AD9775EB AD [Analog Devices], AD9775EB Datasheet - Page 33

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

The complex carrier synthesized in the AD9775 digital modulator
is accomplished by creating two real digital carriers in quadrature.
Carriers in quadrature cannot be created with the modulator
running at f
with modulation rates of f
Regions A and B of Figures 37 through 42 are the result of the
complex signal described above, when complex modulated in the
AD9775 by +e
signal described above, again with positive frequency components
only, modulated in the AD9775 by –e
ture modulator after the AD9775 inherently modulates by +e
Region A
Region A is a direct result of the upconversion of the complex
signal near baseband. If viewed as a complex signal, only the
images in Region A will remain. The complex Signal A, consisting
of positive frequency components only in the digital domain, has
images in the positive odd Nyquist zones (1, 3, 5...) as well as
images in the negative even Nyquist zones. The appearance and
rejection of images in every other Nyquist zone will become
more apparent at the output of the quadrature modulator. The
A images will appear on the real and the imaginary outputs of
the AD9775, as well as on the output of the quadrature modula-
tor, where the center of the spectral plot will now represent the
quadrature modulator LO, and the horizontal scale now repre-
sents the frequency offset from this LO.
Region B
Region B is the image (complex conjugate) of Region A. If a spec-
trum analyzer is used to view the real or imaginary DAC outputs
of the AD9775, Region B will appear in the spectrum. However, on
the output of the quadrature modulator, Region B will be rejected.
REV. 0
DAC
j t
/2. As a result, complex modulation only functions
. Regions C and D are the result of the complex
DAC
/4 and f
DAC
j t
. The analog quadra-
/8.
j t
.
–33–
Region C
Region C is most accurately described as a down conversion, as the
modulating carrier is –e
images in Region C will remain. This image will appear on the
real and imaginary outputs of the AD9775, as well as on the
output of the quadrature modulator, where the center of the spec-
tral plot will now represent the quadrature modulator LO and the
horizontal scale will represent the frequency offset from this LO.
Region D
Region D is the image (complex conjugate) of Region C. If a
spectrum analyzer is used to view the real or imaginary DAC
outputs of the AD9775, Region D will appear in the spectrum.
However, on the output of the quadrature modulator, Region D
will be rejected.
Figures 43 through 50 show the measured response of the AD9775
and AD8345 given the complex input signal to the AD9775 in
Figure 43. The data in these graphs was taken with a data rate of
12.5 MSPS at the AD9775 inputs. The interpolation rate of 4× or
8× gives a DAC output data rate of 50 MSPS or 100 MSPS. As a
result, the high end of the DAC output spectrum in these graphs
is the first null point for the SIN(x)/x roll-off, and the asymmetry
of the DAC output images is representative of the SIN(x)/x roll-off
over the spectrum. The internal PLL was enabled for these
results. In addition, a 35 MHz third order low-pass filter was
used at the AD9775/AD8345 interface to suppress DAC images.
An important point can be made by looking at Figures 45 and 47.
Figure 45 represents a group of positive frequencies modulated by
complex +f
frequencies modulated by complex –f
real or imaginary outputs of the AD9775, as shown in Figures 45
and 47, the results look identical. However, the spectrum analyzer
cannot show the phase relationship of these signals. The differ-
ence in phase between the two signals becomes apparent when
they are applied to the AD8345 quadrature modulator, with the
results shown in Figures 46 and 48.
DAC
/4, while Figure 47 represents a group of negative
j t
. If viewed as a complex signal, only the
DAC
/4. When looking at the
AD9775