AD9773-EB datasheet(39/60 Pages) AD

39 / 60 page

AD9773

Rev. D | Page 39 of 60

INPUT

(REAL)

OUTPUT

(REAL)

OUTPUT

(IMAGINARY)

INPUT

(IMAGINARY)

e–jωt = COS

ωt + jSINωt

Figure 77. Implementation of a Complex Modulator

COMPLEX MODULATION AND IMAGE REJECTION

OF BASEBAND SIGNALS

In traditional transmit applications, a two-step upconversion is

done in which a baseband signal is modulated by one carrier to

an intermediate frequency (IF) and then modulated 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, typically with analog components, transmit power is

wasted and the usable bandwidth available in the system is

reduced.

A more efficient method of suppressing the unwanted image

can be achieved by using a complex modulator followed by a

quadrature modulator. Figure 78 is a block diagram of a

quadrature modulator. Note that it is in fact the real output

half of a complex modulator. The complete upconversion can

actually be referred to as two complex upconversion stages,

the real output of which becomes the transmitted signal.

INPUT

(REAL)

OUTPUT

INPUT

(IMAGINARY)

COS

ωt

SIN

ωt

Figure 78. Quadrature Modulator

The entire upconversion from baseband to transmit frequency is

represented graphically in Figure 79. The resulting spectrum

shown in Figure 79 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. It is important to remember that in this application (two

baseband data channels) the image rejection is not dependent on

the data at either of the AD9773 input channels. In fact, image

rejection still occurs with either one or both of the AD9773 input

channels active. Note that by changing the sign of the sinusoidal

multiplying term in the complex modulator, the upper sideband

image could be suppressed while passing the lower one. This is

easily done in the AD9773 by selecting the e+jωt bit (Register 01h, Bit

1). In purely complex terms, Figure 79 represents the two-stage

upconversion from complex baseband to carrier.

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