AD9775EB datasheet(32/48 Pages) AD

32 / 48 page

REV. 0

AD9775

–32–

OUTPUT = REAL

COMPLEX BASEBAND

SIGNAL

= REAL

ej( 1 +

2)t

FREQUENCY

– 1 –

1 +

1/2

Figure 34. Two-Stage Complex Upconversion

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

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.

R(1)

BASEBAND CHANNEL 1

REAL INPUT

IMAGINARY INPUT

R(1)

COMPLEX

MODULATOR 1

BASEBAND CHANNEL N

REAL INPUT

IMAGINARY INPUT

COMPLEX

MODULATOR N

R(N)

BASEBAND CHANNEL 2

REAL INPUT

IMAGINARY INPUT

R(2)

COMPLEX

MODULATOR 2

R(N) = REAL OUTPUT OF N

I(N) = IMAGINARY OUTPUT OF 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)

Figure 35. Synthesis of Multicarrier Complex Signal

REAL

IMAGINARY

MULTIPLE

COMPLEX

MODULATORS

FREQUENCY =

2...

REAL

IMAGINARY

REAL

IMAGINARY

AD9775

COMPLEX

MODULATOR

FREQUENCY =

QUADRATURE

MODULATOR

FREQUENCY =

MULTIPLE

BASEBAND

CHANNELS

REAL

REJECTED IMAGES

– 1 – C – Q

1 +

C +

OUTPUT = REAL

ej( N +

C +

Q)t

COMPLEX BASEBAND

SIGNAL

Figure 36. Image Rejection with Multicarrier Signals

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