AD9772AST datasheet(22/30 Pages) AD

22 / 30 page

REV. 0

AD9772

–22–

FREQUENCY – MHz

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

510

Figure 49. Spectral Plot of AD9772 Reconstructing Four

GSM Modulated Carriers @ fDATA = 52 MSPS, PLLVDD = 0

Although the above IS-136 and GSM spectral plots are repre-

sentative of the AD9772’s performance for a particular set of

test conditions, the following recommendations are offered to

maximize the performance and system integration of the AD9772

into multicarrier applications:

1. To achieve the highest possible CNR, the PLL Clock Multi-

plier should be disabled (i.e., PLLVDD to PLLCOM) and

the AD9772’s clock input driven with a low jitter/phase noise

clock source at twice the input data rate. In this case, the

divide-by-two clock appearing at PLLLOCK should serve as

the master clock for the digital upconverter IC(s) such as the

AD6622. PLLLOCK should be limited to a fanout of one.

2. The AD9772 achieves its optimum noise and distortion

performance when configured for baseband operation along

with a differential output and a full-scale current, IOUTFS, set

to approximately 20 mA.

3. Although the 2

× interpolation filters frequency roll-off pro-

vides a maximum reconstruction bandwidth of 0.422

× f

DATA,

the optimum adjacent image rejection (due to the interpola-

tion process) is achieved (i.e., > 73 dBc) if the maximum

channel assignment is kept below 0.400

× f

DATA.

4. To simplify the subsequent IF stages filter requirements (i.e.,

mixer image and LO rejection), it is often advantageous to

offset the frequency band from dc to relax the transition

band requirements of the IF filter.

5. Oversampling the frequency band often results in improved

SFDR and CNR performance. This implies that the data

input rate to the AD9772 is greater than fPASSBAND/0.4 where

fPASSBAND is the maximum bandwidth in which the AD9772

will be required to reconstruct and place carriers. The im-

proved noise performance results in a reduction in the

TxDAC’s noise spectral density due to the added process

gain realized with oversampling. Also, higher oversampling

ratios provide greater flexibility in the frequency planning.

BASEBAND SINGLE-CARRIER

The AD9772 is also well suited for wideband single-carrier

applications such as WCDMA and multilevel QAM whose

modulation scheme requires wide dynamic range from the re-

construction 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 50 shows a spectral plot of the AD9772 reconstructing a

test vector similar to those encountered in WCDMA applica-

tions with the following exception. WCDMA applications pre-

scribe 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 50 re-

veals, the AD9772 is capable of approximately 78 dB ACPR

performance when one accounts for the additive noise/distortion

contributed by the FSEA30 spectrum analyzer.

FREQUENCY – MHz

8.1922

–20

–40

–60

–80

–100

–120

10.2402 12.2882 14.3362 16.3842 18.4322 20.4802 22.5282 24.5762

ACPR LOW =

78.3dB

CH PWR =

–12.25dBm

ACPR UP =

77.7dB

Figure 50. AD9772 Achieves 78 dB ACPR Performance

Reconstructing a “WCDMA-Like” Test Vector with

fDATA = 65.536 MSPS and PLLVDD = 0

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