AD9773-EB datasheet(41/60 Pages) AD

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AD9773

Rev. D | Page 41 of 60

COMPLEX BASEBAND

SIGNAL

OUTPUT = REAL

= REAL

FREQUENCY

1/2

–

ω1– ω2DC

× ej(ω1 + ω2)t

ω1 + ω2

Figure 80. Two-Stage Complex Upconversion

IMAGE REJECTION AND SIDEBAND SUPPRESSION

OF MODULATED CARRIERS

As shown in Figure 79, image rejection can be achieved by

applying baseband data to the AD9773 and following the

AD9773 with a quadrature modulator. To process multiple

carriers while still maintaining image reject capability, each

carrier must be complex modulated. As Figure 81 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 AD9773.

A system in which multiple baseband signals are complex

modulated and then applied to the AD9773 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 AD9773 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 AD9773 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

COMPLEX

MODULATOR 1

COMPLEX

MODULATOR 2

COMPLEX

MODULATOR N

R(1)

R(2)

R(N)

MULTICARRIER

REAL OUTPUT =

R(1) + R(2) + . . .R(N)

(TO REAL INPUT OF AD9773)

MULTICARRIER

IMAGINARY OUTPUT =

I(1) + I(2) + . . .I(N)

(TO IMAGINARY INPUT OF AD9773)

R(N) = REAL OUTPUT OF N

I(N) = IMAGINARY OUTPUT OF N

Figure 81. Synthesis of Multicarrier Complex Signal

MULTIPLE

BASEBAND

CHANNELS

MULTIPLE

COMPLEX

MODULATORS

FREQUENCY =

1, ω2...ωN

REAL

IMAGINARY

REAL

OUTPUT = REAL

IMAGINARY

REAL

IMAGINARY

AD9773

COMPLEX

MODULATOR

FREQUENCY =

QUADRATURE

MODULATOR

FREQUENCY =

COMPLEX BASEBAND

SIGNAL

REJECTED IMAGES

ej(ωN + ωC + ωQ)t

–

1 – ωC – ωQ

1 + ωC + ωQ

Figure 82. Image Rejection with Multicarrier Signals

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