Datasheet

AD9775
Rev. E | Page 35 of 56
MODULATION, INTERPOLATION = 8×
With Control Register 0x01, Bit 7 and Bit 6 set to 11, the
interpolation rate of the AD9775 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 through Figure 74 represent the spectral
response of the AD9775 DAC output with 8× interpolation in the
various modulation modes to a narrow-band baseband signal.
Looking at
Figure 63 through Figure 74, the user can see how
higher interpolation rates reduce the complexity of the recon-
struction 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
02858-071
4
Figure 71. 8× Interpolation, Modulation Disabled
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
f
OUT
(× f
DATA
)
1023
02858-072
4
Figure 72. 8× Interpolation, Modulation = f
DAC
/2
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
43120 5678
f
OUT
(× f
DATA
)
02858-073
Figure 73. 8× Interpolation, Modulation = f
DAC
/4
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
43120 5678
f
OUT
(× f
DATA
)
02858-074
Figure 74. 8× Interpolation, Modulation = f
DAC
/8