In/out Connection Design
High isolation between ports is a
characteristic of Agilent FBAR
duplexers, and values often
exceed 50 dB. To achieve these
isolation numbers, the user’s
motherboard must have 60 dB or
more of isolation between each
pair of the three input/output
lines. This is measured as shown
in Figure 17, using copper pins to
short out all three of the lines to
ground. Isolation is then mea-
sured between the Tx, Rx and
Ant lines, with 60 dB as a mini-
mum acceptable level.
In order to achieve 60 dB of
isolation in the motherboard
itself using microstrip transmis-
sion lines, extreme care must be
taken in the design due to the
poorly contained field lines in
microstrip. Ground areas be-
tween the lines and other good
techniques will have to be used.
The use of coplanar waveguide
over a groundplane (CPWG)
provides for higher line-to-line
isolation.
ε
r
CPWG (Coplanar waveguide)
Such transmission lines have
excellent shielding, and line-to-
line values in excess of 80 dB can
easily be achieved using this
transmission line. Via holes are
used to bring the signal down to
the stripline conductor from the
in/out pads on the surface.
In order to obtain the maximum
isolation between lines in the
buried stripline layer, fill in the
area between lines with large
patches of copper, connected to the
upper (and other) ground planes.
Solder Mask and Solder Stencil
Solder mask is used on all
motherboards, to prevent solder
from adhering to places where it
is not desired. In mounting the
duplexer, it serves as an aid to
alignment during reflow soldering.
The motherboard solder mask
(shown in green in Figures 15, 16
and 17) is 4.6 mm x 4.6 mm,
slightly smaller than the duplexer
and corresponding to the metal
pattern on the underside. This
will contain the solder during
reflow, and prevent the duplexer
from rotating or slipping out of
alignment while it is floating on
molten solder.
A solder stencil is used to print a
pattern of solder paste onto the
motherboard, with the duplexer
placed upon this paste before the
reflow process begins. The design
of the solder stencil is critical to
obtaining good yields in reflow
soldering.
The solder stencil pattern
recommended by Agilent Tech-
nologies is shown in Figure 18,
along with the recommended
opening in the solder mask on
the top surface.
Design guidelines for CPWG
transmission lines can be found
in AppCAD, the design/analysis
software found on the Web at
http://www.agilent.com/view/
appcad
Better performance can be ob-
tained using symmetrical stripline,
where a buried conductor has a
ground plane above and below.
ε
r
Stripline
Shorting pin, soldered to in/out pad and to ground on
both sides, three places.
Figure 17. Motherboard Isolation Test Method.
7
AGILENT [ AGILENT TECHNOLOGIES, LTD. ]
