AD9772EB datasheet(22/32 Pages) AD

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REV. A

AD9772A

–22–

APPLICATIONS

MULTICARRIER

The AD9772A’s wide dynamic range performance makes it well

suited for next generation base station applications in which it

reconstructs multiple modulated carriers over a designated

frequency band. Cellular multicarrier and multimode radios are

often referred to as software radios since the carrier tuning and

modulation scheme is software programmable and performed

digitally. The AD9772A is the recommended TxDAC in

Analog Device’s SoftCell chipset which comprises the AD6622,

Quadrature Digital Upconverter IC, along with its compan-

ion Rx Digital Downconverter IC, the AD6624, and 14-bit,

65 MSPS ADC, the AD6644. Figure 30 shows a generic soft-

ware radio Tx signal chain based on the AD9772A/AD6622.

Figure 31 shows a spectral plot of the AD9772A operating at

64.54 MSPS reconstructing eight IS-136 modulated carriers

spread over a 25 MHz band. For this particular test scenario,

the AD9772A exhibited 74 dBc SFDR performance along with

a carrier-to-noise ratio (CNR) of 73 dB. Figure 32 shows a spectral

plot of the AD9772A operating at 52 MSPS reconstructing four

equal GSM carriers spread over a 15 MHz band. The SFDR

and CNR (in 100 kHz BW) measured to be 76 dBc and 83.4 dB

respectively along with a channel power of –13.5 dBFS. Note,

the test vectors were generated using Rohde & Schwarz’s

WinIQSIM software.

JTAG

OTHER AD6622s FOR

INCREASED CHANNEL

CAPACITY

AD9772A

PLLLOCK

CLK

SUMMATION

SPORT

RCF

CIC

FILTER

NCO

QAM

SPORT

RCF

CIC

FILTER

NCO

QAM

SPORT

RCF

CIC

FILTER

NCO

QAM

SPORT

RCF

CIC

FILTER

NCO

QAM

CLK

PORT

AD6622

Figure 30. Generic Multicarrier Signal Chain Using the

AD6622 and AD9772A

FREQUENCY – MHz

–40

–50

–100

–60

–70

–80

–90

510

–30

–20

Figure 31. Spectral Plot of AD9772A Reconstructing

Eight IS-136 Modulated Carriers @ fDATA = 64.54 MSPS,

PLLVDD = 0

FREQUENCY – MHz

–10

–110

–30

–50

–70

–90

–100

–80

–60

–40

–20

Figure 32. Spectral Plot of AD9772A Reconstructing

Four GSM Modulated Carriers @ fDATA = 52 MSPS,

PLLVDD = 0

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

sentative of the AD9772A’s performance for a particular set of

test conditions, the following recommendations are offered to

maximize the performance and system integration of the AD9772A

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 AD9772A’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 AD9772A 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

fDATA,

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

fDATA.

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 AD9772A is greater than fPASSBAND/0.4

where fPASSBAND is the maximum bandwidth in which the

AD9772A will be required to reconstruct and place carriers.

The improved 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.

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