Datasheet
MAX2021
High-Dynamic-Range, Direct Up-/Downconversion
650MHz to 1200MHz Quadrature Mod/Demod
14
Maxim Integrated
varying temperature, LO frequency, and baseband
drive conditions. See the
Typical Operating
Characteristics
for details. Note that the resistor value is
chosen to be 100Ω with a corner frequency 1 / (2πRC)
selected to adequately filter the f
LO
and 2f
LO
leakage,
yet not affecting the flatness of the baseband response
at the highest baseband frequency. The common-
mode f
LO
and 2f
LO
signals at I+/I- and Q+/Q- effective-
ly see the RC networks and thus become terminated in
50Ω (R/2). The RC network provides a path for absorb-
ing the 2f
LO
and f
LO
leakage, while the inductor pro-
vides high impedance at f
LO
and 2f
LO
to help the
diplexing process.
RF Demodulator
The MAX2021 can also be used as an RF demodulator
(see Figure 3), downconverting an RF input signal
directly to baseband. The single-ended RF input
accepts signals from 650MHz to 1200MHz with power
levels up to +30dBm. The passive mixer architecture
produces a conversion loss of typically 9.2dB. The
downconverter is optimized for high linearity and
excellent noise performance, typically with a
+35.2dBm IIP3, a P1dB of greater than +30dBm, and a
9.3dB noise figure.
A wide I/Q port bandwidth allows the port to be used as
an image-reject mixer for downconversion to a quadra-
ture IF frequency.
The RF and LO inputs are internally matched to 50Ω.
Thus, no matching components are required, and only
DC-blocking capacitors are needed for interfacing.
Demodulator Output Port Considerations
Much like in the modulator case, the four baseband
ports require some form of DC return to establish a com-
mon mode that the on-chip circuitry drives. This can be
achieved by directly DC-coupling to the baseband ports
(staying within the ±3.5V common-mode range),
through an inductor to ground, or through a low-value
resistor to ground. Figure 4 shows a typical network that
would be used to connect to each baseband port for
demodulator operation. This network provides a com-
mon-mode DC return, implements a high-frequency
diplexer to terminate unwanted RF terms, and also pro-
vides an impedance transformation to a possible higher
impedance baseband amplifier.
The network C
a
, R
a
, L
a
and C
b
form a highpass/low-
pass network to terminate the high frequencies into a
load while passing the desired lower IF frequencies.
Elements L
a
, C
b
, L
b
, C
c
, L
c
, and C
d
provide a possible
impedance transformer. Depending on the impedance
being transformed and the desired bandwidth, a fewer
number of elements could be used. It is suggested that
L
a
and C
b
always be used since they are part of the
high frequency diplexer. If power matching is not a
concern then this would reduce the elements to just the
diplexer.
Resistor R
b
provides a DC return to set the common
mode voltage. In this case, due to the on-chip circuitry,
the voltage would be approx 0V DC. It can also be
used to reduce the load impedance of the next stage.
Inductor L
d
can provide a bit of high frequency gain
peaking for wideband IF systems. Capacitor C
e
is a DC
block.
Typical values for C
a
, R
a
, L
a
, and C
b
would be 1.5pF,
50Ω, 11nH, and 4.7pF, respectively. These values can
change depending on the LO, RF, and IF frequencies
used. Resistor R
b
is in the 50Ω to 200Ω range
The circuitry presented in Figure 4 does not allow for LO
leakage at RF port nulling. Depending on the LO at RF
leakage requirement, a trim voltage might need to be
introduced on the baseband ports to null the LO leakage.
ADC
90
0
RF LO
MAX2021
DIPLEXER/
DC RETURN
MATCHING
ADC
DIPLEXER/
DC RETURN
MATCHING
Figure 3. MAX2021 Demodulator Configuration