AD9763AST datasheet(17/28 Pages) AD

17 / 28 page

REV. B

AD9763

–17–

I and Q digital data can be fed into the AD9763 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 AD9763. 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

AD9763 can be synchronized to the I and Q data stream. The

internal timing of the AD9763 routes the selected I and Q data

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

tion filter precedes the AD9763, the symbol rate will be half that

of the system clock driving the digital data stream and the IQWRT

and IQCLK pins on the AD9763.

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,

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 AD9763, when used with the AD8346,

reconstructing a wideband CDMA signal at 1.8 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 42. CDMA Signal, 8 M Chips Sampled at

65 MSPS, Recreated at 2.4 GHz, Adjacent Channel

Power > 60 dBm

(“Q DAC”)

IOUTA

IOUTB

QOUTA

QOUTB

DCOM

FSADJ2

REFIO

SLEEP

RSET2

3.74k

0.1 F

CLK2

Q DATA

INPUT

I DATA

INPUT

DVDD

AVDD

500

200

CFILTER

500

2.5k

200

0.1 F

VPBF

BBIP

BBIN

BBQP

BBQN

AD8346

LOIPP

LOIPN

VOUT

NOTE: 500

RESISTOR NETWORK - OHMTEK ORN SERIES

2.5k

RESISTOR NETWORK

GAINCTRL

CLK1

FSADJ1

RSET1

3.83k

RCAL

200

500

INPUT

LATCHES

PHASE

SPLITTER

200

500

200

CFILTER

500

200

DAC

LATCHES

DAC

LATCHES

DAC

(“I DAC”)

INPUT

LATCHES

WRT1

WRT2

ACOM

AD9763

IOUTFS

11mA

Figure 41. Baseband QAM Implementation Using an AD9763 and AD8346

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