Datasheet

AD9773
Rev. D | Page 35 of 60
MODULATION, INTERPOLATION = 2×
With Control Register 01h, Bit 7 and Bit 6 set to 01, the inter-
polation rate of the AD9773 is 2×. Modulation is achieved by
multiplying successive samples at the interpolation filter output
by the sequence (+1, −1).
Figure 63 to Figure 66 represent
the spectral response of the AD9773 DAC output with 2×
interpolation in the various modulation modes to a narrow
band baseband signal (again, the tall rectangles in the graphic).
The advantage of interpolation becomes clear in
Figure 63 to
Figure 66, where it can be seen that 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 to 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
02857-063
Figure 63. 2x Interpolation, Modulation = Disabled
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
f
OUT
( × f
DATA
)
0.50 1.0 1.5 2.0
02857-064
Figure 64. 2x 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
02857-065
Figure 65. 2x 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
02857-066
Figure 66. 2x Interpolation, Modulation = f
DAC
/8