AD9772A
APPLICATIONS
MULTICARRIER
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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.
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0
5
20
25
10
15
FREQUENCY – MHz
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.
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.
CLK
AD6622
CIC
NCO
SPORT RCF
SPORT RCF
SPORT RCF
PLLLOCK
CLK
FILTER QAM
AD9772A
CIC
FILTER QAM
NCO
SUMMATION
2. The AD9772A achieves its optimum noise and distortion
performance when configured for baseband operation along
CIC
FILTER QAM
NCO
OTHER AD6622s FOR
INCREASED CHANNEL
CAPACITY
with a differential output and a full-scale current, IOUTFS
set to approximately 20 mA.
,
CIC
FILTER QAM
NCO
SPORT RCF
JTAG
ꢂPORT
3. Although the 2ϫ interpolation filters frequency roll-off pro-
vides a maximum reconstruction bandwidth of 0.422 ϫ 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 ϫ fDATA.
Figure 30. Generic Multicarrier Signal Chain Using the
AD6622 and AD9772A
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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.
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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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0
5
10
15
20
25
30
FREQUENCY – MHz
Figure 31. Spectral Plot of AD9772A Reconstructing
Eight IS-136 Modulated Carriers @ fDATA = 64.54 MSPS,
PLLVDD = 0
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REV. A
ADI [ ADI ]
