Data Sheet
AD9763/AD9765/AD9767
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.
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.
–30
CDMA
–40
–50
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.
–60
==
–70
–80
–90
–100
–110
c11
c11
C0
cu1
–120
–130
cu1
C0
CENTER 2.4GHz
3MHz
SPAN 30MHz
FREQUENCY
Figure 85. CDMA Signal, 8 MHz Chip Rate Sampled at 65 MSPS, Recreated at
2.4 GHz, Adjacent Channel Power >60 dBm
Rev. G | Page 33 of 44
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