AD9775 datasheet(35/56 Pages) AD

35 / 56 page

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 fS/2, fS/4, or fS/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

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fOUT (×fDATA)

Figure 71. 8× Interpolation, Modulation Disabled

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–80

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fOUT (×fDATA)

Figure 72. 8× Interpolation, Modulation = fDAC/2

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567

fOUT (×fDATA)

Figure 73. 8× Interpolation, Modulation = fDAC/4

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567

fOUT (×fDATA)

Figure 74. 8× Interpolation, Modulation = fDAC/8

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