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2.4GHz 802.11b Zero-IF Transceivers
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 receiver baseband outputs (RX_BBIP, RX_BBIN,
RX_BBQP, and RX_BBQN) are differential low-imped-
ance 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 com-
mon-mode voltage. The outputs are capable of driving
loads up to 5k ? || 5pF with the full bandwidth baseband
signals at a differential amplitude of 500mV P-P .
Proper board layout is essential to maintain good bal-
ance between I/Q traces. This provides good quadra-
ture phase accuracy.
Receiver Power Detector (MAX2820/MAX2821 Only)
The 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 comparator input of the base-
band IC. The threshold level can be programmed
through the MAX2820/MAX2821 control software.
Transmit Path
Transmitter Baseband Inputs
The transmitter baseband inputs (TX_BBIP, TX_BBIN,
TX_BBQP, and TX_BBQN) are high-impedance differ-
ential 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 base-
band 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 TX_BB inputs. The lines
should be shielded on an inner layer to prevent cou-
pling of RF to these TX I/Q inputs and possible enve-
lope demodulation of the RF signal.
Transmit Path Baseband Lowpass Filtering
The on-chip transmit lowpass filters provide the filtering
necessary to attenuate the unwanted higher-frequency
spurious signal content that arises from the DAC clock
feedthrough and sampling 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 family
accomplishes this through an on-chip calibration
sequence. During this sequence, the net TX baseband
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 differential
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 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 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 connected to VCC_DRVR. The
power-amplifier driver outputs require external imped-
ance matching and differential to single-ended conver-
sion. The balanced to single-ended conversion and
interface to 50 ? is achieved through the use of an off-
chip 4:1 balun transformer, such as one from Murata or
TOKO. In this case, the TX RF output must be imped-
ance-matched to a differential/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 out-
put match should be adjusted until the return loss at the
balun output is >10dB.
20
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