AD9765AST datasheet(33/44 Pages) AD

33 / 44 page

Data Sheet

AD9763/AD9765/AD9767

Rev. G | Page 33 of 44

I and Q digital data can be fed into the AD9763 in two ways. In

dual-port mode, the digital I information drives one input port,

and the digital Q information drives the other input port. If no

interpolation filter precedes the DAC, the symbol rate is 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 streams. The internal timing of the AD9763 routes

the selected I and Q data to the correct DAC output. In interleaved

mode, if no interpolation filter precedes the AD9763, the symbol

rate is half that of the system clock driving the digital data stream

and the IQWRT and IQCLK pins on the AD9763.

CDMA

Code division multiple access (CDMA) is an air transmit/receive

scheme in which 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 discrete multitone

(DMT) waveform, a CDMA waveform containing multiple

subscribers can be characterized as having a high peak to average

ratio (that is, 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 it is implemented by using a spreading code with

particular characteristics.

Distortion in the transmit path can lead to power being transmitted

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, filtering or different

component selection is needed to meet the mask requirements.

Figure 85 shows the results of using the AD9763/AD9765/

AD9767 with the AD8346 to reconstruct a wideband CDMA

signal centered at 2.4 GHz. The baseband signal is sampled at

65 MSPS and has a chip rate of 8 MHz.

cu1

–80

–120

–70

–90

–110

–50

–60

–100

–40

CENTER 2.4GHz

–130

–30

c11

FREQUENCY

cu1

c11

SPAN 30MHz

3MHz

Figure 85. CDMA Signal, 8 MHz Chip Rate Sampled at 65 MSPS, Recreated at

2.4 GHz, Adjacent Channel Power >60 dBm

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