High-Dynamic-Range, Direct Up-/Downconversion
1500MHz to 2500MHz Quadrature Mod/Demod
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 50 Ω 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 50 Ω termi-
nations. This results in an RF output power of -26.6dBm.
C = 2.2pF
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.
50 Ω
MAX2023
RF MODULATOR
As demonstrated in Figure 3, providing an RC termination
on each of the I+, I-, Q+, Q- ports reduces the amount of
I
L = 11nH
50 Ω
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 50 Ω
with a corner frequency 1 / (2 π RC) selected to adequate-
C = 2.2pF
LO
0 °
90 °
RF
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
50 Ω
I+/I- and Q+/Q- effectively see the RC networks and thus
become terminated in 25 Ω (R/2). The RC network pro-
vides a path for absorbing the 2f LO and f LO leakage,
Q
L = 11nH
C = 2.2pF
50 Ω
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
Figure 3. Diplexer Network Recommended for DCS 1800/
PCS 1900 EDGE Transmitter Applications
levels up to +30dBm. The passive mixer architecture
produces a conversion loss of typically 9.5dB. The
MAX2023
DIPLEXER/
DC RETURN
MATCHING
ADC
RF
90
0
LO
DIPLEXER/
DC RETURN
MATCHING
ADC
Figure 4. MAX2023 Demodulator Configuration
13
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