Datasheet

AD9773
Rev. D | Page 37 of 60
MODULATION, INTERPOLATION = 8×
With Control Register 01h, Bit 7 and Bit 6 set to 11, the
interpolation rate of the AD9773 is 8×. Modulation is achieved
by multiplying successive samples at the interpolation filter
output by the sequence (0, +0.707, +1, +0.707, 0, −0.707, −1,
+0.707).
Figure 71 to Figure 74 represent the spectral response
of the AD9773 DAC output with 8× interpolation in the various
modulation modes to a narrow band baseband signal.
Looking at
Figure 63 to Figure 74, the user can see how higher
interpolation rates reduce the complexity of the reconstruction
filter needed at the DAC output. It also becomes apparent that
the ability to modulate by f
S
/2, f
S
/4, or f
S
/8 adds a degree of
flexibility in frequency planning.
The Effects of the Digital Modulation on the DAC Output Spectrum, Interpolation = 8×
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
f
OUT
( × f
DATA
)
1023
02857-071
4
Figure 71. 8x Interpolation, Modulation Disabled
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
f
OUT
( × f
DATA
)
1023
02857-072
4
Figure 72. 8x Interpolation, Modulation = f
DAC
/2
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
4312056
f
OUT
(× f
DATA
)
02857-073
78
Figure 73. 8x Interpolation, Modulation = f
DAC
/4
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
43120 5678
f
OUT
(× f
DATA
)
02857-074
Figure 74. 8x Interpolation, Modulation = f
DAC
/8