Re: [SI-LIST] : Fast edges with limited plane capacitance

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From: [email protected]
Date: Fri Mar 17 2000 - 10:25:52 PST


Listen to the engineer, he's right; however, he apparently did not mention
some other substantial reasons.

1. Any current carrying surface conductor will radiate in accord with its
antenna efficiency. This efficiency is related to the relative match to the
free space resistance of 120*PI = 377 ohms. For example, if you double the
Zo of a surface trace operating at a given frequency and a given driving
voltage, even though the trace current is reduced (for the fixed driving
voltage), the radiated emissions from that trace will increase by greater
than 6 dB.

2. The stacking of signal traces on the outside of their respective image
planes forces the field lines to strongly couple between the two signal
layers. Resulting in substantially increased crosstalk and reduced noise

3. The excellent RF capacitor formed by a closely spaced ground-power pair
will indeed decouple the higher harmonics of the fast edges you noted;
however (and in general), not all of the transient current demand of todays
microprocessors, ASICs, and the like (which I'm assuming are used in the
circuit) can be met by interplanar capacitance charge storage. Discrete,
surface-mount, bypass capacitors normally supply the bulk of the current out
to a few hundred megahertz. (Howard Johnson supplied a decent discussion of
this topic a short while back [I think] on this reflector.) A superior (but
more expensive) layup would convert your engineer's recommended stackup by
using power-ground sandwiches in place of EACH of the original power and
ground planes.

Per your question "How does a lack of planar capacitance contribute
to increasing EMI?" refer to item 1 above.

Per your question "Is there any validity to the s-G-s-s-P-s 'copper sandwich'
decreasing EMI?" again refer to item 1 above, and recognize that the power
and ground planes (when properly AC-coupled with the previously noted bypass
capacitors) will enclose the buried stripline traces in a "shielded box,"
thereby reducing radiation emissions. Remember to always keep the hottest
traces off the surface layers; i.e., bury them between the planes.

Michael L. Conn
Owner/Principal Consultant
Mikon Consulting

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