AD9775EB datasheet(31/48 Pages) AD

31 / 48 page

REV. 0

AD9775

–31–

The entire upconversion, from baseband to transmit frequency,

is represented graphically in Figure 33. The resulting spectrum

shown in Figure 33 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 AD9775 input channels.

In fact, image rejection will still occur with either one or both of

the AD9775 input channels active. 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. This is easily done in the AD9775 by selecting

the e

+j t bit (Register 01h, Bit 1). In purely complex terms,

Figure 31 represents the two-stage upconversion from complex

baseband to carrier.

REAL CHANNEL (IN)

IMAGINARY CHANNEL (IN)

COMPLEX

MODULATOR

REAL CHANNEL (OUT)

IMAGINARY CHANNEL (OUT)

–FC*

B/2J

–B/2J

TO QUADRATURE

MODULATOR

–FC

A/2J

–A/2J

B/2

A/2

REAL

IMAGINARY

QUADRATURE

MODULATOR

OUT

A/4 + B/4J

A/4 – B/4J

–FQ – FC

REJECTED IMAGES

A/4 + B/4J

A/4 – B/4J

A/4 + B/4J

A/4 – B/4J

–A/4 – B/4J

–A/4 + B/4J

A/2 + B/2J

A/2 – B/2J

–FQ*

–FQ + FC

FQ – FC

FQ + FC

–FQ

*FC = COMPLEX MODULATION FREQUENCY

*FQ = QUADRATURE MODULATION FREQUENCY

Figure 33. Two-Stage Upconversion and Resulting Image Rejection

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