high-frequency noise from coupling back into the supply.
Capacitors C3 and C8 are larger 0.01µF ceramics used
for filtering lower frequency noise on the supply.
DC-Blocking Capacitors
The MAX9993 has internal baluns on the RF, LO1 and
LO2 inputs. These inputs have almost 0
Ω
resistance at
DC, and so DC-blocking capacitors C1, C9, and C10
are used to prevent any external bias from being shunt-
ed directly to ground. Capacitors C12 and C13 are
used to keep DC current from flowing into the trans-
former, as well as providing the flexibility for matching.
LO BIAS and IF BIAS
Bias currents for the integrated IF output amplifier and
the LO buffer are set with resistors R1 (523
Ω
, ±1%) and
R2 (383
Ω
, ±1%), respectively. These values were care-
fully chosen for best linearity and lowest supply current
through testing at the factory. Changing these values,
or using lower tolerance resistors, degrades perfor-
mance.
Current-Limiting Resistors
Resistors R3 and R4 are used for current limiting at the
supply. Resistor R3 dissipates 80mW and R4 dissi-
pates 125mW.
TAP Network
The network at TAP formed by R5 and C5 helps to ter-
minate the second-order intermodulation products at
the RF input to balance the upper and lower side-band
input IP3 performance.
LEXT
The 10nH (±5%) wire-wound inductor, L3, improves
LO-to-IF and RF-to-IF isolation. If isolation is not critical,
then this pin can be grounded.
IF±
The MAX9993 employs a differential IF output to offer
increased IP2 system performance. The EV kit uses a
4:1 balun to transform the 200
Ω
differential output
impedance to a 50
Ω
single-ended output for easy
bench evaluation. Inductive pullups L1 and L2 provide
DC bias to the IF output amplifier, using C11 for supply
filtering and R4 for current limiting. Series capacitors
C12 and C13 work in conjunction with the inductors
and the 4:1 balun transformer (T1) to match the IF out-
put for 40MHz to 200MHz operation.
As the differential IF outputs are relatively high imped-
ance (200
Ω
), they are more susceptible to component
parasitics. It is often good practice to relieve the
ground plane directly underneath large components to
reduce associated shunt-C parasitics.
LO_SEL
The EV kit includes a 47k
Ω
pullup resistor for easy
selection of the LO port. Providing a ground at TP3
selects LO1, and leaving TP3 open selects LO2. To
drive TP3 from an external source, follow the limits
called out in the MAX9993 device data sheet. Logic
voltages should not be applied 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 MAX9993 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 MAX9993
package exposed paddle (EP) conducts heat from the
device and provides a low-impedance electrical connec-
tion to the ground plane. The EP must be attached to the
PC board ground plane with a low thermal and electrical
impedance contact. Ideally, this is achieved by soldering
the backside of the package directly to a top metal
ground plane on the PC board. Alternatively, the EP can
be connected to an internal or bottom-side ground plane
using an array of plated vias directly below the EP.
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.
Modifying the EV Kit
The RF and LO inputs are broadband matched, so
there is no need to modify the circuit for use anywhere
in the 1700MHz to 2200MHz RF range (1400MHz to
2000MHz LO range).
Retuning for a different IF is as simple as scaling the
values of the IF pullup inductors up or down with fre-
quency. The IF output looks like 200
Ω
differential in
parallel with a capacitor. The capacitance is due to the
combination of the IC, PC board, and external IF com-
ponents. The capacitance is approximately 4pF (per
output) to ground. This capacitance is resonated out at
the frequency of interest by the bias inductors L1 and
L2. To determine the inductor value use the following
equation:
The IF output is tuned for operation at approximately
100MHz, so a 470nH inductor is used. For lower IF fre-
quencies (i.e., larger component values), maintain the
component’s Q value at the cost of larger case size,
unless it is unavoidable.
f
L x C
IF
=
1
2
π
Evaluates: MAX9993
MAX9993 Evaluation Kit
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