參數(shù)資料
型號(hào)： AD9755-EB
廠商： Analog Devices Inc
文件頁(yè)數(shù)： 11/28頁(yè)
文件大?。?/td> 0K
描述： BOARD EVAL FOR AD9755
產(chǎn)品培訓(xùn)模塊： DAC Architectures
標(biāo)準(zhǔn)包裝： 1
系列： TxDAC+®
DAC 的數(shù)量： 1
位數(shù)： 14
采樣率（每秒）： 300M
數(shù)據(jù)接口： 并聯(lián)
設(shè)置時(shí)間： 11ns
DAC 型： 電流
工作溫度： -40°C ~ 85°C
已供物品： 板
已用 IC / 零件： AD9755
相關(guān)產(chǎn)品： AD9755ASTZRL-ND - IC DAC 14BIT 300MSPS 48-LQFP
AD9755ASTZ-ND - IC DAC 14BIT 300MSPS 48-LQFP
REV. B
AD9755
–19–
APPLICATIONS
QAM/PSK Synthesis
Quadrature modulation (QAM or PSK) consists of two baseband
PAM (Pulse Amplitude Modulated) data channels. Both channels
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 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 AD9755 allows
extremely wideband (>10 MHz) quadrature carriers to be syn-
thesized at IFs of over 100 MHz. Figure 28 shows an example
of a 25 MSymbol/S QAM signal, raised cosine-like pulse,
oversampled by 8 at a data rate of 200 MSPS modulated onto a
25 MHz carrier and reconstructed using the AD9755.
–30
START 100kHz
–40
–50
–60
–70
–80
–90
–100
–110
–120
–130
12.49MHz/
STOP 125MHz
FREQUENCY (MHz)
REFERENCE
LEVEL
(dBm)
COMMENT A: 25 MSYMBOL, 64 QAM, CARRIER = 25MHz
A
1 [T1]
CH PWR
ACP UP
ACP LOW
–74.25dBm
9.71442886MHz
–77.42dBm
–78.88dBm
–11.83dBm
1
C11
CU1
C0
–74.25dBm
9.71442886MHz
VBW
50kHz
SWT
12.5s UNIT dBm
1RM
C11
Figure 28. Reconstructed 64-QAM Signal at 25 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 channel.
In Figure 28, the adjacent channel power ratio (ACPR) at the
output of the AD9755 is measured to be 65 dB. The limitation 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 condi-
tions measured in Figure 28, this point occurs in Figure 29 at
–10 dBFS. This shows that the data record is actually degrading
the measured ACPR by up to 10 dB.
AMPLITUDE (dBFS)
40
0
ACPR
(dB)
50
–20
–5
60
70
80
–15
–10
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 AD9755 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 60 dB.
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