series-terminated
> clock line
> that was immediately submerged to a stripline which went to a
> connector on
> the edge of a PCB. The optional card that would use the
> connector was NOT
> installed. The connector receptacle pin rose above the PCB about
> 3/16 inch.
> This small 3/16-inch stub coupled harmonic energy to the SURFACE
> of the PCB
> (a ground plane) at the SURFACE resonant frequency of the PCB. Surface
> currents could be measured all over the surface ground plane
> (with a small
> Fischer Communications probe). NASTY radiation resulted.
> Simple removal of the series termination resistor at the clock driver end
> reduced system radiation over 30 dB.
> The 20H rule had nothing to do with suppressing this radiation.
> The moral?
> Avoid inducing surface currents any way you can.
> Michael L. Conn
> Owner/Principal Consultant
> Mikon Consulting
> *** Serving Your Needs with Technical Excellence ***
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