High-Dynamic-Range, Direct Up-/Downconversion

750MHz to 1200MHz Quadrature Mod/Demod

signals to compensate for any changes in LO drive lev-

els. The two-stage LO amplifier allows a wide input

power range for the LO drive. The MAX2021 can toler-

ate LO level swings from -6dBm to +3dBm.

I/Q Modulator

The MAX2021 modulator is composed of a pair of

matched double-balanced passive mixers and a balun.

The I and Q differential baseband inputs accept signals

from DC to 300MHz with differential amplitudes up to

MAX2021 as either a direct RF modulator or as an

image-reject mixer. The wide common-mode compli-

ance range allows for direct interface with the base-

band DACs. No active buffer circuitry is required

between the baseband DACs and the MAX2021 for

cdma2000 and WCDMA applications.

The I and Q signals directly modulate the 0° and 90° LO

signals and are upconverted to the RF frequency. The out-

puts of the I and Q mixers are combined through a balun

to produce a singled-ended RF output.

Applications Information

LO Input Drive

The LO input of the MAX2021 is internally matched to

50

Ω, and requires a single-ended drive at a 750MHz to

1200MHz frequency range. An integrated balun con-

verts the singled-ended input signal to a differential sig-

nal at the LO buffer differential input. An external

DC-blocking capacitor is the only external part required

at this interface. The LO input power should be within

the -6dBm to +3dBm range. An LO input power of

-3dBm is recommended for best overall peformance.

Baseband I/Q Input Drive

Drive the MAX2021 I and Q baseband inputs differen-

tially for best performance. The baseband inputs have

a 53

Ω differential input impedance. The optimum

source impedance for the I and Q inputs is 100

Ω differ-

ential. This source impedance achieves the optimal sig-

nal transfer to the I and Q inputs, and the optimum

output RF impedance match. The MAX2021 can accept

input power levels of up to +20dBm on the I and Q

inputs. Operation with complex waveforms, such as

CDMA carriers or GSM signals, utilize input power lev-

els that are far lower. This lower power operation is

made necessary by the high peak-to-average ratios of

these complex waveforms. The peak signals must be

kept below the compression level of the MAX2021. The

input common-mode voltage should be confined to the

-3.5V to +3.5V DC range.

The MAX2021 is designed to interface directly with

Maxim high-speed DACs. This generates an ideal total

transmitter lineup, with minimal ancillary circuit elements.

Such DACs include the MAX5875 series of dual DACs,

and the MAX5895 dual interpolating DAC. These DACs

have ground-referenced differential current outputs.

Typical termination of each DAC output into a 50

Ω load

resistor to ground, and a 10mA nominal DC output cur-

rent results in a 0.5V common-mode DC level into the

modulator I/Q inputs. The nominal signal level provided

by the DACs will be in the -12dBm range for a single

CDMA or WCDMA carrier, reducing to -18dBm per car-

rier for a four-carrier application.

The I/Q input bandwidth is greater than 50MHz at

-0.1dB response. The direct connection of the DAC to

the MAX2021 ensures the maximum signal fidelity, with

no performance-limiting baseband amplifiers required.

The DAC output can be passed through a lowpass filter

to remove the image frequencies from the DAC’s output

response. The MAX5895 dual interpolating DAC can be

operated at interpolation rates up to x8. This has the

benefit of moving the DAC image frequencies to a very

high, remote frequency, easing the design of the base-

band filters. The DAC’s output noise floor and interpola-

tion filter stopband attenuation are sufficiently good to

ensure that the 3GPP noise floor requirement is met for

large frequency offsets, 60MHz for example, with no fil-

tering required on the RF output of the modulator.

Figure 1 illustrates the ease and efficiency of interfacing

the MAX2021 with a Maxim DAC, in this case the

MAX5895 dual 16-bit interpolating-modulating DAC.

MAX5895

DUAL 16-BIT INTERP DAC

RF MODULATOR

I/Q GAIN AND

OFFSET ADJUST

BBI

LO

BBQ

FREQ

50

Ω

50

Ω

FREQ

50

Ω

50

Ω

0

°

90

°

∑

MAX2021

Figure 1. MAX5895 DAC Interfaced with MAX2021

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