AD9772AST datasheet(23/30 Pages) AD

23 / 30 page

REV. 0

AD9772

–23–

DIRECT IF

As discussed in the Digital Modes of Operation section, the

AD9772 can be configured to transform digital data represent-

ing baseband signals into IF signals appearing at odd multiples

of the input data rate (i.e., N

× f

DATA where N = 1, 3, . . .). This

is accomplished by configuring the MOD1 and MOD0 digital

inputs HIGH. Note, the maximum DAC update rate of 400 MSPS

limits the data input rate in this mode to 100 MSPS when the

“zero-stuffing operation” is enabled (i.e., MOD1 High). Appli-

cations requiring higher IFs (i.e., 140 MHz) using higher data

rates should disable the “zeros-stuffing” operation. Also, to

minimize the effects of the PLL Clock Multipliers phase noise

as shown in Figure 27, an external low jitter/phase noise clock

source equal to 4

× f

DATA is recommended.

Figure 51 shows the actual output spectrum of the AD9772

reconstructing a 16-QAM test vector with a symbol rate of

5 MSPS. The particular test vector was centered at fDATA/4 with

fDATA = 100 MSPS, and fDAC = 400 MHz. For many applica-

tions, the pair of images appearing around fDATA will be more

attractive since they have the flattest passband and highest signal

power. Higher images can also be used with the realization that

these images will have reduced passband flatness, dynamic

range, and signal power, thus reducing the CNR and ACP per-

formance. Figure 52 shows a dual tone SFDR amplitude sweep

at the various IF images with fDATA = 100 MSPS and fDAC =

400 MHz and the two tones centered around fDATA/4. Note,

since an IF filter is assumed to precede the AD9772, the SFDR

was measured over a 25 MHz window around the images occur-

ring at 75 MHz, 125 MHz, 275 MHz and 325 MHz.

FREQUENCY – MHz

–20

–40

–60

–80

–100

–120

100

150

200

250

300

350

400

Figure 51. Spectral Plot of 16-QAM Signal in Direct IF

Mode at fDATA = 100 MSPS

AOUT – dBFS

–15

–12

–9

–6

–3

IF @ 275MHz

IF @ 75MHz

IF @ 125MHz

IF @ 325MHz

Figure 52. Dual Tone “Windowed” SFDR vs. AOUT at

fDATA = 100 MSPS

Regardless of what image is selected for a given application, the

adjacent images must be sufficiently filtered. In most cases, a

SAW filter providing differential inputs represents the opti-

mum device for this purpose. For single-ended SAW filters, a

balanced-to-unbalanced RF transformer is recommended. The

AD9772’s high output impedance provides a certain amount of

flexibility in selecting the optimum resistive load, RLOAD, as well

as any matching network.

For many applications, the data update rate to the DAC (i.e.,

fDATA) must be some fixed integer multiple of some system

reference clock (i.e., GSM – 13 MHz). Furthermore, these

applications prefer to use standard IF frequencies which offer a

large selection of SAW filter choices of varying passbands (i.e.,

70 MHz). These applications may still benefit from the AD9772’s

direct IF mode capabilities when used in conjunction with a

digital upconverter such as the AD6622. Since the AD6622 can

digitally synthesize and tune up to four modulated carriers, it is

possible to judiciously tune these carriers in a region which may

fall within an IF filter’s passband upon reconstruction by the

AD9772. Figure 53 shows an example in which four carriers

were tuned around 18 MHz with a digital upconverter operating

at 52 MSPS such that when reconstructed by the AD9772 in

the IF MODE, these carriers fall around a 70 MHz IF.

FREQUENCY – MHz

–10

–20

–30

–40

–50

–60

67.5

72.5

–70

–80

–90

–100

Figure 53. Spectral Plot of Four Carriers at 70 MHz IF with

fDATA = 52 MSPS, PLLVDD = 0

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