AD9775EB datasheet(33/48 Pages) AD

33 / 48 page

REV. 0

AD9775

–33–

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 modulation only functions

with modulation rates of fDAC/4 and fDAC/8.

Regions A and B of Figures 37 through 42 are the result of the

complex signal described above, when complex modulated in the

AD9775 by +e

j t. Regions C and D are the result of the complex

signal described above, again with positive frequency components

only, modulated in the AD9775 by –e

j t. The analog quadra-

ture modulator after the AD9775 inherently modulates by +e

j t.

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 will 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 will become

more apparent at the output of the quadrature modulator. The

A images will appear on the real and the imaginary outputs of

the AD9775, as well as on the output of the quadrature modula-

tor, where the center of the spectral plot will now represent the

quadrature modulator LO, and the horizontal scale now repre-

sents the frequency offset from this LO.

Region B

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

trum analyzer is used to view the real or imaginary DAC outputs

of the AD9775, Region B will appear in the spectrum. However, on

the output of the quadrature modulator, Region B will be rejected.

Region C

Region C is most accurately described as a down conversion, as the

modulating carrier is –e

j t. If viewed as a complex signal, only the

images in Region C will remain. This image will appear on the

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

output of the quadrature modulator, where the center of the spec-

tral plot will now represent the quadrature modulator LO and the

horizontal scale will represent 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 will appear in the spectrum.

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

will be rejected.

Figures 43 through 50 show the measured response of the AD9775

and AD8345 given the complex input signal to the AD9775 in

Figure 43. 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

×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 Figures 45 and 47.

Figure 45 represents a group of positive frequencies modulated by

complex +fDAC/4, while Figure 47 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 Figures 45

and 47, the results look identical. However, the spectrum analyzer

cannot show the phase relationship of these signals. The differ-

ence in phase between the two signals becomes apparent when

they are applied to the AD8345 quadrature modulator, with the

results shown in Figures 46 and 48.

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