AD9786BSVRL datasheet(33/56 Pages) AD

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AD9786

Rev. B | Page 33 of 56

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 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, fDAC, 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 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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