2.4GHz 802.11b Zero-IF Transceivers

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19

Receive Gain Control and DC Offset Nulling

The MAX2820/MAX2821 receive path gain is varied

through an external voltage applied to the pin RX_AGC.

ance analog input designed for direct connection to the

RX_AGC DAC output of the baseband IC. The gain-

control range, which is continuously variable, is typical-

ly 70dB. The gain-control characteristic is shown in the

Typical Operating Characteristics section graph

Receiver Voltage Gain vs. Gain-Control Voltage.

Some local noise filtering through a simple RC network

at the input is permissible. However, the time constant

of this network should be kept sufficiently low in order

not to limit the desired response time of the RX gain-

control function.

Receiver Baseband Amplifier Outputs

The MAX2820/MAX2821 receiver baseband outputs

(RX_BBIP, RX_BBIN, RX_BBQP, and RX_BBQN) are

differential low-impedance buffer outputs. The outputs

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

the in-phase (I) and quadrature-phase (Q) ADC inputs

of the baseband IC. The RX I/Q outputs are internally

biased to +1.2V common-mode voltage. The outputs

are capable of driving loads up to 5k

Ω || 5pF with the

full bandwidth baseband signals at a differential ampli-

Proper board layout is essential to maintain good bal-

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

ture phase accuracy.

Receiver Power Detector

The MAX2820/MAX2821 receiver level detector is a

digital output from an internal threshold detector that is

used to determine when to change the LNA gain state.

In most applications, it is connected directly to a com-

parator input of the baseband IC. The threshold level

can be programmed through the MAX2820/MAX2821

control software.

ADDRESS

DATA BIT

CONTENT

DEFAULT

DESCRIPTION

D11:D9

2C(2:0)

111

VGA DC Offset Nulling Parameter 2

D8:D6

1C(2:0)

111

VGA DC Offset Nulling Parameter 1

D5:D4

DL(1:0)

01

RX Level Detector Midpoint Select

D3

SF(0)

0

Special Function Select (not presently used)

• 0 = OFF

• 1 = ON

0 1 0 0

D2:D0

BW(2:0)

010

Receive Filter -3dB Frequency Select (frequencies are

approximate)

• 000 = 8.5MHz

• 001 = 8.0MHz

• 010 = 7.5MHz

• 011 = 7.0MHz

• 100 = 6.5MHz

• 101 = 6.0MHz

Table 7. Receive Settings Register (RECEIVE)

ADDRESS

DATA BIT

CONTENT

DEFAULT

DESCRIPTION

D11:D4

X

X

Reserved

0 1 0 1

D3:D0

PA(3:0)

0000

PA Bias Select:

• 1111 = Highest PA bias

• …………

• 0000 = Lowest PA bias

Table 8. Transmit Settings Register (TRANSMIT)