Datasheet

AD9773
Rev. D | Page 34 of 60
MODULATION, NO INTERPOLATION
With Control Register 01h, Bit 7 and Bit 6 set to 00, the
interpolation function on the AD9773 is disabled.
Figure 59 to
Figure 62 show the DAC output spectral characteristics of the
AD9773 in the various modulation modes, all with the
interpolation filters disabled. The modulation frequency is
determined by the state of Control Register 01h, Bits 5 and 4.
The tall rectangles represent the digital domain spectrum of a
baseband signal of narrow bandwidth.
By comparing the digital domain spectrum to the DAC SIN(x)/x
roll-off, an estimate can be made for the characteristics required
for the DAC reconstruction filter. Note also, per the previous
discussion on amplitude modulation, that the spectral com-
ponents (where modulation is set to f
S
/4 or f
S
/8) are scaled by a
factor of 2. In the situation where the modulation is f
S
/2, the
modulated spectral components add constructively, and there is
no scaling effect.
The Effects of the Digital Modulation on the DAC Output Spectrum, Interpolation Disabled
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
0 0.2 0.4 0.6 0.8 1.0
f
OUT
( × f
DATA
)
02857-059
Figure 59. No Interpolation, Modulation Disabled
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
0 0.2 0.4 0.6 0.8 1.0
f
OUT
( × f
DATA
)
02857-060
Figure 60. No Interpolation, Modulation = f
DAC
/2
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
0 0.2 0.4 0.6 0.8 1.0
f
OUT
( × f
DATA
)
02857-061
Figure 61. No Interpolation, Modulation = f
DAC
/4
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
0 0.2 0.4 0.6 0.8 1.0
f
OUT
( × f
DATA
)
02857-062
Figure 62. No Interpolation, Modulation = f
DAC
/8