AD6121

–10–

REV. B

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

THEORY OF OPERATION

The AD6121 consists of high dynamic range IF amplifiers with

voltage controlled gain, a divide-by-two quadrature generator,

an I and Q demodulator, a low dropout regulator and power-

down control inputs (Figure 27).

The AD6121 accommodates both the desired CDMA signal

and an interferer 42 dB larger—approximately 6 mV p-p for the

desired signal and 700 mV p-p for the interferer—as specified in

the CDMA system.

IF Amplifiers and Gain Control

The IF gain is provided by two sections: a CDMA or FM input

stage followed by three cascaded IF amplifiers. The CDMA and

FM input stages use differential, continuously-variable attenua-

tors based on Analog Devices’ patented X-AMP™ topology.

These low noise attenuators consist of a differential R-2R ladder

network, linear interpolator, and a fixed gain amplifier. The

bulk of the IF gain is provided by three cascaded, wideband

2

I

Q

ROOFING

FILTER

IF

OUTPUT

DEMODULATOR

INPUT

IF AMPLIFIERS

INPUT STAGE

QUADRATURE DEMODULATOR

CDMA

INPUT

FM

INPUT

CDMA/FM

SELECT

GAIN

CONTROL

VOLTAGE

INPUT

1.23V

REFERENCE

OUTPUT

GAIN

CONTROL

VOLTAGE

REFERENCE

INPUT

POWER-

DOWN 2

POWER-

DOWN 1

IOUT

IOUT

QOUT

QOUT

LOCAL

OSCILLATOR

INPUT

VPOS

VREG

LOW

DROPOUT

REGULATOR

GAIN CONTROL

SCALE FACTOR

PTAT

TEMPERATURE

COMPENSATION

AD6121

Figure 27. Functional Block Diagram

VGAIN – Volts

16

0.5

2.5

1

1.5

2

8

10

4

14

6

12

2

0

°C,

+25

°C and +85°C

amplifiers. The gain and input bandwidth of the AD6121 are

identical for both the FM and CDMA operating modes. When

the CDMA/FM pin is high, CDMA mode is enabled. When the

pin is low, FM mode is enabled.

The IF amplifiers operate in two different configurations, one

with the I and Q demodulator powered up and another with the

I and Q demodulator powered down. The I and Q demodulator

power setting is configured with pin PD2. The power-down

control is further discussed in the section of this data sheet

entitled Power-Down Control.

When the demodulator is powered up, the outputs of the IF

amplifiers are internally dc-biased and there is no need for exter-

nal pull-up inductors. A roofing filter is required (see section

entitled Roofing Filter in this data sheet) when using the IF

amplifiers with the I and Q demodulator powered up. Under

these conditions, the IF amplifiers and the low noise attenuator

input stage has +41.4 dB of gain.

When the I and Q demodulator is powered down, the IF ampli-

fiers have open collector outputs resulting in the need for pull-

up inductors. Under this configuration, and with the output of

the IF amplifiers loaded with 1 k

Ω, the gain of the IF amplifiers

and low noise attenuator input stage is +47 dB. The pull-up

inductors should be chosen so that the parasitic capacitance

seen at the output of the IF amplifiers is resonated at the fre-

quency of interest. Figure 28 shows how to configure the pull-up

inductors at the output of the IF amplifiers. The 10 nF capaci-

tors are used for ac coupling.

In order to resonate the parasitic capacitors, rearrange Equation

1 to solve for L.

f

0

1

2

=

π

(1)

capacitance in Farads, and L is the total shunt inductor value in

henrys.