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MAX9985 Evaluation Kit
8) Set the DC supply to +5V and set a current limit to
approximately 500mA if possible. Connect supplies
to the EV kit through the ammeter. Turn on the sup-
ply. Readjust the supply to get +5V at the EV kit.
There will be a voltage drop across the ammeter
when the mixer is drawing current.
9) Select LO2 by grounding LOSEL.
10) Enable the LO and the RF sources.
Testing the Mixer
Adjust the center and span of the spectrum analyzer to
observe the IF output tone at 99MHz. The level should be
approximately -2dBm (6dB conversion gain, 3dB pad
loss). The spectrum analyzer’s absolute magnitude accu-
racy is typically not better than ±1dB. Use the power
meter to get an accurate output power measurement.
Disconnect the GND connection to LOSEL. It will be
pulled high by a pullup resistor on the board. This
selects LO1. Observe the new IF output at 100MHz.
Reconfigure the test setup using a combiner or hybrid
to sum the two RF inputs to do a 2-tone IP3 measure-
ment if desired. Terminate the unused LO input in 50 ? .
Detailed Description
The MAX9985 is a highly integrated dual-channel
downconversion mixer. RF and LO baluns are integrat-
ed on-chip, as well as an LO buffer, IF amplifier, and a
single-pole/double throw (SPDT) LO input select
switch. The MAX9985 EV kit circuit consists mostly of
supply decoupling capacitors and DC-blocking capaci-
tors, resulting in a simple design-in.
DC-Blocking Capacitors
The MAX9985 has internal baluns on the RFMAIN,
RFDIV, LO1, and LO2 inputs. These inputs have almost
0 ? resistance at DC, so DC-blocking capacitors C1,
C8, C14, and C16 are used to prevent any external
bias from being shunted directly to ground. Capacitors
C10, C11, C19, and C20 are used to keep DC current
from flowing into the transformer, as well as providing
the flexibility for matching.
LO Buffer and IF Amplifier Bias
Bias currents for the integrated LO buffers and the IF
output amplifiers are set with resistors. These values
were carefully chosen for best linearity and lowest sup-
ply current through testing at the factory. Changing
these values, or using lower tolerance resistors,
degrades performance. Refer to the MAX9985 IC data
sheet Typical Operating Characteristics section to see
how the performance varies with these resistor values.
Output Voltage Drop Resistors
Resistors R3 and R6 are not required on this EV kit and
are set to 0 ? .
TAP Network
The network at TAP, formed by capacitors C2, C3, C6,
and C7, helps terminate the 2nd-order intermodulation
products at the RF inputs.
IND_EXT
The 30nH low-ESR wire-wound inductors (L3 and L6)
improve LO-to-IF and RF-to-IF isolation. If isolation is
not critical, then the corresponding pins can be
grounded. Approximately 100mA flows through L3 and
L6, thus requiring a low-ESR inductor. To ensure stable
operation, the load impedance presented to the mixer
must be such that any capacitances on the IF- and IF+
ports to ground do not exceed several picofarads.
IF Outputs
The MAX9985 employs a differential IF output to offer
increased IIP2 system performance; the EV kit uses a
4:1 balun to transform the 200 ? differential output
impedance to a 50 ? single-ended output for conve-
nient bench evaluation. Inductive pullups provide DC
bias to the IF output amplifiers. Series capacitors C10,
C11, C19, and C20 work in conjunction with the induc-
tors and the 4:1 balun transformer (T1 and T2) to match
the IF outputs for 100MHz operation.
As the differential IF outputs are relatively high imped-
ance (200 ? ), they are more susceptible to component
parasitics. It is often good practice to minimize the
ground plane directly underneath large components to
reduce associated shunt-C parasitics.
LOSEL
The EV kit includes a 47k ? pullup resistor (R7) for easy
selection of the LO port. Providing a ground at TP3
selects LO2, and leaving TP3 open selects LO1. To
drive TP3 from an external source, follow the limits
called out in the MAX9985 IC data sheet. Do not apply
logic voltages to TP3 without the +5V supply applied.
Doing so can cause the on-chip ESD diodes to conduct
and could damage the part.
Layout Considerations
The MAX9985 EV kit board can be a guide for your
board layout. Pay close attention to thermal design and
close placement of parts to the IC. The package ’s
exposed paddle (EP) conducts heat from the part and
provides a low-impedance electrical connection. The
EP MUST be attached to the PCB ground plane with a
low thermal and electrical impedance contact. Ideally,
this can be achieved by soldering the backside package
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