Complete Dual-Band
Quadrature Transmitters
______________________________________________________________________________________ 23
High-frequency bypass capacitors are required close
to the pins with a dedicated via to ground. The 48-pin
QFN-EP package provides minimal inductance ground
by using an exposed paddle under the part. Provide at
least five low-inductance vias under the paddle to
ground, to minimize ground inductance. Use a solid
ground plane wherever possible. Any cutout in the
ground plane may act as slot radiator and reduce its
shield effectiveness.
Keep the RF LO traces as short as possible to reduce
LO radiation and susceptibility to interference.
IF Tank Design
The low-band tank (TANKL+, TANKL-) and high-band
tank (TANKH+, TANKH-) are fully differential. The exter-
nal tank components are shown in Figure 6. The fre-
quency of oscillation is determined by the following
equation:
C
INT
= Internal capacitance of TANK port
C
D
= Capacitance of varactor
C
VAR
= Equivalent variable tuning capacitance
C
PAR
= Parasitic capacitance due to PC board pads
and traces
C
CENT
= External capacitor for centering oscillation fre-
quency
C
C
= External coupling capacitor to the varactor
Table 10 shows possible component values for various
oscillation frequencies.
Internal to the IC, the charge pump will have a leakage
of less than 10nA. This is equivalent to a 300M& shunt
resistor. The charge-pump output must see an
extremely high DC resistance of greater than 300M&.
This minimizes charge-pump spurs at the comparison
frequency. Make sure there is no solder flux under the
varactor or loop filter.
Layout Issues
The MAX2361/MAX2363/MAX2365 EV kit can be used
as a starting point for layout. For best performance,
take into consideration power-supply issues as well as
the RF, LO, and IF layout.
Power-Supply Layout
To minimize coupling between different sections of the
IC, the ideal power-supply layout is a star configuration,
which has a large decoupling capacitor at a central
V
CC
node. The V
CC
traces branch out from this node,
each going to a separate V
CC
node in the MAX2361/
MAX2363/MAX2365 circuit. At the end of each trace is
a bypass capacitor with impedance to ground less than
1& at the frequency of interest. This arrangement pro-
2 (C
C
C
C ) L
C
C
C
2 C + C
OSC
INT
CENT
VAR
PAR
VAR
D
C
D
=
+
+
+
=
?/DIV>
1
?/DIV>
t
CS
t
CH
t
CWL
t
CWH
DI
CLK
CS
t
ES
B19 (MSB)
B18
B0
A3
A1
A0 (LSB)
t
CS
> 50ns
t
CH
> 10ns
t
CWH
> 50ns
t
ES
> 50ns
t
CWL
> 50ns
t
EW
> 50ns
t
EW
Figure 5. 3-Wire Interface Diagram
L
C
D
C
CENT
C
PAR
C
C
C
C
C
D
MAX2361
MAX2363
MAX2365
C
INT
-R
n
Figure 6. Tank Port Oscillator
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