MAX2440/MAX2441/MAX2442
900MHz Image-Reject Receivers
10 ______________________________________________________________________________________
To disable the prescaler entirely, leave PREGND and
PREOUT floating. Also tie the MOD and DIV1 pins to
GND. Disabling the prescaler does not affect operation
of the VCO stage.
Power Management
MAX2440/MAX2441/MAX2442 supports three different
power-management features to conserve battery life.
The VCO section has its own control pin (VCOON),
which also serves as a master bias pin. When VCOON is
high, the LO, quadrature LO phase shifters, and
prescaler or LO buffer are all enabled. The VCO can be
powered up prior to receiving to allow it to stabilize. With
VCOON high, bringing RXON high enables the receive
path, which consists of the LNA, image-reject mixers,
and IF output buffer. When this pin is low, the receive
path is inactive.
To disable all chip functions and reduce the supply
current to typically less than 0.5µA, pull VCOON, DIV1,
MOD, and RXON low.
Applications Information
Oscillator Tank
The on-chip oscillator requires a parallel-resonant tank
circuit connected across TANK and TANK. Figure 2
shows an example of an oscillator tank circuit. Inductor
L4 provides DC bias to the tank ports. Inductor L3,
capacitor C26, and the series combination of capacitors
C2, C3, and both halves of the varactor diode capaci-
tance set the resonant frequency, as follows:
where C
D1
is the capacitance of one varactor diode.
Choose tank components according to your application
needs, such as phase-noise requirements, tuning
range, and VCO gain. High-Q inductors, such as air-
core micro springs, yield low phase noise. Use a low-
tolerance inductor (L3) for predictable oscillation
frequency. Resistors R6 and R7 can be chosen from 0
to 20Ω to reduce the Q of parasitic resonance due to
series package inductance (L
T
). Keep R6 and R7 as
small as possible to minimize phase noise, yet large
enough to ensure oscillator start-up in fundamental
mode. Oscillator start-up will be most critical with high
tuning bandwidth (low tank Q) and high temperature.
Capacitors C2 and C3 couple in the varactor. Light
coupling of the varactor is a way to reduce the effects
of high varactor tolerance and increase loaded Q. For a
wider tuning range; use larger values for C2 and C3 or
a varactor with a large capacitance ratio. Capacitor
C26 is used to trim the tank oscillator frequency. Larger
values for C26 will help negate the effect of stray PCB
capacitance and parasitic inductor capacitance (L3).
Choose a low tolerance capacitor for C26.
For applications that require a wide tuning range and
low phase noise, a series coupled resonant tank may
be required, as shown in Figure 4. This tank will use the
package inductance in series with inductors L1, L2,
and capacitance of varactor D1 to set the net equiva-
lent inductance which resonates in parallel with the
internal oscillator capacitance. Inductors L1 and L2
may be implemented as microstrip inductors, saving
component cost. Bias is provided to the tank port
through chokes L3 and L5. R1 and R3 should be cho-
sen large enough to de-Q the parasitic resonance due
to L3 and L5, but small enough to minimize the voltage
drop across them due to bias current. Values for R1
and R3 should be kept between 0Ω and 50Ω. Proper
high-frequency bypassing (C1) should be used for the
bias voltage to eliminate power-supply noise from
entering the tank.
SEE FIGURE 1 FOR R6, R7, C2, C3, C26, AND L3 COMPONENT VALUES.
