Datasheet

AD9775
Rev. E | Page 38 of 56
IMAGE REJECTION AND SIDEBAND SUPPRESSION
OF MODULATED CARRIERS
As shown in Figure 79, 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 80 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 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 82,
which also describes the transfer function of this system and the
spectral output. Note the similarity of the transfer functions
given in
Figure 82 and Figure 80. Figure 82 adds an additional
complex modulator stage for the purpose of summing multiple
carriers at the AD9775 inputs. Also, as in
Figure 79, the image
rejection is not dependent on the real or imaginary baseband
data on any channel. Image rejection on a channel occurs 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 imaginary inputs to the AD9775 must be kept at least
3 dB below full scale when operating with the complex modu-
lator. Overranging in the complex modulator results in severe
distortion at the DAC output.
COMPLEX BASEBAND
SIGNAL
OUTPUT = REAL
= REAL
FREQUENCY
1
1/2 1/2
–ω1–ω2DC
×
e
j(ω1 + ω2)t
ω1 + ω2
02858-080
Figure 80. Two-Stage Complex Upconversion
BASEBAND CHANNEL 1
REAL INPUT
BASEBAND CHANNEL 2
REAL INPUT
BASEBAND CHANNEL
N
REAL INPUT
IMAGINARY INPUT
IMAGINARY INPUT
IMAGINARY INPUT
COMPLEX
MODULATOR 1
COMPLEX
MODULATOR 2
COMPLEX
MODULATOR N
R(1)
R(1)
R(2)
R(2)
R(N)
R(N)
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)
R(N) = REAL OUTPUT OF N
I(N) = IMAGINARY OUTPUT OF N
02858-081
Figure 81. Synthesis of Multicarrier Complex Signal
MULTIPLE
BASEBAND
CHANNELS
MULTIPLE
COMPLEX
MODULATORS
FREQUENCY = ω
1
, ω
2
...ω
N
REAL
IMAGINARY
REAL
OUTPUT = REAL
IMAGINARY
REAL REAL
IMAGINARY
AD9775
COMPLEX
MODULATOR
FREQUENCY = ω
C
QUADRATURE
MODULATOR
FREQUENCY = ω
Q
COMPLEX BASEBAND
SIGNAL
REJECTED IMAGES
DC
×
e
j(ω
N
+ ω
C
+ ω
Q
)t
–ω
1
– ω
C
– ω
Q
ω
1
+ ω
C
+ ω
Q
02858-082
Figure 82. Image Rejection with Multicarrier Signals