AD9773AST datasheet(17/19 Pages) AD

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AD9773

PRELIMINARY TECHNICAL DATA

a(t)

b(t)

real

input

output

imaginary

input

output

complex filter

= (c+jd)

× a(t)-d × b(t)

× a(t)+c × b(t)

complex input

= (a+jb)

Figure 11. Realization of a Complex Filter

Figure 12. Implementation of Complex Modulator

Figure 13. Quadrature Modulator

ejωt = cos

ωt + jsinωt

cos

ωt

sin

ωt

output (real)

output

(imaginary)

input (real)

input

(imaginary)

cos

ωt

sin

ωt

output

input (real)

input

(imaginary)

Figure 10. Identical Baseband Signals, Amplitude

Modulated with Sine and Cosine Carriers.

A more efficient method of suppressing the unwanted

image can be achieved by using a complex modulator

followed by a quadrature modulator. Figure 13 shows a

block diagram of of a quadrature modulator. Note that

it is in fact the real output half of a complex modulator.

using the architecture defined in figure 11, a system can

be realized which operates on complex signals, giving a

complex (real and imaginary) output.

The complete upconversion can actually be referred to

as two complex upconversion stages, the real output of

which becomes the transmitted signal.

The entire upconversion, from baseband to transmit

frequency, is represented graphically in figure 14.The

resulting spectrum shown in figure 14 represents the

complex data consisting of the baseband real and

imaginary channels, now modulated onto orthogonal

(cosine and negative sine) carriers at the transmit

frequency. Note that by changing the sign of the

sinusoidal multiplying term in the complex modulator,

the upper sideband image could have been suppressed

while passing the lower one.

Aejωt/2

Ae-jωt/2

Aejωt/2j

Ae-jωt/2j

sinusoidal

m odulation

cosinusoidal

m odulation

A (am plitude)

baseband

signal

If a complex modulation function (e+jωt) is desired, the

real and imaginary components of the system corre-

spond to the real and imaginary components of e+jωt, or

cos

ωt and sinωt.As Figure 12 shows, the complex

modulation function can be realized by applying these

components to the structure of the complex system

defined in Figure 11.

COMPLEX MODULATION AND IMAGE

REJECTION

In many applications, a two step upconversion is done

in which the baseband signal is modulated by one

carrier to an IF (intermediate frequency) and then mod-

ulated a second time to the transmit frequency. Although

this approach has several benefits, a major drawback is

that two images are created near the transmit frequency.

Only one image is needed, the other being an exact

duplicate. Unless the unwanted image is filtered, typi-

cally with analog components, transmit power is wasted

and the usable bandwidth available in the system is

reduced.

REV. PrA

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