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MAX2441EAI

900MHz Image-Reject Receivers

Maxim Integrated 

900MHz Image-Reject Receivers

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:

fr

=

2π

1

(L3)(CEFF )

C EFF

=

1

+





1

C2

+

1

C3

+

2

CD1 

C26

where CD1 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 (LT). 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.

MAX2440

MAX2441

MAX2442

TANK

LT

LT

TANK

VCC

L4

100nH

R7

C2

L3

C26

C3

R5

1kΩ

1/2 D1

R8

47kΩ

1/2 D1

VCO_CTRL

C1

47pF

R6

R4

1kΩ

D1 = ALPHA SMV1299-004

SEE FIGURE 1 FOR R6, R7, C2, C3, C26, AND L3 COMPONENT VALUES.

Figure 2. Oscillator Tank Schematic, Using the On-Chip VCO

10 ______________________________________________________________________________________

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