Rainbow Electronics MAX19993 - Manual (Page 20)

MAX19993
Dual, SiGe, High-Linearity, 1200MHz to 1700MHz
Downconversion Mixer with LO Buffer/Switch
20
Applications Information
Input and Output Matching
The RF and LO inputs are internally matched to 50I.
No matching components are required. The RF port
input return loss is typically better than 19dB over the
1400MHz to 1700MHz RF frequency range and return
loss at the LO ports are 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 200I (differential). For
evaluation, an external low-loss 4:1 (impedance ratio)
balun transforms this impedance to a 50I single-ended
output. See the Typical Application Circuit.
Reduced-Power Mode
Each channel of the device has two pins (LO_ADJ_D/
LO_ADJ_M, IFD_SET/IFM_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 Q1% resistors are
not readily available, substitute with Q5% resistors.
Significant reductions in power consumption can also
be realized by operating the mixer with an optional 3.3V
supply voltage. Doing so reduces the overall power
consumption by approximately 46%. See the 3.3V Supply
AC Electrical Characteristics table and the relevant 3.3V
curves in the Typical Operating Characteristics section.
IND_EXT_ Inductors
The default application circuit calls for connecting
IND_EXT_ (pins 15 and 31) to ground through a 0I
resistor (0603) as close as possible to the pin. For
improved RF-to-IF and LO-to-IF isolation, contact the
factory for details.
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
such that any capacitance from both IF- and IF+ to
ground does not exceed several picofarads. For the best
performance, 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 connect 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 MAX19993 evaluation kit can be used
as a reference for board layout. 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 MAX19993’s 36-pin TQFN-
EP package provides a low thermal-resistance path to
the die. It is important that the PCB on which the device
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.