MAX2424/MAX2426
900MHz Image-Reject Receiver
with Transmit Mixer
10 ______________________________________________________________________________________
_______________Detailed Description
The following sections describe each of the functional
blocks shown in the Functional Diagram.
Receiver
The MAX2424/MAX2426’s receive path consists of a
900MHz low-noise amplifier, an image-reject mixer, and
an IF buffer amplifier.
The LNA’s gain and biasing are adjustable via the LNA-
GAIN pin. Proper operation of this pin provides optimum
performance over a wide range of signal levels. The LNA
has four modes determined by the DC voltage applied on
the LNAGAIN pin. See Table 1, as well as the relevant
Typical Operating Characteristics plots.
At low LNAGAIN voltages, the LNA is shut off and the
input signal capacitively couples directly into mixer to
provide maximum linearity for large-signal operation
(receiver close to transmitter). As the LNAGAIN voltage
increases, the LNA turns on. Between 0.5V and 1V at
LNAGAIN, the LNA is partially biased and behaves like a
Class C amplifier. Avoid this operating mode for applica-
tions where linearity is a concern. As the LNAGAIN volt-
age reaches 1V, the LNA is fully biased into Class A
mode, and the gain is monotonically adjustable for LNA-
GAIN voltages above 1V. See the receiver gain, IP3, and
Noise Figure vs. LNAGAIN plots in the Typical Operating
Characteristics for more information.
The downconverter is implemented using an image-
reject mixer consisting of an input buffer with two out-
puts, each of which is fed to a double-balanced mixer.
A quadrature LO drives the local-oscillator (LO) port of
each mixer. An on-chip oscillator and an external tank
circuit generates the LO. Its signal is buffered and split
into two phase shifters, which provide 90° of phase shift
across their outputs. This pair of LO signals is fed to the
mixers. The mixers’ outputs then pass through a sec-
ond pair of phase shifters, which provide a 90° phase
shift across their outputs. The resulting mixer outputs are
then summed together. The final phase relationship is
such that the desired signal is reinforced and the image
signal is canceled. The downconverter mixer output
appears on the RXOUT pin, a single-ended 330Ω output.
Transmitter
The MAX2424/MAX2426 transmitter consists of a bal-
anced mixer and a PA driver amplifier. The mixer inputs
are accessible via the TXIN and TXIN pins. An equiva-
lent circuit for the TXIN and TXIN pins is shown in
Figure 2. Because TXIN and TXIN are linearly coupled
to the mixer stage, they can accept spectrally shaped
input signals. Typically, the mixer can be used to multi-
ply the LO with a baseband signal, generating BPSK or
ASK modulation. Transmit upconversion can also be
implemented by applying a modulated IF signal to
these inputs. For applications requiring image rejection
on the transmitter, refer to the MAX2420/MAX2421/
MAX2422/MAX2460/MAX2463 data sheet.
Set the common-mode voltage at TXIN, TXIN to 2.3V by
selecting appropriate values for R
A
and R
B
(Figure 1). The
total series impedance of R
A
and R
B
should be approxi-
mately 100kΩ.
Frequency modulation (FM) is realized by modulating
the VCO tuning voltage. Apply the appropriate differen-
tial and common-mode voltages to TXIN and TXIN to
control transmitter output power (Figure 3).
LNA partially biased. Avoid this
mode— the LNA operates in a
Class C manner
LNA capacitively bypassed,
minimum gain, maximum IP3
MODE
LNA at maximum gain
(remains monotonic)
LNA gain is monotonically
adjustable
1.5 < V
LNAGAIN
≤ V
CC
1.0 < V
LNAGAIN
≤ 1.5
0.5 < V
LNAGAIN
< 1.0
0 < V
LNAGAIN
≤ 0.5
LNAGAIN
VOLTAGE (V)
Table 1. LNA Modes
