This appears to be a blend of co-planar and microstrip. Co-planar has no
ground plane, that is, everything is in one plane thus the name "co-planar".
A "microstrip-like" line is flanked by two ground surfaces and there is no
ground plane under it.
Like this
---- - ----

Microstrip is a flat line over a ground plane. Like this
-
-----------

The vias along both sides of this "CPW" T-line appear to be used to
constrain the fields and

The Vias most certainly must enter into the characteristic impedance
equation since the spacing to the line is so close. Therefore the line must
be narrower than pure coplanar.

I was part of a program to design a family of his same type of switch on
GaAs, back in the early 90's and the co-planar line was used.

73, Steve, k9DCi




"Mike Andrews" wrote in message
...
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