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MAX2426EAI

900MHz Image-Reject Receivers with Transmit Mixer

Maxim Integrated 

900MHz Image-Reject Receiver

with Transmit Mixer

VCC

R1

i

TXIN

MAX2424

MAX2426

1.5µA

R2

2M

TXIN

1.5µA

R3

RT = R1 + R2 + R3

VDIFF = VTXIN - VTXIN

Figure 3. Biasing TXIN and TXIN for FM

For example, if VCC = 3.3V and POUT = -8dBm, choose

RT = 100kΩ for sufficient current through the divider, so

that bias currents for TXIN and TXIN have little effect

over temperature. Set VTXIN = 2.3V to satisfy common-

mode voltage range requirements at VCC = 3.3V.

Use the Transmit Output Power vs. Input Voltage graph

in the Typical Operating Characteristics to determine

the input voltage (in mVp-p) required to produce the

desired output. Divide this value by 2√2 and use it for

VDIFF. A -8dBm transmitter output requires 250mVp-p /

2√2 = 88.4mV.

VTXIN = 2.3V + 0.0884V = 2.3884V

RT = R1 + R2 + R3

Solve for resistors R1, R2, and R3 with the following

equations:

R3 = V TXIN x RT

VCC

( ) R2 = VTXIN – V TXIN x RT

VCC

R1 = RT – R2 – R3

Since the transmit and receive sections typically require

different LO frequencies, it is not recommended to have

both transmit and receive active at the same time.

Phase Shifter

The MAX2424/MAX2426 uses passive networks to pro-

vide quadrature phase shifting for the receive IF and LO

signals. Because these networks are frequency selec-

tive, both the RF and IF frequency operating ranges are

limited. Image rejection degrades as the IF and RF

moves away from the designed optimum frequencies.

The MAX2424/MAX2426’s phase shifters are arranged

such that the LO frequency is higher than the RF carrier

frequency (high-side injection).

Local Oscillator (LO)

The on-chip LO is formed by an emitter-coupled differ-

ential pair. An external LC resonant tank sets the oscil-

lation frequency. A varactor diode is typically used to

create a voltage-controlled oscillator (VCO). See the

Applications Information section for an example VCO

tank circuit.

The LO may be overdriven in applications where an

external signal is available. The external LO signal

should be about 0dBm from 50Ω, and should be AC

coupled into either the TANK or TANK pin. Both TANK

and TANK require pull-up resistors to VCC. See the

Applications Information section for details.

The local oscillator resists pulling caused by changes

in load impedance that occur as the part is switched

from standby mode, with just the oscillator running to

either transmit or receive mode. The amount of LO

pulling is affected if a signal is present at the RXIN port

in transmit mode. The most common cause of pulling is

imperfect isolation in an external transmit/

receive (T/R) switch. The AC Electrical Characteristics

table contains specifications for this case as well.

Prescaler

The on-chip prescaler operates in two different modes:

as a dual-modulus divide-by-64/65, or as an oscillator

buffer amplifier. The DIV1 pin controls this function.

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 drive mode, the division ratio is con-

trolled by the MOD pin. Drive MOD high to operate the

prescaler in divide-by-64 mode. Drive MOD and DIV1

low to operate the prescaler in divide-by-65 mode.

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