AD9775EB datasheet(30/48 Pages) AD

30 / 48 page

REV. 0

AD9775

–30–

fOUT, NORMALIZED TO fDATA WITH ZERO STUFFING

DISABLED – Hz

–50

–40

–30

–20

–10

0.5

1.0

1.5

2.0

ZERO STUFFING

ENABLED

ZERO STUFFING

DISABLED

Figure 29. Effect of Zero Stuffing on DAC’s SIN(x)/

x Response

To improve upon the pass-band flatness of the desired image,

the zero stuffing mode can be enabled by setting the control

fDAC/fDATA by a factor of 2 by doubling the DAC sample rate and

inserting a midscale sample (i.e., 1000 0000 0000 0000) after

every data sample originating from the interpolation filter. This

is important as it will affect the PLL divider ratio needed to keep

the VCO within its optimum speed range. Note that the zero

stuffing takes place in the digital signal chain at the output of the

digital modulator before the DAC.

The net effect is to increase the DAC output sample rate by a

factor of 2

× with the “0” in the SIN(x)/x DAC transfer function

occurring at twice the original frequency. A 6 dB loss in ampli-

tude at low frequencies is also evident, as can be seen in Figure 29.

It is important to realize that the zero stuffing option by itself

does not change the location of the images but rather their ampli-

tude, pass-band flatness, and relative weighting. For instance, in

the previous example, the pass-band amplitude flatness of the

image at 3

× f

DATA/4 is now improved to 0.59 dB while the signal

level has increased slightly from –10.5 dBFS to –8.1 dBFS.

INTERPOLATING (COMPLEX MIX MODE)

(Control Register 01h, Bit 2)

In the complex mix mode, the two digital modulators on the

AD9775 are coupled to provide a complex modulation function.

In conjunction with an external quadrature modulator, this

complex modulation can be used to realize a transmit image

rejection architecture. The complex modulation function can be

programmed for e

+j t or e–j t to give upper or lower image rejec-

tion. As in the real modulation mode, the modulation frequency

can be programmed via the SPI port for fDAC/2, fDAC/4, and

fDAC/8, where fDAC represents the DAC output rate.

OPERATIONS ON COMPLEX SIGNALS

Truly complex signals cannot be realized outside of a computer

simulation. However, two data channels, both consisting of real

data, can be defined as the real and imaginary components of a

complex signal. I (real) and Q (imaginary) data paths are often

defined this way. By using the architecture defined in Figure 30,

a system can be realized that operates on complex signals,

giving a complex (real and imaginary) output.

If a complex modulation function (e

+j t) is desired, the real and

imaginary components of the system correspond to the real and

imaginary components of e

+j t, or cos t and sin t. As Figure

31 shows, the complex modulation function can be realized

by applying these components to the structure of the com-

plex system defined in Figure 30.

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 IF (intermediate frequency) and then modulated a second

time to the transmit frequency. Although this approach has

several benefits, a major drawback is that two images are cre-

ated 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 32 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 actu-

ally be referred to as two complex upconversion stages, the real

output of which becomes the transmitted signal.

a(t)

INPUT

OUTPUT

COMPLEX FILTER

= (c + jd)

IMAGINARY

b(t)

a(t) + c

b(t)

c(t)

b(t) + d

b(t)

Figure 30. Realization of a Complex System

OUTPUT

(IMAGINARY)

OUTPUT

(REAL)

INPUT

(REAL)

INPUT

(IMAGINARY)

e–j t = COS t + jSIN t

Figure 31. Implementation of a Complex Modulator

OUTPUT

INPUT

(REAL)

INPUT

(IMAGINARY)

COS t

SIN t

Figure 32. Quadrature Modulator

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