AD9772EB datasheet(23/32 Pages) AD

23 / 32 page

REV. A

AD9772A

–23–

BASEBAND SINGLE-CARRIER

The AD9772A is also well suited for wideband single-carrier

applications such as WCDMA and multilevel QAM whose

modulation scheme requires wide dynamic range from the

reconstruction DAC to achieve the out-of-band spectral mask as

well as the in-band CNR performance. Many of these applica-

tions strategically place the carrier frequency at one quarter of

the DAC’s input data rate (i.e., fDATA/4) to simplify the digital

modulator design. Since this constitutes the first fixed IF fre-

quency, the frequency tuning is accomplished at a later IF stage.

To enhance the modulation accuracy as well as reduce the shape

factor of the second IF SAW filter, many applications will often

specify the passband of the IF SAW filter be greater than the

channel bandwidth. The trade-off is that the TxDAC must now

meet the particular application’s spectral mask requirements

within the extended passband of the 2nd IF, which may include

two or more adjacent channels.

Figure 33 shows a spectral plot of the AD9772A reconstructing

a test vector similar to those encountered in WCDMA applica-

tions with the following exception. WCDMA applications

prescribe a root raised cosine filter with an alpha = 0.22, which

limits the theoretical ACPR of the TxDAC to about 70 dB. This

particular test vector represents white noise that has been band-

limited by a “brickwall” bandpass filter with the same passband

such that its maximum ACPR performance is theoretically

83 dB and its peak-to-rms ratio is 12.4 dB. As Figure 33 reveals,

the AD9772A is capable of approximately 78 dB ACPR per-

formance when one accounts for the additive noise/distortion

contributed by the FSEA30 spectrum analyzer.

–30

CENTER 16.25MHz

SPAN 6MHz

600kHz

–40

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–90

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–120

–130

C11

Cu1

Figure 33. AD9772A Achieves 78 dB ACPR Performance

Reconstructing a “WCDMA-Like” Test Vector with fDATA =

65.536 MSPS and PLLVDD = 0

DIRECT IF

As discussed in the Digital Modes of Operation section, the

AD9772A 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

fDATA 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 9, an external low jitter/phase noise clock

source equal to 4

× f

DATA is recommended.

Figure 34 shows the actual output spectrum of the AD9772A

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 understanding

that these images will have reduced passband flatness, dynamic

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

formance. Figure 35 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 AD9772A, the

SFDR was measured over a 25 MHz window around the images

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

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 optimum

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

to-unbalanced RF transformer is recommended. The AD9772A’s

high output impedance provides a certain amount of flexibility

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

matching network.

FREQUENCY – MHz

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–40

–20

100

200

300

400

Figure 34. Spectral Plot of 16-QAM Signal in Direct IF

Mode at fDATA = 100 MSPS

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