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AD9775

Rev. E | Page 39 of 56

The complex carrier synthesized in the AD9775 digital

modulator is accomplished by creating two real digital carriers

in quadrature. Carriers in quadrature cannot be created with

the modulator running at fDAC/2. As a result, complex modula-

tion only functions with modulation rates of fDAC/4 and fDAC/8.

Regions A and B of Figure 83 to Figure 88 are the result of the

complex signal described previously, when complex modulated

in the AD9775 by +ejωt. Regions C and D are the result of the

complex signal described previously, again with positive

frequency components only, modulated in the AD9775 by –ejωt.

The analog quadrature modulator after the AD9775 inherently

modulates by +ejωt.

Region A

Region A is a direct result of the upconversion of the complex

signal near baseband. If viewed as a complex signal, only the

images in Region A remain. The complex Signal A, consisting

of positive frequency components only in the digital domain,

has images in the positive odd Nyquist zones (1, 3, 5, …), as

well as images in the negative even Nyquist zones. The

appearance and rejection of images in every other Nyquist zone

becomes more apparent at the output of the quadrature

modulator. The A images appear on the real and the imaginary

outputs of the AD9775, as well as on the output of the quadrature

modulator, where the center of the spectral plot now represents

the quadrature modulator LO, and the horizontal scale now

represents the frequency offset from this LO.

Region B

Region B is the image (complex conjugate) of Region A. If a

spectrum analyzer is used to view the real or imaginary DAC

outputs of the AD9775, Region B appears in the spectrum.

However, on the output of the quadrature modulator, Region B

is rejected.

Region C

Region C is most accurately described as a downconversion, as

the modulating carrier is –ejωt. If viewed as a complex signal, only

the images in Region C remain. This image appears on the real

and imaginary outputs of the AD9775, as well as on the output of

the quadrature modulator, where the center of the spectral plot

now represents the quadrature modulator LO and the horizontal

scale represents the frequency offset from this LO.

Region D

Region D is the image (complex conjugate) of Region C. If a

spectrum analyzer is used to view the real or imaginary DAC

outputs of the AD9775, Region D appears in the spectrum.

However, on the output of the quadrature modulator, Region D

is rejected.

Figure 89 to Figure 96 show the measured response of the AD9775

and AD8345 given the complex input signal to the AD9775 in

Figure 89. The data in these graphs was taken with a data rate of

12.5 MSPS at the AD9775 inputs. The interpolation rate of 4× or 8×

gives a DAC output data rate of 50 MSPS or 100 MSPS. As a result,

the high end of the DAC output spectrum in these graphs is the

first null point for the sin(x)/x roll-off, and the asymmetry of the

DAC output images is representative of the sin(x)/x roll-off over the

spectrum. The internal PLL was enabled for these results. In

addition, a 35 MHz third-order low-pass filter was used at the

AD9775/AD8345 interface to suppress DAC images.

An important point can be made by looking at Figure 91 and

Figure 93. Figure 91 represents a group of positive frequencies

modulated by complex +fDAC/4, while Figure 93 represents a

group of negative frequencies modulated by complex −fDAC/4.

When looking at the real or imaginary outputs of the AD9775,

as shown in Figure 91 and Figure 93, the results look identical.

However, the spectrum analyzer cannot show the phase

relationship of these signals. The difference in phase between

the two signals becomes apparent when they are applied to the

AD8345 quadrature modulator, with the results shown in

Figure 92 and Figure 94.

–100

–80

–60

–40

–20

–0.5

–1.5

–1.0

–2.0

0.5

1.0

1.5

2.0

(LO)

fOUT (×fDATA)

Figure 83. 2× Interpolation, Complex fDAC/4 Modulation

–100

–80

–60

–40

–20

–1.0

–3.0

–2.0

–4.0

1.0

2.0

3.0

4.0

(LO)

fOUT (×fDATA)

Figure 84. 4× Interpolation, Complex fDAC/4 Modulation

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