AD9775 datasheet(38/56 Pages) AD

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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

1/2

–

ω1– ω2DC

× ej(ω1 + ω2)t

ω1 + ω2

Figure 80. Two-Stage Complex Upconversion

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 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

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

AD9775

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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