2.4GHz 802.11b Zero-IF Transceivers

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Transmit Path

Transmitter Baseband Inputs

The MAX2820/MAX2821 transmitter baseband inputs

(TX_BBIP, TX_BBIN, TX_BBQP, and TX_BBQN) are

high-impedance differential analog inputs. The inputs

are designed to be directly connected (DC-coupled) to

the in-phase (I) and quadrature-phase (Q) DAC outputs

of the baseband IC. The inputs must be externally

biased to +1.2V common-mode voltage. Typically, the

DAC outputs are current outputs with external resistor

loads to ground. I and Q are nominally driven by a

Proper board layout is essential to maintain good bal-

ance between I/Q traces. This provides good quadra-

ture phase accuracy by maintaining equal parasitic

capacitance on the lines. In addition, it is important not

to expose the TX I/Q circuit board traces going from the

digital baseband IC to the MAX2820/MAX2821. The

lines should be shielded on an inner layer to prevent

coupling of RF to these TX I/Q inputs and possible

envelope demodulation of the RF signal.

Transmit Path Baseband Lowpass Filtering

The MAX2820/MAX2821 on-chip transmit lowpass fil-

ters provide the filtering necessary to attenuate the

unwanted higher-frequency spurious signal content

that arises from the DAC clock feedthrough and sam-

pling images. In addition, the filter provides additional

attenuation of the second sidelobe of signal spectrum.

The filter frequency response is set on-chip. No user

adjustment or programming is required. The Typical

Gain vs. Frequency profile is shown in the Typical

Operating Characteristics.

Transmitter DC Offset Calibration

In a zero-IF system, in order to achieve low LO leakage

at the RF output, the DC offset of the TX baseband sig-

nal path must be reduced to as near zero as possible.

Given that the amplifier stages, baseband filters, and

TX DAC possesses some finite DC offset that is too

large for the required LO leakage specification, it is

necessary to “null” the DC offset. The MAX2820/

MAX2821 accomplish this through an on-chip calibra-

tion sequence. During this sequence, the net TX base-

band signal path offsets are sampled and cancelled in

the baseband amplifiers. This calibration occurs in the

first ~2.2µs after TX_ON is taken high. During this time,

it is essential that the TX DAC output is in the 0V differ-

ential state. The calibration corrects for any DAC offset.

However, if the DAC is set to a value other than the 0V

state, then an offset is erroneously sampled by the

MAX2820/MAX2821 TX offset calibration. The TX DAC

output must be put into the 0V differential state at or

before the time TX_ON is taken high.

Power Amplifier Driver Output

The MAX2820/MAX2821 TX_RF outputs are high-

impedance RF differential outputs directly connected to

the driver amplifier. The outputs are essentially open-

collector outputs with an on-chip inductor choke con-

nected to VCC_DRVR. The power amplifier driver

outputs require external impedance matching and dif-

ferential to single-ended conversion. The balanced to

single-ended conversion and interface to 50

Ω is

achieved through the use of an off-chip 4:1 balun trans-

former, such as one from Murata or Toko. In this case,

the TX RF output must be impedance-matched to a dif-

ferential/balanced impedance of 200

Ω. The Typical

Application Circuit shows the balun, inductors, and

capacitors that constitute the matching network of the

power amplifier driver outputs. The output match

should be adjusted until the return loss at the balun out-

put is > 10dB.

Transmit Gain Control

The transmit gain-control input provides a direct analog

control over the transmit path gain. The transmit gain of

the MAX2820/MAX2821 is controlled by an external

voltage at pin TX_GC. The typical gain-control charac-

teristic is provided in the Typical Operating

Characteristics graph Transmitter Gain Control vs.

Gain-Control Voltage. The input is a high-impedance

analog input designed to directly connect to to the DAC

output of the baseband IC. Some local noise filtering

through a simple RC network at the input is permissi-

ble. However, the time constant of this network should

be kept sufficiently low so the desired response time of

the TX gain-control function is not limited.

During the TX turn-on sequence, internally the gain is

set at the minimum while the TX baseband offset cali-

bration is taking place. The RF output is effectively

“blanked” for the first 2.2µs after TX_ON is taken high.

After 2.2µs, the “blanking” is released, and the gain-

control amplifier ramps to the gain set by the external

voltage applied to the TX_GC input.

PA Bias DAC Output

The MAX2820/MAX2821 provide a programmable ana-

log current source output for use in biasing the RF

power amplifier, such as the MAX2242. The output is

essentially an open-drain output of a current source

DAC. The output is designed to directly connect to the

bias current pin on the power amplifier. The value of the

current is determined by the 4 bits programmed into the

internal register on the MAX2820/MAX2821. This pro-

grammability permits optimizing of the power amplifier

idle current based on the output power level of the PA.

Care must be taken in the layout of this line. Avoid run-

ning the line in parallel with the RF line. RF might couple