AD9775 datasheet(36/56 Pages) AD

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AD9775

Rev. E | Page 36 of 56

ZERO STUFFING

(Control Register 0x01, Bit 3)

As shown in Figure 75, a 0 or null in the output frequency

response of the DAC (after interpolation, modulation, and DAC

reconstruction) occurs at the final DAC sample rate (fDAC). This

is due to the inherent sin(x)/x roll-off response in the digital-to-

analog conversion. In applications where the desired frequency

content is below fDAC/2, this may not be a problem. Note that at

fDAC/2 the loss due to sin(x)/x is 4 dB. In direct RF applications,

this roll-off may be problematic due to the increased pass-band

amplitude variation as well as the reduced amplitude of the

desired signal.

Consider an application where the digital data into the AD9775

represents a baseband signal around fDAC/4 with a pass band of

fDAC/10. The reconstructed signal out of the AD9775 would

experience only a 0.75 dB amplitude variation over its pass band.

However, the image of the same signal occurring at 3 × fDAC/4

suffers from a pass-band flatness variation of 3.93 dB. This image

may be the desired signal in an IF application using one of the

various modulation modes in the AD9775. This roll-off of image

frequencies can be seen in Figure 59 to Figure 74, where the effect

of the interpolation and modulation rate is apparent as well.

–50

–40

–30

–20

–10

fOUT, NORMALIZED TO fDATA WITH ZERO STUFFING DISABLED (Hz)

0.5

1.0

1.5

2.0

ZERO STUFFING

ENABLED

ZERO STUFFING

DISABLED

Figure 75. Effect of Zero Stuffing on DAC’s sin(x)/x Response

To improve upon the pass-band flatness of the desired image,

the zero stuffing mode can be enabled by setting the control

fDAC/fDATA by a factor of 2 by doubling the DAC sample rate and

inserting a midscale sample (that is, 1000 0000 0000 0000) after

every data sample originating from the interpolation filter. This

is important as it affects the PLL divider ratio needed to keep

the VCO within its optimum speed range. Note that the zero

stuffing takes place in the digital signal chain at the output of

the digital modulator before the DAC.

The net effect is to increase the DAC output sample rate by a

factor of 2× with the 0 in the sin(x)/x DAC transfer function

occurring at twice the original frequency. A 6 dB loss in

amplitude at low frequencies is also evident (see Figure 75).

Note that the zero-stuffing option by itself does not change the

location of the images but rather their amplitude, pass-band

flatness, and relative weighting. For instance, in the previous

example, the pass-band amplitude flatness of the image at

3 × fDATA/4 improved to +0.59 dB while the signal level increased

slightly from −10.5 dBFS to −8.1 dBFS.

INTERPOLATING (COMPLEX MIX MODE)

(Control Register 0x01, Bit 2)

In the complex mix mode, the two digital modulators on the

AD9775 are coupled to provide a complex modulation function.

In conjunction with an external quadrature modulator, this

complex modulation can be used to realize a transmit image

rejection architecture. The complex modulation function can be

programmed for e+jωt or e−jωt to give upper or lower image

rejection. As in the real modulation mode, the modulation

frequency ω can be programmed via the SPI port for fDAC/2,

fDAC/4, and fDAC/8, where fDAC represents the DAC output rate.

OPERATIONS ON COMPLEX SIGNALS

Truly complex signals cannot be realized outside of a computer

simulation. However, two data channels, both consisting of real

data, can be defined as the real and imaginary components of a

complex signal. I (real) and Q (imaginary) datapaths are often

defined this way. By using the architecture defined in Figure 76,

a system can be realized that operates on complex signals,

giving a complex (real and imaginary) output.

If a complex modulation function (e+jωt) is desired, the real and

imaginary components of the system correspond to the real and

imaginary components of e+jωt or cosωt and sinωt. As Figure 77

shows, the complex modulation function can be realized by

applying these components to the structure of the complex

system defined in Figure 76.

a(t)

= (c + jd)

b(t)

c(t)

× b(t) + d × b(t)

b(t)

× a(t) + c × b(t)

INPUT

OUTPUT

INPUT

OUTPUT

COMPLEX FILTER

IMAGINARY

Figure 76. Realization of a Complex System

INPUT

(REAL)

OUTPUT

(REAL)

OUTPUT

(IMAGINARY)

INPUT

(IMAGINARY)

e–jωt = COS

ωt + jSINωt

Figure 77. Implementation of a Complex Modulator

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