Datasheet

AD9777
Rev. C | Page 35 of 60
MODULATION, INTERPOLATION = 2×
With Control Register 01h, Bit 7 and Bit 6 set to 01, the
interpolation rate of the AD9777 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 AD9777 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 significant point is
that the interpolation filtering is done prior 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 Digital Modulation on DAC Output Spectrum, Interpolation = 2x
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
f
OUT
( × f
DATA
)
0.50 1.0 1.5 2.0
02706-063
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
02706-064
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
02706-065
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
02706-066
Figure 66. 2× Interpolation, Modulation = f
DAC
/8