AD9775EB AD [Analog Devices], AD9775EB Datasheet - Page 32

• Manufacturer Part Number: AD9775EB
• Description: 14-Bit, 160 MSPS 2X/4X/8X Interpolating Dual TxDAC+ D/A Converter
• Manufacturer: AD [Analog Devices]

AD9775
IMAGE REJECTION AND SIDEBAND SUPPRESSION OF
MODULATED CARRIERS
As shown in Figure 33, image rejection can be achieved by applying
baseband data to the AD9775 and following the AD9775 with a
quadrature modulator. To process multiple carriers while still
maintaining image reject capability, each carrier must be complex
modulated. As Figure 34 shows, single- or multiple-complex
modulators can be used to synthesize complex carriers. These
complex carriers are then summed and applied to the real and
Figure 34. Two-Stage Complex Upconversion
= REAL
OUTPUT = REAL
IMAGINARY
COMPLEX BASEBAND
BASEBAND
CHANNELS
MULTIPLE
REAL
– 1 –
SIGNAL
1/2
2
FREQUENCY =
BASEBAND CHANNEL N
BASEBAND CHANNEL 2
BASEBAND CHANNEL 1
MODULATORS
–
IMAGINARY INPUT
IMAGINARY INPUT
IMAGINARY INPUT
MULTIPLE
COMPLEX
1
1
–
DC
REAL INPUT
REAL INPUT
REAL INPUT
C
–
e j( 1 + 2)t
1
Figure 36. Image Rejection with Multicarrier Signals
Figure 35. Synthesis of Multicarrier Complex Signal
,
Q
2
...
N
OUTPUT = REAL
FREQUENCY
1 +
COMPLEX BASEBAND
MODULATOR 2
MODULATOR 1
MODULATOR N
IMAGINARY
1/2
COMPLEX
2
COMPLEX
COMPLEX
REAL
SIGNAL
REJECTED IMAGES
DC
R(1)
R(1)
R(2)
R(2)
R(N)
R(N)
FREQUENCY =
–32–
MODULATOR
AD9775
COMPLEX
R(N) = REAL OUTPUT OF N
I(N) = IMAGINARY OUTPUT OF N
e j(
imaginary inputs of the AD9775. A system in which multiple
baseband signals are complex modulated and then applied to
the AD9775 real and imaginary inputs followed by a quadrature
modulator is shown in Figure 36, which also describes the transfer
function of this system and the spectral output. Note the simi-
larity of the transfer functions given in Figure 36 and Figure 34.
Figure 36 adds an additional complex modulator stage for the
purpose of summing multiple carriers at the AD9775 inputs. Also,
as in Figure 33, the image rejection is not dependent on the real
or imaginary baseband data on any channel. Image rejection on
a channel will occur if either the real or imaginary data, or both,
is present on the baseband channel.
It is important to remember that the magnitude of a complex signal
can be 1.414× the magnitude of its real or imaginary components.
Due to this 3 dB increase in signal amplitude, the real and imagi-
nary inputs to the AD9775 must be kept at least 3 dB below full
scale when operating with the complex modulator. Overranging
in the complex modulator will result in severe distortion at the
DAC output.
N
+
C
C
+
Q
)t
MULTICARRIER
REAL OUTPUT =
R(1) + R(2) + ...R(N)
(TO REAL INPUT OF AD9775)
MULTICARRIER
IMAGINARY OUTPUT =
I(1) + I(2) + ...I(N)
(TO IMAGINARY INPUT OF AD9775)
1
IMAGINARY
+
REAL
C
+
Q
FREQUENCY =
QUADRATURE
MODULATOR
Q
REAL
REV. 0