Datasheet

AD9772A
Rev. C | Page 29 of 40
APPLICATIONS INFORMATION
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 fre-
quency band. Cellular multicarrier and multimode radios are
often referred to as software radios because the carrier tuning
and modulation scheme is software programmable and performed
digitally. The AD9772A is the recommended TxDAC® in the
Analog Devices, Inc., SoftCell® chipset, which comprises the
AD6622 (a quadrature digital upconverter IC), the AD6624
(an Rx digital downconverter IC that acts as a companion to the
AD6622), and the AD6644 (a 14-bit, 65 MSPS ADC). Figure 53
shows a generic software radio Tx signal chain using the
AD9772A and
AD6622.
Figure 54 shows a spectral plot of the AD9772A operating at
64.54 MSPS, reconstructing eight IS-136-modulated carriers spread
over a 25 MHz band. In this example, the AD9772A exhibits an
SFDR performance of 74 dBc and a carrier-to-noise ratio (CNR) of
73 dB.
Figure 55 shows a spectral plot of the AD9772A operating at
52 MSPS, reconstructing four equal GSM-modulated carriers
spread over a 15 MHz band. The SFDR and CNR (in 100 kHz BW)
are measured to be 76 dBc and 83.4 dB, respectively, and have a
channel power of −13.5 dBFS. The test vectors were generated
using the Rohde & Schwarz WinIQSIM software.
JTAG
OTHER AD6622s FOR
INCREASED CHANNEL
CAPACITY
AD9772A
PLLLOCK CLK+/
CLK–
SUMMATION
SPORT RCF
CIC
FILTER
NCO
QAM
SPORT RCF
SPORT RCF
SPORT RCF
CLK
µPORT
AD6622
CIC
FILTER
CIC
FILTER
CIC
FILTER
NCO
QAM
NCO
QAM
NCO
QAM
02253-053
Figure 53. Generic Multicarrier Signal Chain Using the
AD6622 and AD9772A
FREQUENCY (MHz)
–40
–50
–100
0
AMPLITUDE (dBm)
–60
–70
–80
–90
5101520 3025
–30
–
20
02253-054
Figure 54. Spectral Plot of AD9772A Reconstructing Eight IS-136-Modulated
Carriers @ f
DATA
= 64.54 MSPS, PLLVDD = 0
FREQUENCY (MHz)
–
10
–110
0
AMPLITUDE (dBm)
–30
–50
–70
–90
5
10 15
20 25
–100
–80
–60
–40
–20
02253-055
Figure 55. Spectral Plot of AD9772A Reconstructing Four GSM-Modulated
Carriers @ f
DATA
= 52 MSPS, PLLVDD = 0
Although the above IS-136 and GSM spectral plots are
representative of the AD9772A’s performance for a 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
multiplier should be disabled (that is, PLLVDD to
PLLCOM) and the AD9772A clock input should be driven
with a low jitter, low phase noise clock source at twice the
input data rate. In this case, the divide-by-2 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 1.
2. The AD9772A achieves its optimum noise and distortion
performance when the device is configured for baseband
operation and the differential output and full-scale current,
I
OUTFS
, are set to approximately 20 mA.
3. Although the frequency roll-off of the 2× interpolation
filter provides a maximum reconstruction bandwidth of
0.422 × f
DATA
, the optimum adjacent image rejection (due to
the interpolation process) can be achieved (that is, > 73 dBc) if
the maximum channel assignment is kept below 0.400 × f
DATA
.
4. To simplify the filter requirements (that is, mixer image
and LO rejection) of the subsequent IF stages, 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 f
PASSBAND
/0.4 Hz, where
f
PAS SB AND
is the maximum bandwidth that the AD9772A is
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, and higher oversampling ratios provide greater
flexibility in the frequency planning.