AD9775EB AD [Analog Devices], AD9775EB Datasheet - Page 30

• Manufacturer Part Number: AD9775EB
• Description: 14-Bit, 160 MSPS 2X/4X/8X Interpolating Dual TxDAC+ D/A Converter
• Manufacturer: AD [Analog Devices]

AD9775
To improve upon the pass-band flatness of the desired image,
the zero stuffing mode can be enabled by setting the control
register bit to a Logic “1.” This option increases the ratio of
f
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
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
tion. As in the real modulation mode, the modulation frequency
f
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.
DAC
DAC
can be programmed via the SPI port for f
/f
/8, where f
DATA
Figure 29. Effect of Zero Stuffing on DAC’s SIN(x)/
x Response
–10
–20
–30
–40
–50
10
0
by a factor of 2 by doubling the DAC sample rate and
0
DATA
DAC
f
OUT
ZERO STUFFING
+j t
/4 is now improved to 0.59 dB while the signal
, NORMALIZED TO f
represents the DAC output rate.
DISABLED
or e
0.5
–j t
to give upper or lower image rejec-
DISABLED – Hz
DATA
1.0
WITH ZERO STUFFING
ZERO STUFFING
ENABLED
1.5
DAC
/2, f
DAC
2.0
/4, and
–30–
If a complex modulation function (e
imaginary components of the system correspond to the real and
imaginary components of e
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.
Figure 31. Implementation of a Complex Modulator
Figure 30. Realization of a Complex System
(IMAGINARY)
a(t)
b(t)
Figure 32. Quadrature Modulator
(IMAGINARY)
(REAL)
INPUT
INPUT
INPUT
COMPLEX FILTER
INPUT
(REAL)
INPUT
INPUT
IMAGINARY
= (c + jd)
SIN t
e
–j t
OUTPUT
OUTPUT
= COS t + jSIN t
+j t
90
, or cos t and sin t. As Figure
90
c(t)
b(t)
+j t
) is desired, the real and
COS t
b(t) + d
a(t) + c
OUTPUT
(REAL)
OUTPUT
(IMAGINARY)
OUTPUT
b(t)
b(t)
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