Datasheet

AD9777
Rev. C | Page 41 of 60
COMPLEX BASEBAND
SIGNAL
OUTPUT = REAL
= REAL
FREQUENCY
1
1/2 1/2
–ω1–ω2DC
×
e
j(ω1 + ω2)t
ω1 + ω2
02706-080
Figure 80. Two-Stage Complex Upconversion
IMAGE REJECTION AND SIDEBAND
SUPPRESSIONS OF MODULATED CARRIERS
As shown in Figure 79, image rejection can be achieved by
applying baseband data to the AD9777 and following the
AD9777 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 AD9777.
A system in which multiple baseband signals are complex
modulated and then applied to the AD9777 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 summing multiple carriers at the
AD9777 inputs. In addition, 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 AD9777 must be kept at least
3 dB below full scale when operating with the complex
modulator. Overranging in the complex modulator results in
severe distortion at the DAC output.
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 AD9777)
MULTICARRIER
IMAGINARY OUTPUT =
I(1) + I(2) + . . .I(N)
(TO IMAGINARY INPUT OF AD9777)
R(N) = REAL OUTPUT OF N
I(N) = IMAGINARY OUTPUT OF N
02706-081
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
AD9777
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
02706-082
Figure 82. Image Rejection with Multicarrier Signals