Datasheet

MAX2021
High-Dynamic-Range, Direct Up-/Downconversion
650MHz to 1200MHz Quadrature Mod/Demod
13
Maxim Integrated
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 -23.2dBm.
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 2, providing an RC termina-
tion on each of the I+, I-, Q+, Q- ports reduces the
amount of LO leakage present at the RF port under
RF-MODULATOR
LO
RF
100Ω
100Ω
L = 40nH
C = 6.8pF
∑
L = 40nH
I
Q
100Ω
100Ω
C = 6.8pF
C = 6.8pF
0°
90°
MAX2021
Figure 2. Diplexer Network Recommended for GSM 900
Transmitter Applications
12
I
DAC
50I
31dB 31dB17dB
RFOUT
50I
12
Q
DAC
50I
50I
0
90
C
MAX2021
MAX5873
DUAL DAC
45, 80,
OR
95MHz
LO
LOOPBACK
OUT
(FEEDS BACK
INTO Rx CHAIN
FRONT END)
Rx OFF SPI
CONTROL
SPI
LOGIC
MAX2058
RF DIGITAL VGA
MAX9491
VCO + SYNTH
Figure 1. Transmitter Lineup