AD9765AST datasheet(18/28 Pages) AD

18 / 28 page

REV. B

AD9765

–18–

I and Q digital data can be fed into the AD9765 in two different

ways. In dual port mode, The digital I information drives one

input port, while the digital Q information drives the other input

port. If no interpolation filter precedes the DAC, the symbol

rate will be the rate at which the system clock drives the CLK

and WRT pins on the AD9765. In interleaved mode, the digital

input stream at Port 1 contains the I and the Q information in

alternating digital words. Using IQSEL and IQRESET, the

AD9765 can be synchronized to the I and Q data stream. The

internal timing of the AD9765 routes the selected I and Q data

to the correct DAC output. In interleaved mode, if no interpola-

tion filter precedes the AD9765, the symbol rate will be half that

of the system clock driving the digital data stream and the

IQWRT and IQCLK pins on the AD9765.

CDMA

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

receive scheme where the signal in the transmit path is modu-

lated with a pseudorandom digital code (sometimes referred to

as the spreading code). The effect of this is to spread the trans-

mitted signal across a wide spectrum. Similar to a DMT wave-

form, a CDMA waveform containing multiple subscribers can

be characterized 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 imple-

mented 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. Regula-

tory bodies define a spectral mask outside of the transmit band,

(“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

INPUT

LATCHES

500

DAC

LATCH

DAC

(“I DAC”)

INPUT

LATCHES

WRT1

WRT2

ACOM

AD9765

LOIPP

LOIPN

PHASE

SPLITTER

REFIN

VGAIN

GAIN

CONTROL

TXOPP

TXOPN

GAIN

CONTROL

SCALE

FACTOR

TEMPERATURE

COMPENSATION

MODOPN

MODOPP

VCC

+3V

500

IOUTB

DAC

LATCH

634

Figure 44. CDMA Transmit Application Using AD9765 and AD6122

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 43 shows the AD9765, when used with the AD8346,

reconstructing a wideband CDMA signal at 2.4 GHz. The

baseband signal is being sampled at 65 MSPS and has a chip

rate of 8M chips.

–80

–120

–70

–90

–110

–50

–60

–100

–40

CENTER 2.4GHz

3MHz

SPAN 30MHz

–130

–30

c11

cu1

c11

FREQUENCY

Figure 43. CDMA Signal, 8 M Chips Sampled at 65 MSPS,

Recreated at 2.4 GHz, Adjacent Channel Power > 60 dBm

Figure 44 shows an example of the AD9765 used in a W-CDMA

transmitter application using the AD6122 CDMA 3 V IF sub-

system. The AD6122 has functions, such as external gain con-

trol and low distortion characteristics, needed for the superior

Adjacent Channel Power (ACP) requirements of W-CDMA.

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