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MAX2021ETX

High-Dynamic-Range, Direct Up-/Downconversion 750MHz to 1200MHz Quadrature Mod/Demod

Maxim Integrated 

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.

RF Output

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 89mVP-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-

C = 6.8pF

I

L = 40nH

100Ω

MAX2021

RF-MODULATOR

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 (2fLO)

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 fLO

and 2fLO leakage, yet not affecting the flatness of the

baseband response at the highest baseband frequency.

The common-mode fLO and 2fLO signals at I+/I- and

Q+/Q- effectively see the RC networks and thus become

terminated in 50Ω (R/2). The RC network provides a path

for absorbing the 2fLO and fLO leakage, while the induc-

tor provides high impedance at fLO and 2fLO to help the

diplexing process.

RF Demodulator

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.

100Ω

C = 6.8pF LO

0°

90°

∑

Q

L = 40nH

100Ω

100Ω

C = 6.8pF

Figure 2. Diplexer Network Recommended for GSM 900

Transmitter Applications

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.

Power Scaling with Changes

to the Bias Resistors

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,

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