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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
d
c11
cu1
C0
C0
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
R
3.74k
0.1 F
CLK2
Q DATA
INPUT
I DATA
INPUT
DVDD
AVDD
500
200
C
FILTER
500
2.5k
2.5k
2.5k
200
0.1 F
5V
VPBF
BBIP
BBIN
BBQP
BBQN
AD8346
LOIPP
LOIPN
VOUT
NOTE: 500 RESISTOR NETWORK - OHMTEK ORN SERIES
2.5k RESISTOR NETWORK
GAINCTRL
CLK1
FSADJ1
R
3.83k
R
CAL
200
500
+
INPUT
LATCHES
PHASE
SPLITTER
200
500
200
C
FILTER
500
200
DAC
LATCHES
DAC
DAC
LATCHES
DAC
(
“
I DAC
”
)
INPUT
LATCHES
WRT1
WRT2
ACOM
AD9763
U1
U2
I
OUTFS
11mA
Figure 41. Baseband QAM Implementation Using an AD9763 and AD8346