Datasheet

MAX2023
High-Dynamic-Range, Direct Up-/Downconversion
1500MHz to 2500MHz Quadrature Mod/Demod
13
The I/Q input power levels and the insertion loss of the
device determine the RF output power level. The input
power is a function of the delivered input I and Q volt-
ages to the internal 50Ω termination. For simple sinu-
soidal baseband signals, a level of 89mV
P-P
differential
on the I and the Q inputs results in a -17dBm input
power level delivered to the I and Q internal 50Ω termi-
nations. This results in an RF output power of -26.6dBm.
External Diplexer
LO leakage at the RF port can be nulled to a level less
than -80dBm by introducing DC offsets at the I and Q
ports. However, this null at the RF port can be compro-
mised by an improperly terminated I/Q IF interface. Care
must be taken to match the I/Q ports to the driving DAC
circuitry. Without matching, the LO’s second-order (2f
LO
)
term may leak back into the modulator’s I/Q input port
where it can mix with the internal LO signal to produce
additional LO leakage at the RF output. This leakage
effectively counteracts against the LO nulling. In addi-
tion, the LO signal reflected at the I/Q IF port produces a
residual DC term that can disturb the nulling condition.
As demonstrated in Figure 3, providing an RC termination
on each of the I+, I-, Q+, Q- ports reduces the amount of
LO leakage present at the RF port under varying temper-
ature, LO frequency, and baseband termination condi-
tions. See the
Typical Operating Characteristics for
details. Note that the resistor value is chosen to be 50Ω
with a corner frequency 1 / (2πRC) selected to adequate-
ly filter the f
LO
and 2f
LO
leakage, yet not affecting the flat-
ness of the baseband response at the highest baseband
frequency. The common-mode f
LO
and 2f
LO
signals at
I+/I- and Q+/Q- effectively see the RC networks and thus
become terminated in 25Ω (R/2). The RC network pro-
vides a path for absorbing the 2f
LO
and f
LO
leakage,
while the inductor provides high impedance at f
LO
and
2f
LO
to help the diplexing process.
RF Demodulator
The MAX2023 can also be used as an RF demodulator
(see Figure 4), downconverting an RF input signal
directly to baseband. The single-ended RF input
accepts signals from 1500MHz to 2500MHz with power
levels up to +30dBm. The passive mixer architecture
produces a conversion loss of typically 9.5dB. The
RF MODULATOR
LO
RF
50Ω
50Ω
L = 11nH
C = 2.2pF
∑
L = 11nH
I
Q
50Ω
50Ω
C = 2.2pF
C = 2.2pF
0°
90°
MAX2023
Figure 3. Diplexer Network Recommended for DCS 1800/
PCS 1900 EDGE Transmitter Applications
ADC
90
0
RF LO
MAX2023
DIPLEXER/
DC RETURN
MATCHING
ADC
DIPLEXER/
DC RETURN
MATCHING
Figure 4. MAX2023 Demodulator Configuration