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參數(shù)資料
| 型號： | AD9751AST |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | DAC |
| 英文描述： | 10-Bit, 300 MSPS High-Speed TxDAC+ D/A Converter |
| 中文描述： | PARALLEL, WORD INPUT LOADING, 0.011 us SETTLING TIME, 10-BIT DAC, PQFP48 |
| 封裝： | MS-026BBC, LQFP-48 |
| 文件頁數(shù)： | 18/26頁 |
| 文件大?。?/td> | 714K |
| 代理商： | AD9751AST |
REV. 0
AD9751
–18–
APPLICATIONS
QAM/PSK Synthesis
Quadrature modulation (QAM or PSK) consists of two baseband
PAM (Pulse Amplitude Modulated) data channels. Both chan-
nels are modulated by a common frequency carrier. However,
the carriers for each channel are phase-shifted 90
°
from each
other. This orthogonality allows twice the spectral efficiency
(data for a given bandwidth) of digital data transmitted via AM.
Receivers can be designed which selectively choose the ‘in phase’
and ‘quadrature’ carriers, and then recombine the data. The
recombination of the QAM data can be mapped as points repre-
senting digital words in a two dimensional constellation as shown
in Figure 27. Each point, or symbol, represents the transmission
of multiple bits in one symbol period.
0100
0101
0001
0000
0110
0111
0011
0010
1110
1111
1011
1010
1100
1101
1001
1000
Figure 27. 16 QAM Constellation, Gray Coded (Two 4-Level
PAM Signals with Orthogonal Carriers)
Typically, the I and Q data channels are quadrature-modulated
in the digital domain. The high data rate of the AD9751 allows
extremely wide band (>10 MHz) quadrature carriers to be syn-
thesized. Figure 28 shows an example of a 25 MSymbol/S
QAM signal, oversampled by eight at a data rate of 200 MSPS;
modulated onto a 25 MHz carrier and reconstructed using the
AD9751. The power in the reconstructed signal is measured
to be –12.08 dBm. In the first adjacent band, the power is
–73.67 dBm, while in the second adjacent band the power is
–76.91 dBm.
–
30
START 100kHz
–
40
–
50
–
60
–
70
–
80
–
90
–
100
–
110
–
120
–
130
12.49MHz/
STOP 125MHz
R
–
1RM
–
74.49bBM,
+9.71442886MHz
–
73.67dBm
–
76.91dBm
–
12.08dBm
1 [T1]
CH PWR
ACP UP
ACP LOW
C11
C0
Cu1
C11
C0
Cu1
MARKER 1 [T1] RBW 5kHz RF ATT 0dB
–
74.49dBm VBW 50kHz
9.71442886MHz SWT 12.5 s UNIT dBm
COMMENT A: 25 MSYMBOL, 64 QAM, CARRIER = 25MHz
1
Figure 28. Reconstructing Raised Cosine Signal at
120 MHz IF
A figure of merit for wideband signal synthesis is the ratio of signal
power in the transmitted band to the power in an adjacent chan-
nel. In Figure 29, the adjacent channel power ratio (ACPR) at
the output of the AD9751 is measured to be 62 dB. The limita-
tion on making a measurement of this type is often not the DAC
but the noise inherent in creating the digital data record using
computer tools. To find how much this is limiting the perceived
DAC performance, the signal amplitude can be reduced, as is
shown in Figure 29. The noise contributed by the DAC will
remain constant as the signal amplitude is reduced. When the
signal amplitude is reduced to the level where the noise floor
drops below that of the spectrum analyzer, ACPR will fall off at
the same rate that the signal level is being reduced. Under the
conditions measured in Figure 28, this point occurs in Figure 29
at –4 dBFS. This shows that the data record is actually degrad-
ing the measured ACPR by up to 4 dB.
AMPLITUDE
–
dBFS
80
60
40
0
–
10
–
20
A
–
70
50
–
15
–
5
Figure 29. ACPR vs. Amplitude for QAM Carrier
A single-channel active mixer such as the Analog Devices AD8343
can then be used for the hop to the transmit frequency. Figure 30
shows an applications circuit using the AD9751 and the AD8343.
The AD8343 is capable of mixing carriers from dc to 2.5 GHz.
Figure 31 shows the result of mixing the signal in Figure 28 up
to a carrier frequency of 800 MHz. ACPR measured at the
output of the AD8343 is shown in Figure 31 to be 58 dB.
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
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| AD9751ASTZKL1 | 制造商:Rochester Electronics LLC 功能描述: 制造商:Analog Devices 功能描述: |
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