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AD6122ACPRL

CDMA 3 V Transmitter IF Subsystem with Integrated Voltage Regulator

Analog Devices 

AD6122

VCC

I INPUT

LOCAL

OSCILLATOR

INPUT

Q INPUT

COMMON-MODE

REFERENCE

OUTPUT

VPOS

QUADRATURE

MODULATOR

OUTPUT

QUADRATURE MODULATOR

ATTENUATOR

،2

LOW

DROPOUT

REGULATOR

VREG

AD6122

GAIN

CONTROL

SCALE

FACTOR

IF AMPLIFIER

INPUT

IF AMPLIFIERS

TRANSMIT

OUTPUT

TEMPERATURE

COMPENSATION

POWER- POWER-

DOWN 1 DOWN 2

1.23 V

REFERENCE

OUTPUT

GAIN CONTROL

REFERENCE

VOLTAGE

INPUT

GAIN CONTROL

VOLTAGE

INPUT

Figure 21. Block Diagram

THEORY OF OPERATION

The CDMA Transmitter IF Subsystem (Figure 21) consists of

an I and Q modulator with a divide-by-two quadrature genera-

tor, high dynamic range IF amplifiers with voltage-controlled

gain, a low dropout regulator and power-down control inputs.

I and Q Modulator

The I and Q modulator accepts differential quadrature baseband

inputs from CDMA baseband converters. The LO is injected at

twice the IF frequency. A divide-by-two quadrature generator

followed by dual polyphase filters ensures ± 1° quadrature accu-

racy (Figure 22).

For 500 mV p-p differential I and Q input signals, the output

power of the modulator will be –21 dBm referred to 1 kΩ when

the output of the modulator is loaded with a 1 kΩ differential

load. With the maximum input conditions stated above, the

modulator outputs are a 225 µA p-p differential current; conse-

quently, the output load will greatly affect the output power of

the modulator.

2 ؋ IF

LO INPUT

180؇

I

،2

I

POLYPHASE

FILTERS

QUADRATURE

OUTPUT TO

MODULATOR

Q

،2

Q

Figure 22. Simplified Quadrature Generator Circuit

The I and Q modulator also provides a common mode reference

signal at the MODCMREF pin. This voltage is a dc voltage set

to 1.408 V when a 2.7 V supply is used. It is used to dc bias

the output of the DAC that provides I and Q inputs to the

modulator.

IF Amplifiers and Gain Control

The IF amplifiers provide an 86 dB linear in dB gain control

range. The input stage uses a differential, continuously variable

attenuator based on Analog Devices’ patented X-AMP™ topol-

ogy. This low noise attenuator consists of a differential R-2R

ladder network, linear interpolator and a fixed gain amplifier.

The IF amplifier’s input impedance is 1 kΩ differential. Similar

to the I and Q modulator’s output, the IF amplifier’s output is a

differential current, which will vary depending upon the gain

control voltage. In order to achieve the specified gain, the out-

put of the IF amplifiers should be loaded with a 1 kΩ differen-

tial load.

The gain control circuits contain both temperature compensa-

tion circuitry and a choice of internal or external reference for

adjusting the gain scale factor. The gain control input accepts

an external gain control voltage input from a DAC. It provides

97 dB of gain control range with a nominal 75 dB/V scale factor.

The external gain control input signal should be a clean signal.

It is recommended to filter this signal in order to eliminate the

noise that results from the DAC. If a noisy signal is used for the

gain control voltage, VGAIN inband and adjacent channel noise

peaking can occur at the output of the AD6122. A simple RC

filter can be employed, but care should be taken with its design.

If too big a resistor is used, a large voltage drop may occur

across the resistor, resulting in lower gain than expected (as a

result of a lower voltage reaching the AD6122). An RC filter

with a 20 kHz bandwidth, employing a 1 kΩ resistor is appropri-

ate. This results in an 8.2 nF capacitor. The resulting circuit

is shown in Figure 23. Note that the input resistance at the

VGAIN pin is approximately 100 kΩ.

FROM

BASEBAND

CONVERTER

1k⍀

8.2nF

AD6122

VGAIN

109k⍀

X-AMP is a trademark of Analog Devices, Inc.

REV. B

–11–

Figure 23. Gain Voltage Filtering

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