AD9709AST datasheet(17/27 Pages) AD

17 / 27 page

REV. 0

AD9709

–17–

CDMA

Carrier Division Multiple Access, or CDMA, is an air transmit/

receive scheme where the signal in the transmit path is modulated

with a pseudorandom digital code (sometimes referred to as the

spreading code). The effect of this is to spread the transmitted

signal across a wide spectrum. Similar to a DMT waveform, a

CDMA waveform containing multiple subscribers can be char-

acterized as having a high peak to average ratio (i.e., crest factor),

thus demanding highly linear components in the transmit signal

path. The bandwidth of the spectrum is deﬁned by the CDMA

standard being used, and in operation is implemented by using

a spreading code with particular characteristics.

Distortion in the transmit path can lead to power being trans-

mitted out of the deﬁned band. The ratio of power transmitted

in-band to out-of-band is often referred to as Adjacent Channel

Power (ACP). This is a regulatory issue due to the possibility

of interference with other signals being transmitted by air.

Regulatory bodies deﬁne a spectral mask outside of the transmit

band, and the ACP must fall under this mask. If distortion in

the transmit path causes the ACP to be above the spectral mask,

then ﬁltering, or different component selection is needed to

meet the mask requirements.

Figure 42 shows the AD9709/AD8346 application circuit of

Figure 41 reconstructing a wideband, or W-CDMA test vector

with a bandwith of 8 MHz, centered at 2.4 GHz and being

sampled at 62.5 MHz. The IF frequency at the DAC output

is 15.625 MHz. ACPR for the given test vector is measured

at greater than 54 dB.

FREQUENCY

CENTER 2.4GHz

3MHz

SPAN 30MHz

cu1

c11

–130

–120

–110

–100

–90

–80

–70

–60

–50

–40

–30

cu1

c11

Figure 42. CDMA Signal, 8 M Chips Sampled at 65 MSPS,

Recreated at 2.4 GHz, Adjacent Channel Power > 54 dBm

Figure 43 shows an example of the AD9709 used in a W-CDMA

transmitter application using the AD6122 CDMA 3 V IF sub-

system. The AD6122 has functions, such as external gain

control and low-distortion characteristics, needed for the

superior Adjacent Channel Power (ACP) requirements of

W-CDMA.

(“Q DAC”)

IOUTA

QOUTA

QOUTB

DCOM

FSADJ2

REFIO

SLEEP

RSET2

1.9k

0.1 F

CLK2

Q DATA

INPUT

I DATA

INPUT

DVDD

AVDD

500

IIPP

IIPN

IIQP

IIQN

AD6122

CLK1

FSADJ1

RSET1

RCAL

220

500

DAC

LATCH

DAC

(“I DAC”)

INPUT

LATCHES

WRT1

WRT2

ACOM

AD9709

LOIPP

LOIPN

PHASE

SPLITTER

REFIN

VGAIN

GAIN

CONTROL

TXOPP

TXOPN

GAIN

CONTROL

SCALE

FACTOR

TEMPERATURE

COMPENSATION

MODOPN

MODOPP

VCC

500

IOUTB

INPUT

LATCHES

500

DAC

LATCH

DAC

634

Figure 43. CDMA Transmit Application Using AD9709 and AD6122

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