AD9773-EB datasheet(35/60 Pages) AD

35 / 60 page

AD9773

Rev. D | Page 35 of 60

MODULATION, INTERPOLATION = 2×

With Control Register 01h, Bit 7 and Bit 6 set to 01, the inter-

polation rate of the AD9773 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 AD9773 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 input data rate

frequency are suppressed by >70 dB.

Another significant point is that the interpolation filtering is

done previous 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 fS/4 modulation mode, there is no

true stop band as the band edges coincide with each other. In

the fS/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 the Digital Modulation on the DAC Output Spectrum, Interpolation = 2×

–100

–80

–60

–40

–20

fOUT (×fDATA)

0.5

1.0

1.5

2.0

Figure 63. 2x Interpolation, Modulation = Disabled

–100

–80

–60

–40

–20

fOUT (×fDATA)

0.5

1.0

1.5

2.0

Figure 64. 2x Interpolation, Modulation = fDAC/2

–100

–80

–60

–40

–20

fOUT (×fDATA)

0.5

1.0

1.5

2.0

Figure 65. 2x Interpolation, Modulation = fDAC/4

–100

–80

–60

–40

–20

fOUT (×fDATA)

0.5

1.0

1.5

2.0

Figure 66. 2x Interpolation, Modulation = fDAC/8

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