Joel Kolstad wrote:
I've noticed that various data sheet application circuits for items such as RF
switches, amplifiers, etc. (e.g.,
http://www.hittite.com/product_info/.../hmc349lp4c.pd f )
use what I would call "microstrip traces surrounded by a ground pour 'guard'
to reduce coupling to adjacent traces." Someone else, however, has suggested
that the application circuits were really designed as co-planar waveguides
(with grounds). Anyone else want to venture an opinion?

Their Application Note 17 ("Design techniques enhance isolation in
switch assemblies") talks explicitly about grounded-coplanar-waveguide
techniques. The evaluation boards for the "349" switches certainly
appear to use the CPW techniques discussed in AppNote 17, including
closely-spaced plated thru-holes to tie top and bottom ground planes
together.

For a 62.5 mil board, a 50 ohm microstrip's width is around 115 mils using
FR-4 (k=4.7). Using TxLine 2003, for a CPW w/ground I have to reduce the gap
to ~50 mils before the signal trace width reduces ~10% to ~104 mils. I take
this to imply that there's not a lot of coupling between the signal trace and
the copper pours (instead of the ground plane) until the gap width is
comparable to the board thickness.

In general, my impression has been that the use of copper flooding is more to
provide isolation between adjacent traces than to change the form of the
transmission line, and the use of the picket fenced vias was to insure that a
large chunk of copper pour didn't suddently turn into a resonator. Does that
sound correct?

If by "picket fenced vias" you mean the projections outward from the
signal vias on the evaluation boards, I take those to be the plated
thru-holesdiscussed in AppNote 17.

Are you seeing something I'm not?

_Interesting_ gadgets. Awfully small, but interesting; may have to
ask for samples and eval boards.

--
Mike Andrews, W5EGO

Tired old sysadmin