AD9751 Analog Devices, AD9751 Datasheet - Page 19

• Manufacturer Part Number: AD9751
• Manufacturer: Analog Devices

Specifications of AD9751

• Resolution (bits): 10bit
• Dac Update Rate: 300MSPS
• Dac Settling Time: 11ns
• Max Pos Supply (v): +3.6V
• Single-supply: Yes
• Dac Type: Current Out
• Dac Input Format: Par

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 to selectively choose the “in phase”
and “quadrature” carriers, and then recombine the data.
The recombination of the QAM data can be mapped as points
representing digital words in a two-dimensional constellation, as
shown in Figure 27. Each point, or symbol, represents the trans-
mission of multiple bits in one symbol period.
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 wideband (>10 MHz) quadrature carriers to be syn-
thesized. Figure 28 shows an example of a 25 MSymbol/S
QAM signal, oversampled by 8 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.
REV. C
Figure 28. Reconstructing Raised Cosine Signal
at 120 MHz IF
COMMENT A: 25 MSYMBOL, 64 QAM, CARRIER = 25MHz
–100
–110
–120
–130
–30
–40
–50
–60
–70
–80
–90
START 100kHz
1
C11
MARKER 1 [T1]
0100
0110
1110
1100
9.71442886MHz
C11
–74.49dBm
0101
0111
1111
1101
C0
12.49MHz/
VBW
RBW
SWT
0001
0011
1011
1001
CH PWR
ACP UP
ACP LOW
1 [T1]
C0
12.5 s
50kHz
5kHz
Cu1
+9.71442886MHz
0000
0010
1010
1000
RF ATT
UNIT
–74.49bBM,
STOP 125MHz
–73.67dBm
–76.91dBm
–12.08dBm
Cu1
0dB
dBm
1RM
–19–
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
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.
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.
Figure 29. ACPR vs. Amplitude for QAM Carrier
80
70
60
50
40
–20
–15
AMPLITUDE (dBFS)
–10
–5
AD9751
0