AD9786 datasheet(34/60 Pages) AD

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AD9786

Rev. 0 | Page 34 of 60

MODULATION MODES

Table 30. Single Channel Modulation

MODDUAL

CHANNEL

MOD[1]

MOD[0]

Mode

I Channel, no modulation

I Channel, modulation by fDAC/2

I Channel, modulation by fDAC/4

I Channel, modulation by fDAC/8

Q Channel, no modulation

Q Channel, modulation by fDAC/2

Q Channel, modulation by fDAC/4

Q Channel, modulation by fDAC/8

Either channel of the AD9786’s interpolation filter channels can

be routed to the DAC and modulated. In single channel

operation the input data may be modulated by a real sinusoid;

the input data and the modulating sinusoid will 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

will add in phase resulting in constructive signal summation.

As the modulating sinusoidal frequency becomes a larger

fraction of the DAC update rate, fDAC, the more the sinc function

of the DAC shapes the modulated signal bandwidth, and the

closer the first image moves. As the AD9786 interpolation

filter’s pass band represents a large portion of the input data’s

Nyquist band, under certain modulation and interpolation

modes it is possible for modulated signal bands to touch or

overlap images if sufficient interpolation is not used.

Figure 59 shows the effects of fDAC/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

fSIN

2 fSIN

4 fSIN

8 fSIN

fDAC/2

fSIN

2 fSIN

4 fSIN

8 fSIN

fDAC/4

Overlap

Touching

2 fSIN

4 fSIN

fDAC/8

Overlap

Touching

6 fSIN

Table 32. Modulated Pass-Band Edges Sinc Shaping

(Lower/Upper)

Interpolation

Modulation

None

×2

×4

×8

None

–2.8241

–0.6708

–0.1657

–0.0413

fDAC/2

–0.0701

–1.1932

–2.3248

–3.0590

–22.5378

–9.1824

–6.1190

–4.9337

fDAC/4

Overlap

Touching

–0.2921

–0.5974

–1.9096

–1.3607

fDAC/8

Overlap

Touching

–0.0727

–0.4614

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