Datasheet

AD9786
Rev. B | Page 33 of 56
MODULATION MODES
Table 30. Single-Channel Modulation
MODDUAL CHANNEL MOD[1] MOD[0] Mode
0 0 0 0 I channel, no modulation
0 0 0 1 I channel, modulation by f
DAC
/2
0 0 1 0 I channel, modulation by f
DAC
/4
0 0 1 1 I channel, modulation by f
DAC
/8
0 1 0 0 Q channel, no modulation
0 1 0 1 Q channel, modulation by f
DAC
/2
0 1 1 0 Q channel, modulation by f
DAC
/4
0 1 1 1 Q channel, modulation by f
DAC
/8
Either channel of the AD9786 interpolation filter channels can
be routed to the DAC and modulated. In single-channel
operation, the input data can be modulated by a real sinusoid;
the input data and the modulating sinusoid contain both
positive and negative frequency components. A double side-
band output results when modulating two real signals. At the
DAC output, the positive and negative frequency components
add in phase, resulting in constructive signal summation.
As the modulating sinusoidal frequency becomes a larger
fraction of the DAC update rate, f
DAC,
the sinc function of the
DAC shapes the modulated signal bandwidth more, and the
first image moves closer.
Because the AD9786 interpolation filter pass band represents a
large portion of the input data Nyquist band, it is possible for
modulated signal bands to touch or overlap images if sufficient
interpolation is not used under certain modulation and
interpolation modes.
Figure 59 shows the effects of f
DAC
/8 modulation when using 8×
interpolation. Figure 60 to Figure 62 show the effects of real
modulation under all interpolation modes. The sinc shaping at
the corners of the modulated signal band and the bandwidth to
the first image for those cases whose pass bands do not touch or
overlap are tabulated.
Table 31. Synthesis Bandwidth vs. Interpolation Modes
Interpolation
Modulation None ×2 ×4 ×8
None f
SIN
2 f
SIN
4 f
SIN
8 f
SIN
f
DAC
/2 f
SIN
2 f
SIN
4 f
SIN
8 f
SIN
f
DAC
/4 Overlap Touching 2 f
SIN
4 f
SIN
f
DAC
/8 Overlap Overlap Touching 6 f
SIN
Table 32. Modulated Pass-Band Edges Sinc Shaping
(Lower/Upper)
Interpolation
Modulation None ×2 ×4 ×8
None 0 0 0 0
–2.8241 –0.6708 –0.1657 –0.0413
f
DAC
/2 –0.0701 –1.1932 –2.3248 –3.0590
–22.5378 –9.1824 –6.1190 –4.9337
f
DAC
/4 Overlap Touching –0.2921 –0.5974
–1.9096 –1.3607
f
DAC
/8 Overlap Overlap Touching –0.0727
–0.4614