Applications Information
Agilent’s ACMD-7401 duplexers
provide high RF performance in
a very small package. However, in
order to achieve all the perfor-
mance available from the
duplexer, care must be taken in
the design of the board onto
which it is mounted. The purpose
of this information is to provide
Agilent’s recommendations on
the design of that board (called
the motherboard in this note).
Areas where care in design must
be observed are
thermal ground,
RF ground, in/out connection
design,
and
solder mask/solder
stencil design.
These four design
areas, which are sometimes
interrelated, will be considered
one at a time below.
Thermal Ground
FBAR resonators have a negative
temperature coefficient of
frequency — as temperature goes
up, the frequency response of the
filter shifts down in frequency.
See Figure 12. Typical coeffi-
cients are 57 KHz/° C for the Tx
filter and 40 KHz/° C for the Rx
filter. In Figure 13, the same data
are presented with the scale
narrowed down to the upper end
of the Tx band. Note that all
these data are taken at low input
power levels (+10 dBm).
When input power is +29 dBm,
heating in the Tx filter due to RF
losses causes the filter membranes
to heat up beyond 85°C. This, in
turn, causes the filter response to
shift further left (down in fre-
quency), resulting in increased
insertion loss at the high end of
the Tx band (1910 MHz). Agilent
Technologies takes this into
account in the manufacture and
final test of the duplexer — all
specifications for insertion loss
(and other parameters) will be met
at the specified input power level
and motherboard temperature.
Note that high power/high tem-
perature testing done at Agilent is
performed with the duplexer
soldered down to a test board
having a very good heat sink.
The motherboard must be de-
signed to remove heat from the
duplexer with the lowest possible
thermal resistance. Mount the
duplexer on a large surface of
1
/ ounce copper ground plane
2
(as shown in Figure 14), to enable
the heat to be removed in all
directions. Via holes, necessary
for RF grounding, should be filled
with copper plating to further
remove heat from the duplexer’s
Tx filter and dump it into a
second ground plane located in a
lower layer of the motherboard.
FBAR duplexers have extremely
low thermal mass and must be
properly heat sunk, as well as
isolated from external sources of
heat (such as a nearby power
amplifier). Failure to provide an
adequate thermal design to cool
0
-10
S31 (Ant-Rx), (dB)
-20
-30
-30°C
25°C
85°C
-40
-50
-60
1.82
1.86
1.90
1.94
1.98
2.02
FREQUENCY (GHz)
Figure 12. Tx Filter Response with Temperature.
0.0
-0.5
S31 (TX to Ant Loss), (dB)
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
-4.0
-4.5
-5.0
1.845
1.855
1.865
1.875
1.885
1.895
1.905
1.915
-30°C
25°C
85°C
FREQUENCY (GHz)
Figure 13. Tx Filter Response with Temperature (expanded).
5
AGILENT [ AGILENT TECHNOLOGIES, LTD. ]
