REV. 0
AD9752
–17–
AD9752
(“I DAC”)
AD9752
(“Q DAC”)
IOUTA
IOUTB
QOUTA
QOUTB
DCOM
FSADJ
REFIO
SLEEP
RSET2
1.9k
0.1 F
CLK
Q DATA
INPUT
I DATA
INPUT
DVDD
AVDD
100W
500
100
CFILTER
100
CFILTER
100
500
634
0.1 F
+5V
VPBF
BBIP
BBIN
BBQP
BBQN
AD8346
PHASE
SPLITTER
LOIP
LOIN
VOUT
500mV p-p WITH
VCM=1.2V
NOTE: 500
RESISTOR NETWORK - OHMTEK ORN5000D
100
RESISTOR NETWORK - TOMC1603-100D
REFLO
ACOM
REFLO
AVDD
REFIO
FSADJ
RSET1
2k
RCAL
220
U1
U2
AVDD
1.82V
LATCHES
500
DAC
+
LATCHES
Figure 37. Baseband QAM Implementation Using Two AD9752s
AD9752
0
90
AD9752
CARRIER
FREQUENCY
12
TO
MIXER
DSP
OR
ASIC
NYQUIST
FILTERS
QUADRATURE
MODULATOR
Figure 36. Typical Analog QAM Architecture
In this implementation, it is much more difficult to maintain
proper gain and phase matching between the I and Q channels.
The circuit implementation shown in Figure 37 helps improve
upon the matching and temperature stability characteristics
between the I and Q channels, as well as showing a path for up-
conversion using the AD8346 quadrature modulator. Using a
single voltage reference derived from U1 to set the gain for both
the I and Q channels will improve the gain matching and stabil-
ity. RCAL can be used to compensate for any mismatch in gain
between the two channels. This mismatch may be attributed to
the mismatch between RSET1 and RSET2, effective load resistance
of each channel, and/or the voltage offset of the control ampli-
fier in each DAC. The differential voltage outputs of U1 and U2
are fed into the respective differential inputs of the AD8346 via
matching networks.
Using the same matching techniques described above, Figure 38
shows an example of the AD9752 used in a W-CDMA transmit-
ter application using the AD6122 CDMA 3 V transmitter IF
subsystem. The AD6122 has functions, such as external gain
control and low distortion characteristics, needed for the supe-
rior Adjacent Channel Power (ACP) requirements of W-CDMA.
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 cause the ACP to be above the spectral mask,
then filtering, or different component selection is needed to
meet the mask requirements.