High-Dynamic-Range, Direct Up-/Downconversion

1500MHz to 2300MHz Quadrature Mod/Demod

10

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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

MAX2023 quadrature modulator.

GSM Transmitter Applications

The MAX2023 is an ideal modulator for a zero-IF (ZIF),

single-carrier GSM transmitter. The device’s wide dynamic

range enables a very efficient overall transmitter architec-

ture. Figure 2 illustrates the exceptionally simple complete

lineup for a high-performance GSM/EDGE transmitter.

The single-carrier GSM transmit lineup generates base-

band I and Q signals from a simple 12-bit dual DAC

such as the MAX5873. The DAC clock rate can be a

multiple of the GSM system clock rate of 13MHz. The

ground-referenced outputs of the dual DAC are filtered

by simple discrete element lowpass filters to attenuate

both the DAC images and the noise floor. The I and Q

baseband signals are then level shifted and amplified

by a MAX4395 quad operational amplifier, configured

as a differential input/output amplifier. This amplifier

can deliver a baseband power level of greater than

+15dBm to the MAX2023, enabling very high RF output

power levels. The MAX2023 will deliver up to +5dBm

for GSM vectors with full conformance to the required

system specifications with large margins. The excep-

tionally low phase noise of the MAX2023 allows the cir-

cuit to meet the GSM system level noise requirements

with no additional RF filters required, greatly simplifying

the overall lineup.

The output of the MAX2023 drives a MAX2059 RF VGA,

which can deliver up to +15dBm of GSM carrier power

and includes a very flexible digitally controlled attenuator

with over 56dB of adjustment range. This accommodates

the full static and dynamic power-control requirements,

with extra range for lineup gain compensation.

RF Output

The MAX2023 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

MAX2023’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 -165dBc/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-

∑

0

°

90

°

45, 80,

OR

95MHz LO

31dB

I

Q

12

12

LOOPBACK

OUT

(FEEDS BACK

INTO Rx CHAIN

FRONT-END)

Rx

OFF

SPI

CONTROL

31dB

17dB

MAX2021/MAX2023

SPI

LOGIC

MAX5873

DUAL DAC

MAX4395

QUAD AMP

MAX2058/MAX2059

RF DIGITAL VGAs

MAX9491

VCO + SYNTH

RFOUT

Figure 2. Complete Transmitter Lineup for GSM/EDGE DCS/PCS-Band Base Stations