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參數(shù)資料
| 型號(hào)： | MAX2820-MAX2821 |
| 廠商： | Maxim Integrated Products, Inc. |
| 英文描述： | 2.4GHz 802.11b Zero-IF Transceivers |
| 中文描述： | 基于2.4GHz 802.11b零IF收發(fā)器 |
| 文件頁數(shù)： | 20/23頁 |
| 文件大?。?/td> | 392K |
| 代理商： | MAX2820-MAX2821 |
M
2.4GHz 802.11b Zero-IF Transceivers
20
______________________________________________________________________________________
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
400mV
P-P
differential baseband signal.
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
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
. 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
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| MAX2821AETM+D | 功能描述:射頻接收器 RoHS:否 制造商:Skyworks Solutions, Inc. 類型:GPS Receiver 封裝 / 箱體:QFN-24 工作頻率:4.092 MHz 工作電源電壓:3.3 V 封裝:Reel |
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