From: Zabinski, Patrick J. (firstname.lastname@example.org)
Date: Thu Nov 09 2000 - 19:34:10 PST
Prior to starting the experiment, I thought there would be some
kind of relationship between capacitance and edge rate, but within
the limits of what I had available, my original thought was wrong.
To provide a little more background, the test board consisted of
--------- ---- Split plane
--- Upper Stripline
--- Lower Stripline
-------------- Solid Plane
The striplines ran orthogonally to the split. The traces were
10" long, and the upper and lower planes were tied together
with vias only near the end of the lines (thus, the two planes
were connected 5" from the split).
I left the soldermask off the split plane to allow easy addition
of stitching caps, etc.
We kept our measurements to TDR and TDT, with 35 psec, 250 psec,
500 psec, 750 psec, and 1000 psec (1 nsec) edge rates.
The reason for the upper and lower striplines was to see if it
made a difference. The theory going in was that the upper striplines
were coupled much tighter to the split plane, so they would see
more of an affect than the lower striplines. We quickly found
the lower striplines to behave just fine with the split. In
the majority of measurements, we could not even detect the split.
However, the upper striplines demonstrated a strong coupling to
the split plane, and we could easily see the discontinuities/coupling.
From here, we then tried to insert stitching caps along the split.
I epoxied (non-conductive) four different value caps (0805 package
size) to wooden sticks: 0.001, 0.01, 0.1, and 1.0 uF. I verified
the caps were not shorted and kept their appropriate capacitance
values, after which I played around a while. After much experimentation,
I could not detect any difference between the effects from
inserting the different capacitance values. This was quite unexpected.
I then took the blade of a small screwdriver (calling this an "ideal"
cap), and used it to short the planes at different locations. What
surprised me even more was that the screwdriver produced the
same effects as the caps. After I came to this conclusion, I again
double-checked my caps to make sure they were "caps" and not "shorts".
Anyway, within the limits of my experiments, I was not able to detect
a difference in performance using different cap values.
> Very interesting. Did you select your cap value appropriate
> to the edge
> rate of the signals, or was it a constant value? I have not tried the
> "mirror plane" underneath the gap before. Did you make the
> Co of the mirror
> plane consistent with the edge rate or not?
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