Evaluates: MAX2021
nulling. In addition, the LO signal reflected at the I/Q IF
port produces a residual DC term that can disturb the
nulling condition.
As shown in Figure 1, providing an RC termination on
each of the I+, I-, Q+, Q- ports reduces the amount of
LO leakage present at the RF port under varying temp-
erature, LO frequency, and baseband drive conditions.
Note that the resistor value is chosen to be 100Ω with a
corner frequency 1 / (2πRC) selected to adequately filter
the f
LO
and 2f
LO
leakage, yet not affecting the flatness
of the baseband response at the highest baseband
frequency. The common-mode f
LO
and 2f
LO
signals at
I+/I- and Q+/Q- effectively see the RC networks and
thus become terminated in 50Ω (R/2). The RC network
provides a path for absorbing the 2f
LO
and f
LO
leakage,
while the inductor provides high impedance at f
LO
and
2f
LO
to help the diplexing process.
The MAX2021 EV kit includes flexibility for a diplexer
network to be installed if desired. See Figure 3 for
details on the EV kit schematic.
Layout Considerations
The MAX2021 evaluation board can be a guide for your
board layout. Pay close attention to thermal design and
close placement of components to the IC. The
MAX2021 package’s exposed paddle (EP) conducts
heat from the device and provides a low-impedance
electrical connection to the ground plane. The EP must
be attached to the PCB ground plane with a low ther-
mal and electrical impedance contact. Ideally, this is
achieved by soldering the backside of the package
directly to a top metal ground plane on the PCB.
Alternatively, the EP can be connected to an internal or
bottom-side ground plane using an array of plated vias
directly below the EP. The MAX2021 EV kit uses nine
evenly spaced 0.016in-diameter, plated through holes
to connect the EP to the lower ground planes.
Depending on the ground plane spacing, large sur-
face-mount pads in the IF path may need to have the
ground plane relieved under them to reduce parasitic
shunt capacitance.
MAX2021 Evaluation Kit
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