From: Mike LaBonte (firstname.lastname@example.org)
Date: Thu Nov 09 2000 - 06:15:37 PST
Would you agree that your nearby solid plane is basically a large,
distributed capacitor, performing essentially the same function
as Abe's stitched capacitors? Admittedly a return current at split
plane A must pass through the dielectric to the solid plane, across
a little bit of solid plane, and then back through the dielectric
to split plane B. But it does form a capacitive coupling that is
available everywhere; therefore no need to calculate capacitor
placement. I have not considered capacitance magnitude differences,
A separate discussion could be held on what happens to the currents
induced in the solid plane.
"Zabinski, Patrick J." wrote:
> In the general sense, I agree with your comments. However, I
> have found exceptions.
> As an example, if your board cross section looks something like:
> --- Microstrip
> -------------- Split Plane
> then the statements you make below are true.
> However, through empirical experimentation, we have found that
> many of the issues you discuss (crosstalk, discontinuity, etc.)
> can be drastically reduced. By inserting a solid plane under the
> split plane, like:
> --- Microstrip
> -------------- Split Plane
> "Thin" dielectric
> -------------- Solid Plane
> we have noticed dramatic improvements in performance. We have not
> taken our studies to the point of being able to engineer the
> effects of the solid plane, but by inserting a solid plane under the
> splits, much of what you discuss is no longer an issue (we did not
> test for EMI, but I suspect it improves as well).
> You also mention the stitching capacitors. We have found them to
> be useful. However, what I did not predict is the frequency
> (spacing) in which you must place them. Using one test board,
> we placed an ideal stitching capacitor (shorting bar) across the split,
> and slid the capacitor along the split. We then injected signals
> of various edge rates ranging from 35 psec to 1 nsec. Prior
> to making the measurements, I predicted that there would be
> a relationship between the edge rate and how far the cap could be
> away from the trace. What I found was that the regardless of
> edge rate, the stitching cap needed to be within 2 mm of the
> trace! This was quite unexpected.
> Most members of this list should be well aware of the potential
> issues associated with splits (it's been discussed SEVERAL
> times), but it does seem like we haven't quite explored
> the entire issue quite yet.
> > Dear Scholars:
> > It is well known that when a high-speed signal crosses a slot
> > of an adjacent reference Ground or Power plane, several
> > undesirable effects can occur. For instance, a disturbance of
> > return current path takes place which can cause a glitch,
> > increased crosstalk and EMI radiation. The rule that routing
> > of high-speed nets over voids or cuts of neighboring plane
> > layers must be avoided is firmly established in the SI
> > literature. Yet, the complexity of modern high speed designs
> > imposes many violations of above guideline.
> > To make matters more complicated, many simulation programs
> > assume continuous Ground and Power planes and do not
> > accurately take into account effects of plane discontinuities
> > on the return current path. It is therefore, important to
> > visually inspect a PCB database for the signals crossing
> > plane slots (and voids), before generating the Gerber files
> > and releasing the design for fabrication.
> > Figure 1 illustrates several concepts associated with such
> > examination. A section of a power plane is shown having gap G
> > (due to presence of multiple powers) and several traces (T1,
> > T2, T3 and T4) of an adjacent signal layer which are routed
> > over the splits. C1 and C2 represent two stiching
> > capacitors. In this example the gap width is 20 mils.
> > Smaller widths (such as 10 mils) can be preferable since the
> > break should be as narrow as feasible. Majority of crossings
> > occurs at 90 degree angle with respect to axes of slot in
> > order to minimize the segment length over the void. Some of
> > the traces contain serpentines but are routed to pass
> > boundaries only once. Stiching capacitors are utilized to
> > minimize undesirable effects of the cuts.
> > Certain rules of thumb have been formulated for determination
> > of the required number and values of stiching capacitors; an
> > example follows:
> > For every five traces which cross a plane slot, insert
> > approximately one or more capacitors within each 0.250 in.
> > 0.01uF is an acceptable value for "stiching" capacitors,
> > though it is preferable to mix several
> > 2251 Lawson Lane
> > Santa Clara, CA 95054
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