Rainbow Electronics MAX19995 - Manual (Page 26)

MAX19995
Dual, SiGe, High-Linearity, 1700MHz to 2200MHz
Downconversion Mixer with LO Buffer/Switch
26 ______________________________________________________________________________________
Differential IF
The MAX19995 has an IF frequency range of 50MHz to
500MHz, where the low-end/high-end frequency
depends on the frequency response of the external IF
components. Note that these differential ports are ideal
for providing enhanced IIP2 performance. Single-
ended IF applications require a 4:1 (impedance ratio)
balun to transform the 200
Ω differential IF impedance
to a 50
Ω single-ended system. After the balun, the
return loss is typically 12.5dB. The user can use a dif-
ferential IF amplifier on the mixer IF ports, but a DC
block is required on both IFD+/IFD- and IFM+/IFM-
ports to keep external DC from entering the IF ports of
the mixer.
Applications Information
Input and Output Matching
The RF and LO inputs are internally matched to 50
Ω.
No matching components are required. The RF port
input return loss is typically better than 16dB over the
RF frequency range of 1700MHz to 2200MHz and
return loss at the LO ports are typically better than
16dB over the entire LO range. RF and LO inputs
require only DC-blocking capacitors for interfacing.
The IF output impedance is 200
Ω (differential). For
evaluation, an external low-loss 4:1 (impedance ratio)
balun transforms this impedance to a 50
Ω single-ended
output (see the Typical Application Circuit ).
Reduced-Power Mode
Each channel of the MAX19995 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. See the
Typical
Operating Characteristics to evaluate the biasing vs.
performance tradeoff. If ±1% resistors are not readily
available, ±5% resistors may be substituted.
Significant reductions in power consumption can also be
realized by operating the mixer with an optional supply
voltage of +3.3V. Doing so reduces the overall power
consumption by up to 62%. See the
+3.3V Supply AC
Electrical Characteristics 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 a 0
Ω
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
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 MAX19995 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 MAX19995’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
MAX19995 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.