AD6122ACPRL AD [Analog Devices], AD6122ACPRL Datasheet - Page 11

• Manufacturer Part Number: AD6122ACPRL
• Description: CDMA 3 V Transmitter IF Subsystem with Integrated Voltage Regulator
• Manufacturer: AD [Analog Devices]

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.
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.
REV. B
X-AMP is a trademark of Analog Devices, Inc.
LO INPUT
Figure 22. Simplified Quadrature Generator Circuit
2
IF
COMMON-MODE
180
VPOS
OSCILLATOR
REFERENCE
OUTPUT
Q INPUT
I INPUT
LOCAL
INPUT
2
2
QUADRATURE MODULATOR
Q
I
POWER-
DOWN 1
2
POLYPHASE
REGULATOR
DROPOUT
FILTERS
LOW
POWER-
DOWN 2
QUADRATURE
QUADRATURE
OUTPUT TO
MODULATOR
MODULATOR
I
Q
Figure 21. Block Diagram
VREG
REFERENCE
OUTPUT
OUTPUT
1.23 V
AD6122
–11–
VCC
GAIN CONTROL
REFERENCE
CONTROL
VOLTAGE
FACTOR
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.
SCALE
INPUT
ATTENUATOR
GAIN
CONVERTER
GAIN CONTROL
BASEBAND
VOLTAGE
Figure 23. Gain Voltage Filtering
INPUT
FROM
IF AMPLIFIER
INPUT
IF AMPLIFIERS
COMPENSATION
1k
TEMPERATURE
8.2nF
VGAIN
TRANSMIT
OUTPUT
109k
AD6122
AD6122