AD9709 datasheet(23/32 Pages) AD

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AD9709

Rev. B | Page 23 of 32

I and Q digital data can be fed into the AD9709 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

AD9709. 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 AD9709 can be synchronized

to the I and Q data streams. The internal timing of the AD9709

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

interleaved mode, if no interpolation filter precedes the AD9709,

the symbol rate is half that of the system clock driving the digital

data stream and the IQWRT and IQCLK pins on the AD9709.

CDMA

Code division multiple access (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 discrete multitone (DMT) wave-

form, 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 45 displays the results of using the application circuit shown

in Figure 44 to reconstruct a wideband CDMA (W-CDMA) test

vector using a bandwidth of 8 MHz that is centered at 2.4 GHz

and sampled at 65 MHz. The IF frequency at the DAC output is

15.625 MHz. The adjacent channel power ratio (ACPR) for the

given test vector is measured at greater than 54 dB.

cu1

–80

–120

–70

–90

–110

–50

–60

–100

–40

CENTER 2.4GHz

–130

–30

c11

FREQUENCY

cu1

c11

SPAN 30MHz

3MHz

Figure 45. CDMA Signal, 8 MHz Chip Rate Sampled at 65 MSPS,

Recreated at 2.4 GHz, Adjacent Channel Power > 54 dB

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