[SI-LIST] Re: A moat @ 10MHz

Ken,

We saw similar results in a set of test boards from several
years ago.  The board was 2x3 inches of 0.062" FR4.  The
bottom was a solid ground plane across the entire surface.
The top had two planes, roughly 2x1.5" each with a complete
separation/split between them.

To bring in the signal, small vias were punched through to the bottom,
such that we could use micro-probes in a Ground-Signal (GS) arrangement,
with the "signal" being the top plane.  We had two such boards,
one with 60 mil gap (i.e., separation between two top
plates) and one with 120 mil gap.

At low frequencies, the two top (call them "power") planes were
very well isolated from one another.  This pretty much held
true until the top planes reached resonant frequency.  At
resonance, the two planes were effectively shorted between
each other, with only marginal S21 loss between them.

As the frequency increased above the first resonance, S21 varied
+/- 5 dB, but the split was quite uneffective in isolating
the two top planes.  Also interesting was that the 60 mil
and 120 mil gaps performed very similar to one another
above the first resonance.  Below resonance, the 120
mil gap did perform better (if I recall, something
like 5-10 dB better than the 60 mil gap).

We ran short of time/funding to work through a good explanation
for what we were seeing in the lab, but we have since noticed
similar behavior when dealing with splits/moats.  From this,
we have tried to minimize the size of the moat/plate such
that its resonant frequency is pushed up as high as we
can get.

Pat


> 
> Giovanni,
> This is indeed artistic.  But the 10GHz field is so totally 
> unaffected by the
> moat, that I find it hard to believe the results are 
> accurate.  What dielectric
> thickness did you use between planes?  (Was it much less than 
> 2mm?  0.1mm might
> be a good thickness for a reasonable power-ground plane pair.)
> -Ken Hayden
> 
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List FAQ wiki page is located at:
                http://si-list.org/wiki/wiki.pl

List technical documents are available at:
                http://www.si-list.org

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu

Other related posts: