5G Pioneer Band FEM
(LNA, PA and Tx/Rx switch) and two
filters (low-pass filter after the PA for
harmonic rejection and bandpass filter
after the LNA).
The FEM is realised as a custom
laminate package suitable for SMT assembly.
A photograph of one of the assembled
FEMs, prior to lidding, is shown
in Figure 2. The three MMICs (PA, LNA
and switch) are assembled into pockets
in the laminate substrate material that
forms the package. The backside of the
ICs therefore sit on the metal base of
the package, providing a good thermal
contact to the PCB on which the FEM is
ultimately mounted, and a low inductance
interconnect to the PCB ground.
It also means that the surface of the die
is approximately level with the laminate
surface inside the package, thus
minimizing RF bond wire lengths and
associated parasitics.
An advantage of the custom laminate
package approach is the ability to integrate
filters within the package structure,
which helps to reduce both the size and
cost of the FEM component. Such filters
have been integrated into both the receive
path (bandpass filter after the LNA)
and the transmit path (low pass filter
after the PA).
The PA die can be seen in the top left
of Figure 2, followed by the integrated
low-pass filter. This was designed to
have a very low pass-band loss (around
0.2 dB) and to provide good harmonic
rejection (in excess of 20 dB). The PA
includes on-chip power detection that
can be used to monitor the RF power
transmitted by the FEM. The Tx/Rx
switch at the common port of the FEM,
on the right side, is a PIN diode MMIC,
which is a technology that is well suited
to the realisation of low-loss, high linearity
mm-wave switches 3.
The LNA die is in the bottom right
of the module, with the RF path moving
from top to bottom of the die in the
Y-direction. The band pass filter after the
LNA is clearly visible as a coupled line
structure. The coupled sections have
been designed as curves rather than
straight lines to allow for a more compact
implementation. The shaped tracks at
the input and output of the coupled line
structure are part of the filter, and are
critical in ensuring optimum performance.
The RF ports of the FEM package
were carefully designed for optimum RF
performance of the transition from the
PCB, on which the component will be
mounted, to the internal short 50Ω routing
lines within the package. The parasitics
of the PCB pads on the motherboard
Figure 4: Measured receive path small-signal performance compared to simulated.
Figure 5: Measured receive path NF of 3 units compared to simulated.
Figure 6: Measured transmit path small-signal performance compared to
simulated.
22 MW November - December 2017 www.mwee.com
