Datasheet

AD9775
Rev. E | Page 33 of 56
MODULATION, INTERPOLATION = 2×
With Control Register 0x01, Bit 7 and Bit 6 set to 01, the
interpolation rate of the AD9775 is 2×. Modulation is achieved
by multiplying successive samples at the interpolation filter
output by the sequence (+1, −1).
Figure 63 through Figure 66
represent the spectral response of the AD9775 DAC output with
2× interpolation in the various modulation modes to a narrow-
band baseband signal (the tall rectangles in the graphic). The
advantage of interpolation becomes clear in
Figure 63 through
Figure 66, where the images that would normally appear in the
spectrum around the input data rate frequency are suppressed
by >70 dB.
Another significant point is that the interpolation filtering is
done previous to the digital modulator. For this reason, as
Figure 63 through Figure 66 show, the pass band of the
interpolation filters can be frequency shifted, giving the equivalent
of a high-pass digital filter.
Note that when using the f
S
/4 modulation mode, there is no
true stop band as the band edges coincide with each other. In
the f
S
/8 modulation mode, amplitude scaling occurs over only a
portion of the digital filter pass band due to constructive
addition over just that section of the band.
The Effects of the Digital Modulation on the DAC Output Spectrum, Interpolation = 2×
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
f
OUT
(× f
DATA
)
0.50 1.0 1.5 2.0
02858-063
Figure 63. 2× Interpolation, Modulation = Disabled
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
f
OUT
(× f
DATA
)
0.50 1.0 1.5 2.0
02858-064
Figure 64. 2× Interpolation, Modulation = f
DAC
/2
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
f
OUT
(× f
DATA
)
0.50 1.0 1.5 2.0
02858-065
Figure 65. 2× Interpolation, Modulation = f
DAC
/4
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
f
OUT
(× f
DATA
)
0.50 1.0 1.5 2.0
02858-066
Figure 66. 2× Interpolation, Modulation = f
DAC
/8