MAX2021ETX-T Maxim Integrated Products, MAX2021ETX-T Datasheet - Page 10

• Manufacturer Part Number: MAX2021ETX-T
• Description: Modulator / Demodulator High-Dynamic-Range D irect Up-/Downconver
• Manufacturer: Maxim Integrated Products

High-Dynamic-Range, Direct Up-/Downconversion
750MHz to 1200MHz Quadrature Mod/Demod
The MAX5895 DAC has programmable gain and differ-
ential offset controls built in. These can be used to opti-
mize the LO leakage and sideband suppression of the
MAX2021 quadrature modulator.
The MAX2021 utilizes an internal passive mixer archi-
tecture that enables the device to possess an excep-
tionally low-output noise floor. With such architectures,
the total output noise is typically a power summation of
the theoretical thermal noise (KTB) and the noise contri-
bution from the on-chip LO buffer circuitry. As demon-
strated in the Typical Operating Characteristics, the
MAX2021’s output noise approaches the thermal limit
of -174dBm/Hz for lower output power levels. As the
output power increases, the noise level tracks the noise
contribution from the LO buffer circuitry, which is
approximately -168dBc/Hz.
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
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.
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-
Figure 2. Diplexer Network Recommended for GSM 900
Transmitter Applications
10
Q
______________________________________________________________________________________
I
L = 40nH
L = 40nH
C = 6.8pF
C = 6.8pF
C = 6.8pF
100Ω
100Ω
100Ω
100Ω
LO
RF-MODULATOR
0°
External Diplexer
MAX2021
90°
∑
P-P
RF Output
differential
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
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 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 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
and 2f
baseband response at the highest baseband frequency.
The common-mode f
Q+/Q- effectively see the RC networks and thus become
terminated in 50Ω (R/2). The RC network provides a path
for absorbing the 2f
tor provides high impedance at f
diplexing process.
The MAX2021 can also be used as an RF demodulator,
downconverting an RF input signal directly to base-
band. The single-ended RF input accepts signals from
750MHz 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, typi-
cally 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.
Bias currents for the LO buffers are optimized by fine
tuning resistors R1, R2, and R3. Maxim recommends
using ±1%-tolerant resistors; however, standard ±5%
values can be used if the ±1% components are not
readily available. The resistor values shown in the
Typical Application Circuit were chosen to provide
peak performance for the entire 750MHz to 1200MHz
band. If desired, the current can be backed off from
this nominal value by choosing different values for R1,
LO
leakage, yet not affecting the flatness of the
Power Scaling with Changes
LO
LO
and f
and 2f
LO
to the Bias Resistors
leakage, while the induc-
LO
LO
RF Demodulator
and 2f
signals at I+/I- and
LO
to help the
LO
LO
)