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MAX2422

900MHz Image-Reject Transceivers

Maxim Integrated 

900MHz Image-Reject Transceivers

The local oscillator is resistant to LO pulling caused by

changes in load impedance that occur as the part is

switched from standby mode, with just the oscillator run-

ning to either transmit or receive mode. The amount of

LO pulling is affected if there is power at the RXIN port in

transmit mode. The most common cause of this is imper-

fect isolation in an external transmit/receive (T/R) switch.

The AC Electrical Characteristics table contains specifi-

cations for this case as well.

Prescaler

The on-chip prescaler can be used in two different

modes: as a dual-modulus divide-by-64/65, or as oscil-

lator buffer amplifier. The DIV1 pin controls this func-

tion. When DIV1 is low, the prescaler is in dual-modulus

divide-by-64/65 mode; when it is high, the prescaler is

disabled and the oscillator buffer amplifier is enabled.

The buffer typically outputs -8dBm into a 50Ω load. To

minimize shutdown supply current, pull the DIV1 pin

low when in shutdown mode.

In divide-by-64/65 mode, the division ratio is controlled

by the MOD pin. When MOD is high, the prescaler is in

divide-by-64 mode; when it is low, it divides the LO fre-

quency by 65. The DIV1 pin must be at a logic low in

this mode.

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

MAX2420/MAX2421/MAX2422/MAX2460/MAX2463 sup-

ports four different power-management features to con-

serve 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 either transmitting or

receiving, to allow it to stabilize. For transmit-to-receive

switching, the receiver and transmitter sections have

their own enable control inputs, RXON and TXON. 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. The TXON input enables the IF

adjustable-gain amplifier, upconverter mixer, and PA

predriver. VCOON must be high for the transmitter to

operate. When TXON is low, the transmitter is off.

To disable all chip functions and reduce the supply

current to typically less than 0.5µA, pull VCOON, DIV1,

MOD, RXON, and TXON 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 capaci-

tors C2, C3, and both halves of the varactor diode

capacitance set the resonant frequency as follows:

( ) ( ) fr = 2π

1

L3

CEFF





CEFF

=

1

 C2

+

1

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 tol-

erance 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 is most critical with high tun-

ing bandwidth (low tank Q) and high temperature.

Capacitors C2 and C3 couple in the varactor. Light

MAX2420

MAX2421

MAX2422

MAX2460

MAX2463

LT

LT

VCC

L4

100nH

R7

C2

L3

R6

C26

C3

R5

1kΩ

1/2 D1

R8

47kΩ

1/2 D1

VCO_CTRL

C1

47pF

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

______________________________________________________________________________________ 11

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