Rainbow Electronics MAX19995А - Manual (Page 20)

MAX19995A
Applications Information
Input and Output Matching
The RF and LO inputs are internally matched to 50Ω.
No matching components are required. The RF port
input return loss is typically better than 16.5dB over the
RF frequency range of 1700MHz to 2200MHz and
return loss at the LO ports is typically better than 15dB
over the entire LO range. RF and LO inputs require only
DC-blocking capacitors for interfacing.
The IF output impedance is 200Ω (differential). For
evaluation, an external low-loss 4:1 (impedance ratio)
balun transforms this impedance to a 50Ω single-ended
output (see the
Typical Application Circuit ).
Reduced-Power Mode
Each channel of the MAX19995A has two pins
(LO_ADJ_, IF_SET) that allow external resistors to set
the internal bias currents. Nominal values for these
resistors are given in Table 1. Larger value resistors
can be used to reduce power dissipation at the
expense of some performance loss. If ±1% resistors
are not readily available, substitute with ±5% resistors.
Significant reductions in power consumption can also
be realized by operating the mixer with an optional sup-
ply voltage of 3.3V. Doing so reduces the overall power
consumption by up to 54%. See the
3.3V Supply AC
Electrical Characteristics table and the relevant 3.3V
curves in the Typical Operating Characteristics section.
IND_EXT_ Inductors
For applications requiring optimum RF-to-IF and LO-to-
IF isolation, connect low-ESR inductors from IND_EXT_
(pins 15 and 31) to ground. When improved isolation is
not required, connect IND_EXT_ to ground using 0Ω
resistance. See the
Typical Operating Characteristics to
evaluate the isolation vs. inductor value tradeoff.
Layout Considerations
A properly designed PCB is an essential part of any
RF/microwave circuit. Keep RF signal lines as short as
possible to reduce losses, radiation, and inductance.
The load impedance presented to the mixer must be so
that any capacitance from both IF- and IF+ to ground
does not exceed several picofarads. For the best per-
formance, route the ground pin traces directly to the
exposed pad under the package. The PCB exposed
pad MUST be connected to the ground plane of the
PCB. It is suggested that multiple vias be used to con-
nect this pad to the lower-level ground planes. This
method provides a good RF/thermal-conduction path
for the device. Solder the exposed pad on the bottom
of the device package to the PCB. The MAX19995A
evaluation kit can be used as a reference for board lay-
out. Gerber files are available upon request at
www.maxim-ic.com .
Power-Supply Bypassing
Proper voltage-supply bypassing is essential for high-
frequency circuit stability. Bypass each V
CC pin and
TAPMAIN/TAPDIV with the capacitors shown in the
Typical Application Circuit (see Table 1 for component
values). Place the TAPMAIN/TAPDIV bypass capacitors
to ground within 100 mils of the pin.
Exposed Pad RF/Thermal Considerations
The exposed pad (EP) of the MAX19995A’s 36-pin thin
QFN-EP package provides a low thermal-resistance
path to the die. It is important that the PCB on which the
MAX19995A is mounted be designed to conduct heat
from the EP. In addition, provide the EP with a low-
inductance path to electrical ground. The EP MUST be
soldered to a ground plane on the PCB, either directly
or through an array of plated via holes.
Dual, SiGe, High-Linearity, 1700MHz to 2200MHz
Downconversion Mixer with LO Buffer/Switch
20 ______________________________________________________________________________________